Article

Carbonate platform growth and cyclicity at a terminal Proterozoic passive margin, Infra Krol Formation and Krol Group, Lesser Himalaya, India

October 2003
Sedimentology 50(5):921 - 952

October 2003
50(5):921 - 952

DOI:10.1046/j.1365-3091.2003.00589.x

Authors:

Ganqing Jiang

University of Nevada, Las Vegas

Nicholas Christie-Blick

Columbia University

Alan Jay Kaufman

University of Maryland, College Park

Dhiraj Mohan Banerjee

University of Delhi

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Abstract The Infra Krol Formation and overlying Krol Group constitute a thick (< 2 km), carbonate-rich succession of terminal Proterozoic age that crops out in a series of doubly plunging synclines in the Lesser Himalaya of northern India. The rocks include 18 carbonate and siliciclastic facies, which are grouped into eight facies associations: (1) deep subtidal; (2) shallow subtidal; (3) sand shoal; (4) peritidal carbonate complex; (5) lagoonal; (6) peritidal siliciclastic–carbonate; (7) incised valley fill; and (8) karstic fill. The stromatolite-rich, peritidal complex appears to have occupied a location seaward of a broad lagoon, an arrangement reminiscent of many Phanerozoic and Proterozoic platforms. Growth of this complex was accretionary to progradational, in response to changes in siliciclastic influx from the south-eastern side of the lagoon. Metre-scale cycles tend to be laterally discontinuous, and are interpreted as mainly autogenic. Variations in the number of both sets of cycles and component metre-scale cycles across the platform may result from differential subsidence of the interpreted passive margin. Apparently non-cyclic intervals with shallow-water features may indicate facies migration that was limited compared with the dimensions of facies belts. Correlation of these facies associations in a sequence stratigraphic framework suggests that the Infra Krol Formation and Krol Group represent a north- to north-west-facing platform with a morphology that evolved from a siliciclastic ramp, to carbonate ramp, to peritidal rimmed shelf and, finally, to open shelf. This interpretation differs significantly from the published scheme of a basin centred on the Lesser Himalaya, with virtually the entire Infra Krol–Krol succession representing sedimentation in a persistent tidal-flat environment. This study provides a detailed Neoproterozoic depositional history of northern India from rift basin to passive margin, and predicts that genetically related Neoproterozoic deposits, if they are present in the High Himalaya, are composed mainly of slope/basinal facies characterized by fine-grained siliciclastic and detrital carbonate rocks, lithologically different from those of the Lesser Himalaya.

Sedimentary evolution and sequence stratigraphy of Ediacaran high‐grade phosphorite–dolomite‐shale successions of Bocaina Formation (Corumbá Group), Central Brazil: Implications for the Neoproterozoic phosphogenic event

Article

Jul 2023

Extensive phosphorite deposition is observed in the Neoproterozoic after a prolonged hiatus during most of the Mesoproterozoic era. This event is thought to represent an important record of major palaeoenvironmental, palaeoceanographic and biotic changes that shaped Neoproterozoic ecosystems, suggesting close relationships between phosphogenesis and the preservation of key Ediacaran biotas. However, high-grade Ediacaran phosphorite deposits are relatively uncommon, diminishing the opportunity to test current phosphate mineralization-deposition models and their implications for Neoproterozoic research. In this scenario, widespread Ediacaran phosphorite–dolomite–shale successions of Bocaina Formation (Corumbá Group – Central Brazil) are poorly explored in international literature. Nevertheless, recent advances in phosphate exploration gave access to continuous drill core sections and freshly opened mine pits, revealing an unprecedented record of complex phosphatic successions featuring the occurrence of Ediacaran microfossils assigned to the Doushantuo–Pertatataka assemblages. This work seeks to constrain main lithofacies, sequence stratigraphy and depositional settings from these phosphatic successions in order to analyze the sedimentary evolution of the unit under the current Neoproterozoic phosphorite research framework. These results indicate that Bocaina Formation records secular sustained phosphate deposition. These deposits are related to unprecedented, microbialite reef rim phosphorites deposited during a lower accretionary rimmed platform stage, followed by the deposition of Doushantuo-like, whole platform phosphorites associated with a later, drowned platform stage, therefore, reinforcing the evidence for the operation of strong allogeneic controls on phosphate mineralization-concentration. In addition, this study concludes that fossiliferous Ediacaran phosphatic deposits such as the Bocaina Formation are important to understanding Neoproterozoic phosphogenic events, because they may record the transition from a Precambrian to Phanerozoic-like phosphogenesis associated with the instauration of the Ediacaran–Cambrian phosphatic taphonomic window. This evidence hints that the growing dataset from the Bocaina Formation may bring new, exciting perspectives for Neoproterozoic research as a whole.

Stratigraphy and depositional evolution of the terminal Ediacaran platform in the central to northern Sichuan Basin, Southwest China

Article

Jul 2022
PALAEOGEOGR PALAEOCL

The upper Ediacaran Dengying Formation in the Sichuan Basin provides well-preserved sedimentary records for investigating Precambrian microbialites and platform evolution. However, as previous studies have mainly focused on the broad platform-basin architecture, the depositional evolution of the platform has not been fully illuminated in the central to northern Sichuan Basin, where an intraplatform rift trough developed. Therefore, based on drilling wells and outcrops in the area, 20 lithofacies were identified and grouped into eight lithofacies associations indicating peritidal flat, shallow subtidal flat, lagoon, mixed tidal flat, slope, basin, coast and shallow sea environments. Moreover, two transgressive-regressive depositional cycles (Cycle 1 and Cycle 2) were recognized based on the five shallowing-upward (types A–E) and three deepening-upward (types F–H) cycle types. A ramp-like epeiric platform with a gentle slope developed in Cycle 1. The platform is dominated by shallow subtidal grainstones and peritidal microbialites, and the trough is in a slope environment without deep-water basin facies. In the transgression of Cycle 2, a siliciclastic-dominated environment comprising the coast and shallow sea developed on the shelf. Then a rimmed carbonate platform developed in the regression of Cycle 2, with the accumulation of shoal grainstone and peritidal microbialite in the platform margin, lagoonal dolomudstone in the platform interior, and steep slope rhythmite and basinal chert in the distal rift trough. Eustatic sea-level fluctuation and climatic change controlled the lithological transitions between carbonate and siliciclastic deposits. The platform configuration change is closely related to the evolution of the rift trough, which is the result of intense differential subsidence and sedimentation rates between the trough and platform under the effect of pre-existing basement rifts and coeval growth fault activities. A detailed depositional history of the Ediacaran Dengying Formation around the intraplatform rift trough was finally obtained, providing a good template for the study of the Precambrian microbialite-dominated platform with a similar tectonic setting.

A newly discovered Neoproterozoic diamictite-cap carbonate couplet from the Western Himalaya: The expansion of the Marinoan snowball Earth glaciation to the northwestern margin of the Indian Plate in North Pakistan

Article

Aug 2022
PRECAMBRIAN RES

One of the central tenets of the snowball Earth hypothesis is the global recognition of the glacial diamictite-cap carbonate couplets in the Cryogenian sedimentary record. However, the scarcity of such peculiar stratigraphic elements in the Neoproterozoic successions of the Himalaya brings into question the severity and global extent of the snowball Earth event and likewise hampered glacio-stratigraphic correlation within the Himalaya and elsewhere. This study provides the first convincing evidence of the glacial diamictite-cap carbonate couplet from the Tanakki Member of the basal Kakul Formation, previously unknown from the Western Himalaya in North Pakistan. Detailed sedimentological analysis of the diamictite from the Tanakki Member reveals deposition in glacially-influenced proximal to distal subaqueous debris apron. The presence of glaciogenic clasts (striated, facetted and bullet-shaped) together with evidence of the ice-rafted dropstones in pervasive facies association provides credence to the glaciogenic affinity. The thin cap carbonate (herein referred to as Tanakki-cap dolomite; TCD) overlying the glacial diamictite record deposition in a deeper shelf (offshore) setting. The lithological, depositional and persistent negative C-isotope characteristics (ca. −3.2 to −5.8‰) combined with regional stratigraphic and available geochronological data allow us to interpret TCD as a ‘Marinoan’ cap carbonate and the underlying diamictite as an expansion of the terminal-Cryogenian (Marinoan) glaciation in the Western Himalaya. Moreover, the analyses of the tectonic and depositional history of the Tanakki Member coupled with the Neoproterozoic paleogeographic evolution of the northern margin of the Indian Plate argue against a previous interpretation of the culminating foreland basin orogeny and instead support deposition in an extensional fault-controlled rift basin. Finally, this study permits us to revise the Neoproterozoic stratigraphic framework of the Western Himalaya by describing the Cryogenian-Ediacaran boundary interval in the region that ultimately helps to overcome the previous glacio-stratigraphic discrepancies in the Neoproterozoic record of the Himalaya.

Article

Jun 2022
PRECAMBRIAN RES

Krol Sandstone-black shale association of the Lesser Himalayan Neoproterozoic succession, Himachal Pradesh, India: An unexplored record of the hothouse aftermath

Article

Apr 2022
MAR PETROL GEOL

Citation of the inferred glacial diamictite-cap carbonate association, as the unequivocal evidence of the Snowball Earth hypothesis, fails to explain the absence of the sedimentary record of the intermediate period between the onset of thawing and the restoration of the optimum temperature for carbonate deposition. The Krol Sandstone-black shale association of the Lesser Himalayan Neoproterozoic succession, represents the initial phase of that hothouse aftermath. The black shale is the product of oceanic anoxia developed during the initial thawing phase of late Cryogenian glaciation. As the melting process continued, remobilization of some pre-existing aeolian sand sheet deposit, lying on the melting ice sheet in the upper shelf region, led to the accumulation of deeper shelf fans, the Krol Sandstone, over this black shale. The petrographic composition, detrital zircon age, and the palaeoflow directions, in conjunction with the available reconstructed paleogeography, suggest that the constituent sand particles of Krol Sandstone were possibly derived from a Late Neoproterozoic aeolian sand sheet of South Australia formed during the coldest phase of the late Cryogenian Elatina glaciation. These findings not only document the sedimentary record of the initial phase of the hothouse aftermath, but also open up a new line of evidence for paleogeographic reconstruction.

Outer to Central Himachal Himalaya Transact - Sedimentary and Tectonic Story Unfolded.

Book

Full-text available

Apr 2022

36th IGC Field Guide Book

Krol Sandstone-Black Shale Association of the Lesser Himalayan Neoproterozoic Succession, Himachal Pradesh, India: An Unexplored Record of the Hothouse Aftermath

Article

Jan 2022

Characteristics of a Tonian reef rimmed shelf before the onset of Cryogenian: Insights from Neoproterozoic Kunihar Formation, Simla Group, Lesser Himalaya

Article

Apr 2020
MAR PETROL GEOL

The Kunihar Formation records evolution of a unique rimmed shelf during the Tonian, before the Cryogenian/Marinoan glaciation of Krol Formation, Lesser Himalaya. The mixed siliciclastic-carbonate system of Neoproterozoic Kunihar Formation provokes interest for outcrop based facies analysis and sequence stratigraphy in order to frame a better understanding on the mechanism and control of its deposition. The formation records a 450 m thick mixed siliciclastic-carbonate rimmed shelf. Five facies associations have been identified- Peritidal-FA1, Lagoonal-FA2, Barrier island/Ooidal shoal-FA3, Reef complex-FA4 and Fore reef-FA5 facies associations. Two systems tracts were delineated - transgressive (TST) and highstand (HST) systems tracts. TST is marked by dominant aggradation of stromatolitic carbonates, while siliciclastic incursion surpassed carbonate deposition during highstand. It conforms to the “lower carbonate-upper clastic” model of mixed siliciclastic-carbonates. During the Precambrian, reef rimmed shelves were considered more abundant in the Archaean and Palaeo-Mesoproterozoic. This study determines that development of rimmed shelves was more common during the Neoproterozoic than previously speculated. Neoproterozoic reef rimmed shelves are concentrated more within the Cryogenian and Ediacaran Period. Stromatolitic reef rimmed shelf of Kunihar Formation is the only rimmed shelf recorded in the Tonian Period till date. Increased microbial stromatolitic accretion was propelled by surge in nutrients due to chemical weathering of underlying tholeiitic basalt (Darla volcanics) associated with the rifting of supercontinent Rodinia. With increasing interest of Tonian-Cryogenian transition as a key period in both planetary and organic evolution, Kunihar Formation represents a terminal reef rimmed shelf system that evolved due to microbial stromatolites in the absence of complex organisms. Subsequent to this, Cryogenian and Ediacaran biota took over as the dominant reef builders.

Neoproterozoic Blaini Formation of Lesser Himalaya, India: Fiction and Fact

Article

Mar 2020
GEOL SOC AM BULL, GSA Bulletin

The long-conceived idea of the glacial origin of Blaini diamictite of Lesser Hima-layan Neoproterozoic succession reached its maxima when the diamictites and capping pink limestone were attributed to the Neoproterozoic Snowball Earth event and its aftermath, respectively. Occurrences of diamictite-limestone association in two different levels have also been correlated with the Sturtian and Marinoan glaciations. Critical review, however, reveals that the interpretations of the glacial origin of diamictites are not well founded. The diamictite-limestone association, which occurs at the lower part of a thick, light brown shale unit and laterally grades into light brown shale, primarily indicates episodic surge events in an otherwise tranquil condition favorable for hemipelagic sedimentation. The lithology, bed geometry, internal organization, and disposition of the diamictite bodies suggest deposition of debris flow fan lobes along fault scarps in a rift setting. Emplacement of subaqueous debris flows is indicated by the associated deposits of entrained turbidity currents. The limestone also bears the signature of claciturbidites. The appearance of diamictite bodies and associated limestone in two distinct levels is not a stratigraphic disposition; on the contrary, the deposits were dislocated and repeated by two successive regional thrust faults. The Chemical Index of Alteration (CIA) values of the light brown shale and the matrix of the diamictites indicate that these sediments formed through prolonged subaerial weathering. The events leading up to development of the rift system and evidence of prolonged weathering within the basin-fill sediments are consistent with supercontinental break up, the prologue of Snowball Earth.

An Empirical Approach for Quick Assessment of Hazard and Safe Height of Steep-Cut Rock Slopes in Garhwal Himalayas

Article

Oct 2023

Accessibility and all-weather functioning of the roads in Himalayas form the backbone of the overall development of the region. All-weather roads are being constructed in the region to cater to the needs of this region and provide quick accessibility to remote areas. Majority of roads are constructed along the hill slopes. Construction of new roads as well as the widening project involves the excavation on hill side of the slope. As a consequence, steep-cut slopes are formed at the side of the road. In principle, adequate analysis and design is warranted while taking up construction activities. However, rigorous slope stability analysis for each section is not always feasible. The field engineers therefore rely on personal experience, judgment and perception, especially when quick decisions are required. Looking at the problem and the importance of hill roads in the Himalayas, urgent need for developing site-specific tools is being felt in organizations dealing with mitigation activities in this region. The tools are required not only at the time of construction but also during the maintenance and emergent situation of natural disasters and also to look out for safe alternate routes. In the field of rock mechanics, rock mass classification systems have been found to be very effective in addressing similar problems. Rock mass rating (Bieniawski, in: Transaction of the South African Institution of Civil Engineers, 1973), slope mass rating (Romana in: International symposium on the role of rock mechanics in excavations for mining and civil works, International Society of Rock Mechanics, Zacatecas, 1985), geological strength index (Marinos et al. in Bull Eng Geol Environ 64:55–65, 2005. https://doi.org/10.1007/s10064-004-0270-5; Morelli in Geotech Geol Eng 35:2803–2816, 2017. https://doi.org/10.1007/s10706-017-0279-8; Marinos et al. in: Underground works under special conditions, 2010. https://doi.org/10.1201/noe0415450287.ch2) and the Q systems (Barton et al. in Rock Mech 6:189–236, 1974. https://doi.org/10.1007/BF01239496) have been used in the past. In the present manuscript, the applicability of the Joint Factor concept (Ramamurthy in Indian Geotech J 8:1–74, 1985; Ramamurthy and Arora in Int J Rock Mech Min Sci 31:9–22, 1994. https://doi.org/10.1016/0148-9062(94)92311-6; Singh et al. in Rock Mech Rock Eng 35:45–64, 2002. https://doi.org/10.1007/s006030200008) is extended to propose an empirical approach for quick assessment of hazard of rock slopes for cut slopes in Garhwal Himalayas. The main objective of the present study is to come out with a methodology to work out an approximate value of the safe height of steep-cut rock slopes. The study has been conducted on steep-cut rock slopes of Garhwal Himalayas. Site investigations followed by desk studies have been done to determine the important parameters of the discontinuities observed in the field. Joint Factor concept is used for evaluating the instability of cut rock slopes. Charts are suggested to assess the hazard and safe height of steep-cut rock slopes based on simple inputs which can be easily obtained in the field. Validation of the proposed approach was done by assessing the stability of cut rock slopes from five different sites. It can be concluded that the observation made in the field and the result obtained from the chart are in agreement with each other.

Carbonate platform architecture and carbon isotope stratigraphy from the neoproterozoic Narji limestone formation, Kurnool Group, India: Decoding clues of relative sea level fluctuations

Article

Dec 2022
MAR PETROL GEOL

From a detail process-based facies and facies association analysis in a spatio-temporal framework, a rimmed shelf platform geometry is proposed for the Neoproterozoic Narji Limestone Formation, Kurnool Group, India. Ten carbonate facies types were delineated and clubbed under seven facies associations belonging to four different paleogeographic settings viz. Forereef (intertidal to shallow subtidal and shallow to deep subtidal), peritidal reef and back-reef lagoon. Whereas crinkly microbial laminites with subordinate domal stromatolites constitute the reef part, the fore slope deposition in the platform largely remained tide-and storm-dominated. Grey to black colored pyrite bearing bedded limestone-shale alternation without any current or wave imprint represents the back-reef lagoon deposition. A high carbonate saturation and productivity in the Narji inner and mid shelf is suggested from an overall dominance of microalgal laminite and microdigitate stromatolites over columnar stromatolites. From documentation of decimeter-to meter-thick shallowing-upward depositional hemicycles, embedded within an overall aggradational Narji lithosuccession, a combined control of sea level and carbonate productivity is inferred in course of Narji sedimentation. A short-duration medium-to large-scale rise in the base level led to the aggradation in shallow water rim of the Narji platform margin; interruption of this overall motif by short thickening- and shallowing-upward progradational hemicycles bear testimony for higher productivity in the Narji mid-to inner shelf areas. The present study intends to correlate shallowing-upward hemicycles with keep-up to catch-up depositional cycles with signature of apparent sea level fall. The control of relative base level variation in Narji depositional history is strengthened with multiple signatures including secular δ13C composition, total organic carbon (TOC) content and siliciclastic content documented from the entire Narji litho-succession.

Carbonate Platform Architecture and Carbon Isotope Stratigraphy from the Neoproterozoic Narji Limestone Formation, Kurnool Group, India: Decoding Clues of Relative Sea Level Fluctuations

Article

Dec 2022
MAR PETROL GEOL

From a detail process-based facies and facies association analysis in a spatio-temporal framework, a rimmed shelf platform geometry is proposed for the Neoproterozoic Narji Limestone Formation, Kurnool Group, India. Ten carbonate facies types were delineated and clubbed under seven facies associations belonging to four different paleogeographic settings viz. forereef (intertidal to shallow subtidal and shallow to deep subtidal), peritidal reef and back-reef lagoon. Whereas crinkly microbial laminites with subordinate domal stromatolites constitute the reef part, the fore slope deposition in the platform largely remained tide-and storm-dominated. Grey to black colored pyrite bearing bedded limestone-shale alternation without any current or wave imprint represents the back-reef lagoon deposition. A high carbonate saturation and productivity in the Narji inner and mid shelf is suggested from an overall dominance of microalgal laminite and microdigitate stromatolites over columnar stromatolites. From documentation of decimeter- to meter-thick shallowing-upward depositional hemicycles, embedded within an overall aggradational Narji lithosuccession, a combined control of sea level and carbonate productivity is inferred in course of Narji sedimentation. A short-duration medium- to large-scale rise in the base level led to the aggradation in shallow water rim of the Narji platform margin; interruption of this overall motif by short thickening- and shallowing-upward progradational hemicycles bear testimony for higher productivity in the Narji mid-to inner shelf areas. The present study intends to correlate shallowing upward hemicycles with keep-up to catch-up depositional cycles with signature of apparent sea level fall. The control of relative base level variation in Narji depositional history is strengthened with multiple signatures including secular δ13C composition, total organic carbon (TOC) content and siliciclastic content documented from the entire Narji litho-succession.

Cambrian−Ordovician magmatic flare-up in NE Gondwana: A silicic large igneous province?

Article

Oct 2022

The origin of the Cambrian−Ordovician tectono-magmatic events affecting NE Gondwana and the adjacent peri-Gondwanan terranes (e.g., Himalaya, Lhasa, Southern Qiangtang, Baoshan, Tengchong, Sibumasu, Helmand, and Karakorum) is controversial. Although its volume is poorly constrained, we propose that an extensive belt of granitic rocks that formed in various pulses between ca. 510 Ma and 460 Ma may represent the remains of a potential silicic large igneous province (LIP), which is referred to here as the Pinghe silicic LIP, with an areal extent of ∼2.5 Mkm2. The putative Pinghe silicic LIP is composed predominantly of S-type granites with subordinate A-type granites and minor intraplate mafic rocks. The recognition of this belt of granitic rocks aids in the refinement of tectonic reconstructions of Gondwana and of models for the rifting of terranes from its northern margin. The Pinghe silicic LIP broadly coincides with the adjacent 511 Ma Kalkarindji LIP in northern Australia, and the plume or mantle upwelling that triggered the Kalkarindji LIP may have been responsible for driving crustal melting that generated the granitic rocks, in a manner analogous to the Karoo−Chon Aike association.

New Facies Model and Carbon Isotope Stratigraphy for an Ediacaran Carbonate Platform From South America (Tamengo Formation—Corumbá Group, SW Brazil)

Article

Full-text available

Jun 2022

The Ediacaran is a period characterized by the diversification of early animals and extensive neritic carbonate deposits. These deposits are still not well understood in terms of facies and carbon isotope composition (δ¹³C). In this study we focus on the Tamengo Formation, in southwestern Brazil, which constitutes one of the most continuous and well-preserved sedimentary record of the late Ediacaran in South America. We present new detailed lithofacies and stable isotopes data from two representative sections (Corcal and Laginha) and revise the paleoenvironmental and stratigraphic interpretation of the Tamengo Formation. The Corcal section consists of neritic deposits including shallow-water limestone beds, alternated with shale and subordinate marl beds. These facies yield specimens of the Ediacaran fossils Cloudina lucianoi and Corumbella werneri. On the other hand, the Laginha section shows more heterogeneous facies, such as impure carbonates, breccias, marls, and subordinate mudstone beds, as well as no evidence of Corumbella werneri. The stable carbon isotope record is also different between the two sections, despite belonging to the same unit. The Corcal section displays higher and more homogeneous δ¹³C values, consistent with those of Ediacaran successions worldwide. The Laginha section, instead, displays more variable δ¹³C values, which suggest the influence of local and post depositional processes. The difference between the two sections was attributed to the different distance from the shore. We propose that the difference is due to topographic variations of the continental platform, which, at the Laginha site, was steeper and controlled by extensional faults. Therefore, the Corcal section is a better reference for the Tamengo Formation, whereas the Laginha is more particular and influenced by local factors. Besides, the lithofacies associations of the Tamengo Formation are like those of the Doushantuo and Dengying formatios, in South China, with no significant biogenic carbonate buildups, and different from those of other important Ediacaran units, such as the Nama Group in Nmibia and the Buah Formation in Oman. Our work highlights the complexity and heterogeneity of Ediacaran carbonate platforms and of their carbon isotopic composition. In addition, we characterize the Corcal section as a possible reference for the Ediacaran in South America.

Evolution of the Lesser Himalaya in space and time

Article

Full-text available

Jul 2021
HIMAL GEOL

The gneisses and associated metasediments and intrusive ca. 1860 Ma Wangtu Granite, designated as the JeoriWangtu-Bandal Gneissic Complex (JWBC), formed the basement for the oldest Lesser Himalayan rocks, when India was part of the Columbia supercontinent. A mega rift extending over the entire length of the present Himalaya developed around ca.1800 Ma. It was accompanied with outpouring of the tholeiitic volcanics, the event was diachronous that younged towards the east. In this shallow marine basin, the Rampur/ Berinag/ Kuncha/Kushma/Ranimata Daling/ Shumar groups accumulated. The lowest thrust sheet, labelled as the Kulu/ Munsiari/Ramgarh Porphery/ Daling/ Jashidanda thrust sheets, were nucleated a bit later or more or less coinciding with the rifting. The thrusting involved the basement rocks, which occur as tectonic slices in all the sectors of the Himalaya. Sedimentation of the Rampur Group and equivalent sequences ceased and after a pronounced hiatus, a carbonate shelf developed during the Mesoproterozoic-Early Neoproterozoic, in which sequences, variously designated as the Shali/Larji/Deoban/Calc “Series”/Dhadang/Baxa/Dedza groups were deposited over the Rampur and equivalent sequences. At that time India formed part of the Rodina. Rifting ca. 830 Ma, demolished the Shali-Baxa carbonate shelf. The succeeding rifted basin, located over the disrupted Mesoproterozoic-Early Neoproterozoic sequences, was also extensive and covered the nascent Kulu and equivalent thrust sheets. More or less simultaneously or with some time lag, the Chaur Granite and the embryonic Jutogh Thrust Sheet were emplaced. In this rifted basin, the Simla/ Jaunsar/ Nathuakhan and allied groups were deposited. The sedimentation in this basin terminated ~730 Ma, its sediments were raised to form an upland, which became abode of the Blaini valley glaciers. The glaciers descended in a marine basin with the Simla (which constituted the Outer Krol Belt) and the Jaunsar (Inner Krol Belt) forming the floors. A post-Blaini marine regression restricted the sea in which the Ediacaran Infra Krol and Krol sequences conformably succeeded the Blaini Formation. At the termination of the Krol sedimentation, the Outer Krol Belt was uplifted, the event coincided with the rifting reported in the Tethyan Bhutan. The sedimentation of the Tal Group continued in the Inner Krol Basin sited over the Jaunsar Group. Another basin (Chilar) was carved towards the SE of the Outer Krol basin, which possibly lasted up to the end of the Ediacaran. The late Cambrian-Early Ordovician Kurgiakh Orogeny terminated the Tal basin. As a component of the Kurgiakh orogeny in its early phase, the Cambrian Tethyan basin together with the underlying Vaikrita rocks was accredited to the Lesser Himalaya Tectogen; the event was associated with Early Palaeozoic granite emplacement and metamorphism. The Early Ordovician witnessed a rather extensive marine transgression, which possibly lasted till the Middle Silurian. Post Middle Silurian, in short intervals during the Permian and Cretaceous, no sedimentation took place in the Lesser Himalayan terrain which remained a positive area; its erosion contributed sediments to the Tethyan basin. The rifting in the Gondwanaland/ Pangea, opened old lineaments, mainly close to the present foothill region to provide pathways to the Permian and Cretaceous seas. The subduction of the Indian Plate commenced during the Thanetian, consequently, the foreland basin was formed at the present foothill region; its embayments extended in the Lesser Himalaya over the folded and deeply eroded pre-Paleocene sequences. The continued subduction of the Indian Plate caused the earlier nucleated thrust sheets to advance southward and as a consequence of which the foreland basin shifted farther south. The Lesser Himalaya formed the provenance of this basin in which the Siwalik succession was deposited. Due to impact of advancing crystalline thrust sheets, the Shali and Simla Group rocks moved partly over the Paleogene of the foreland basin. The Outer and Inner Krol Belts slid on their floors as superficial thrust sheets. The Late Cambrian folds, which are co-axial with the Himalayan folds, got accentuated and also selectively overturned. The Lesser Himalaya part was elevated. Weathering led to extensive erosion and release of the stresses, which periodically activated various faults causing earthquakes.

Challenges to carbonate-evaporite peritidal facies models and cycles: Insights from Lower Cretaceous stromatolite-bearing deposits (Oncala Group, N Spain)

Article

Aug 2020
SEDIMENT GEOL

Peritidal carbonate-evaporite successions, since they are developed in the transition between continental and marine realms, provide essential keys for palaeobathymetric and palaeoclimatic interpretations. As a result, several facies models have been proposed to assist on the interpretation of ancient tidal flat deposits, and peritidal successions have been extensively used for cyclicity analyses. In this study, well-exposed, Lower Cretaceous peritidal deposits (Oncala Group, Cameros Basin, N Spain) are analysed and compared with the most commonly-used present-day analogues (from Shark Bay, the Arabian Gulf and the Bahamas) and with ancient peritidal successions, providing their palaeoenvironmental and palaeoclimatic interpretation, assessing the usefulness and limitations of the facies models, and evaluating the suitability of these deposits for analysis of decimetre to metre-scale cycles. The studied peritidal deposits consist of thinly-bedded to laminated dolostones, dolomitic stromatolites, stromatolite breccias, flat-pebble and edgewise breccias, and calcite and quartz pseudomorphs after anhydrite nodules. Abundant resemblances of the peritidal deposits of the Oncala Group with those of Shark Bay, including that they are largely composed of microbialites and intraclasts, makes the peritidal deposits of the Oncala Group one of the best fossil analogues of this present-day setting. However, the presence of anhydrite nodules indicates pervasive evaporite precipitation in the supratidal zone, which is a feature that does not occur in supratidal flats of Shark Bay, but is characteristic of arid sabkhas of the Arabian Gulf. Nevertheless, the fact that carbonate-evaporite tidal flats of the Oncala Group were laterally related with siliciclastic tidal flats with large freshwater input and broadly inhabited by dinosaurs, suggests that anhydrites precipitated under less arid climates than those of the Arabian Gulf nowadays, pointing to semiarid climatic conditions during deposition. Moreover, the fact that peritidal deposits with anhydrite nodules were exclusively formed in a low-subsidence area of the Cameros Basin suggests that the rate of accommodation space creation also played an important role in their development. Regarding the comparison with other fossil peritidal sediments, the studied deposits show more abundant similarities with Proterozoic and Cambrian successions, composed mainly of stromatolites, microbial laminites, and intraclasts, than with other Mesozoic peritidal deposits, in which bioclasts and burrowing are usually more abundant. This highlights the difficulties for assigning specific features to certain geological ages. Finally, peritidal facies of the Oncala Group may change laterally and vertically to any other facies, showing a patchy lateral distribution of facies and an unsystematic vertical stacking pattern. The sedimentary features of the stromatolite, breccia and thinly-bedded to laminated carbonate facies do not allow their assignment to a unique tidal zone. Moreover, sedimentary features indicative of subaerial exposure, such as anhydrite nodules formed in the capillary zone, occur within any of the carbonate facies and show limited lateral extent. This results in a succession that cannot be clearly subdivided into subsequent shallowing-upward cycles not even by using erosive surfaces or the anhydrite nodule layers as marker horizons of the upper part of the cycles, because their limited lateral extent prevents reliable correlations. Similar composite lateral and vertical facies relationships have been documented both in the present-day analogues and in ancient successions, which suggests that this kind of facies relationships may be common in peritidal successions and highlights the caution that must be taken when trying to perform cyclicity analysis on them.

A broad climatostratigraphy of the Himalaya

Article

Full-text available

Sep 2019
HIMAL GEOL

The rocks in the Himalaya range from Paleoproterozoic to Recent. However, only a rudimentary palaeoclimatic information is available from the Himalaya succession. In fact, no specific and significant study to interpret past climate, save in the Upper Siwalik and Quaternary, has been attempted. Based on published information an attempt is made here to broadly reconstruct the climatostratigraphy of the Himalaya. Paleoproterozoic-1860 Ma-warm and humid; Mesoproterozoic-1500-?1000 Ma-warm, interlude of humid phase. 1840 Ma-pluvial; Neoproterozoic-1000-?950 Ma-warm, ?Pre-Ediacaran <840 pluvial, later part semi-arid, Cryogenian-630-625 Ma-cold, Edicaran-warm, arid at 600 Ma. Paleozoic-Lower Cambrian-? tropical, M. Cambrian-warm, E. Ordovician-warm and arid, L. Ordovician-E-M. Silurian (Ashgill-Wenlock) warm, M. Devonian-uncertain, possibly a bit arid, Givetian-warm-. Tournaisian

A broad climatostratigraphy of the Himalaya

Article

Full-text available

Sep 2019
HIMAL GEOL

Giant ooids in a Neoproterozoic carbonate shelf, Simla Group, Lesser Himalaya, India: An analogue related to Neoproterozoic glacial deposits

Article

Aug 2018
MAR PETROL GEOL

This study is the first comprehensive documentation of giant ooids and related facies associations in the Kunihar Formation, Proterozoic Simla Group Lesser Himalaya, Himachal Pradesh, analysing the understanding of processes and controls on development of giant ooids. Based on field observations, supplemented by outcrop based facies analysis, petrography and delineation of environmental variations, four facies associations have been delineated: (i) Peritidal siliciclastic-carbonate (FA1) (ii) Shelf lagoon (FA2) (iii) Reef complex (FA3) (iv) Fore reef slope (FA4). Deposition of giant ooids and associated facies associations of Kunihar Formation occurred in a carbonate rimmed shelf with high tidal influence. Size of giant ooids from Kunihar Formation is the largest as compared to giant ooids from other geological formations. Kunihar giant ooids developed when normal ooids were washed from ooid shoals (intertidal) into slightly deeper regions (shallow subtidal) resulting in the increased dimension of ooids in suspension due to higher hydrodynamics. Scanning electron microscopy (SEM) studies support microbial origin of ooids, giant ooids and stromatolites of Kunihar Formation. Increased microbial activity in Kunihar Formation is attributed to increase in nutrients by virtue of weathering of underlying Darla volcanics. Abundant carbonate and Microbially induced sedimentary structures (MISS) deposition in lower part of Simla Group points to increased microbial activity which likely increased the volume of oxygen in Neoproterozoic atmosphere ushering in Ice House conditions during the subsequent deposition of Blaini Group. Giant ooids associated with Neoproterozoic glacial deposits throughout the world, occupy stratigraphic positions below, above or between glaciations. Simla Group is another example where giant ooids lie stratigraphically below Marinoan Blaini Tillites. Increased magmatic activity and weathering before and during the Neoproterozoic glaciations increased nutrients in marine waters which increased algal growth. Thus, giant ooids were deposited due to such phases of increased microbial activity before/after glaciation, and during interglacial periods.

Site specific joint spacing distribution of roadcut slopes in a selected stretch of national highway in Indian Garhwal Himalayas

Conference Paper

Oct 2023

Due to extensive road widening project in the Himalayas, the roads experience huge instability problems especially during monsoon season. For the present study, 50 rock slope sites were considered over a stretch of 35 km on a national highway. The various attributes of discontinuities governing the slope failure were observed and the joint spacing in the stretch of the national highway is studied in detail. Detailed scanline survey was carried out and a database comprising more than about 6000 datapoints was generated. It is observed that the joints are spaced between 1 cm to 60 cm for the stretch of road. Statistical analysis of the joint spacing shows that most of the joint sets follow Exponential distribution, Weibull distribution, and lognormal distribution. Slope stability analysis was performed using the joint spacing data for all the slopes. It was observed that 50% of the slopes were susceptible to failure.

Link between Lesser Himalayan Neoproterozoic Krol succession and the Adelaide rift complex of Australia ‒ evidence from the mudstone deposits of Krol succession, Himachal Pradesh, India

Article

Jul 2023
MAR PETROL GEOL

Link between Lesser Himalayan Neoproterozoic Krol Succession and the Adelaide Rift Complex of Australia ‒ Evidence from the Silty Mudstone Deposits of Krol Succession, Himachal Pradesh, India

Article

Jan 2023

Ediacaran stratigraphy and paleogeography in the north Yangtze Block, South China

Article

Dec 2022
SEDIMENT GEOL

Systematic paleontology, acritarch biostratigraphy, and δ 13 C chemostratigraphy of the early Ediacaran Krol A Formation, Lesser Himalaya, northern India

Article

Full-text available

Apr 2022

Acritarch biostratigraphic and δ ¹³ C chemostratigraphic data from the Krol A Formation in the Solan area (Lesser Himalaya, northern India) are integrated to aid inter-basinal correlation of early–middle Ediacaran strata. We identified a prominent negative δ ¹³ C excursion (likely equivalent to EN2 in the lower Doushantuo Formation in the Yangtze Gorges area of South China), over a dozen species of acanthomorphs (including two new species— Cavaspina tiwariae Xiao n. sp., Dictyotidium grazhdankinii Xiao n. sp.), and numerous other microfossils from an interval in the Krol A Formation. Most microfossil taxa from the Krol A and the underlying Infra-Krol formations are also present in the Doushantuo Formation. Infra-Krol acanthomorphs support a correlation with the earliest Doushantuo biozone: the Appendisphaera grandis - Weissiella grandistella - Tianzhushania spinosa Assemblage Zone. Krol A microfossils indicate a correlation with the second or (more likely, when δ ¹³ C data are considered) the third biozone in the lower Doushantuo Formation (i.e., the Tanarium tuberosum - Schizofusa zangwenlongii or Tanarium conoideum - Cavaspina basiconica Assemblage Zone). The association of acanthomorphs with EN2 in the Krol Formation fills a critical gap in South China where chert nodules, and thus acanthomorphs, are rare in the EN2 interval. Like many other Ediacaran acanthomorphs assemblages, Krol A and Doushantuo acanthomorphs are distributed in low paleolatitudes, and they may represent a distinct paleobiogeographic province in east Gondwana. The Indian data affirm the stratigraphic significance of acanthomorphs and δ ¹³ C, clarify key issues of lower Ediacaran bio- and chemostratigraphic correlation, and strengthen the basis for the study of Ediacaran eukaryote evolution and paleobiogeography. UUID: http://zoobank.org/5289fdb2-0e49-4b3b-880f-f5b21acab371 .

Geochemical characteristics of Early Cambrian phosphate bearing sedimentary rocks from the Mussoorie Syncline, India: Implications for paleo-redox conditions

Article

Full-text available

Mar 2022

The phosphate bearing sedimentary rocks of Masrana and Kimoi blocks of Mussoorie Syncline are found to occur associated with a variety of chert-shale-carbonate with some pyrites of Tal Group of Early Cambrian age. Petro-mineralogical studies have revealed that apatite is a dominant phosphate mineral with gangue constituents as calcite, dolomite, variable amounts of quartz, pyrite, hematite as well as some organic structures. Quartz veins are also found cutting across the pellets of collophane. Major element analysis indicates that high P2O5 and CaO values designate high apatite, calcite or gypsum. The antipathetic correlation of P2O5 with most of the major oxides represents, replacement, leaching and weathering of major elements with P2O5 in the voids, cavities and veins during the formation of deposit. The results of trace elements reveal that the phosphorites are more enriched in V, Cr, Co, Ni, Cu, Zn, Ga, Rb, Sr, Y, Zr, Nb, Ba, Pb than Sc, Cs, Hf, Ta, Th, U. Higher concentration of V may be due to the presence of organic matter and its close affinity with P2O5. Vanadium (V) therefore occurs associated with Ni-Cr-Zn-Cu-Ag, which is distinctive of the organic matter obtained from the marine sediments. Rich Concentration of Ba, Cu, Ni and Zn in these phosphorites might be due to the presence of organic matter derived from marine plankton. Ni/Co ratios are low in the phosphate bearing sedimentary rocks (<5), which indicates oxic conditions. The plotted binary diagram of V/(V + Ni) versus Ni/Co also confirms oxic conditions for these samples. In the present study most of the samples have values of V/Cr below 2, whereas two samples show value above 2, indicating dominant oxic environmental conditions with mild reducing conditions. Oxic and anoxic diagenetic environment has been also supported by negative Ce and positive Eu on Post-Archean Australian Shale (PAAS) normalised REE patterns.

Application of re-sampling stochastic framework for rock slopes support design with limited investigation data: Slope case studies along an Indian highway

Article

Full-text available

Jan 2022
ENVIRON EARTH SCI

Rock slope projects are often available with insufficient investigation data for rock properties due to involvement of high costs, manual efforts, geological complexities, etc., in testing. Support estimation performed by traditional approaches for slope failure mitigation could be highly inaccurate due to inaccuracies in the estimated statistics of properties. This article describes a computationally efficient re-sampling stochastic framework to overcome this issue by coupling the Advanced Re-Sampling Reliability Approach (ARRA) with deterministic and Target Reliability Approach (TRA) to estimate the required support for rock-slides mitigation with limited data. The proposed methodology was demonstrated for the support design to mitigate two massive rockslides along a rockslide-prone highway, i.e., Rishikesh-Badrinath National Highway (NH-58) in India. It was concluded that the limited data invokes uncertainties in the statistical parameters (mean and standard deviation) and distributions of properties. Support estimated using traditional approaches with the inaccurate statistics of input properties can result in inaccurate support estimates in the presence of insufficient data. Proposed methodology couples ARRA with traditional methods and evaluates the final support design by quantifying the uncertainty in the reliability index induced by the statistical uncertainties of input properties statistics. This improves the overall accuracy, efficiency, and the designer's confidence in the estimated support.

BIO- AND CHEMOSTRATIGRAPHY OF THE KROL A FORMATION IN LESSER HIMALAYA, INDIA: EARLY EDIACARAN EUKARYOTE EVOLUTION AND ENVIRONMENTAL CHANGE

Conference Paper

Jan 2021

Precambrian and early Cambrian palaeobiology of India: Quo Vadis

Article

Full-text available

Jun 2021

Significance of Precambrian and early Cambrian palaeobiological remains recorded over the last few years on the Indian subcontinent are discussed in the present review. The importance of these remains is assessed in the backdrop of new reports from different parts of the world. Archaean, Proterozoic and early Cambrian successions of India have revealed some of the oldest rock fragments of the world, most primitive eukaryotic remains, bizarre Ediacaran forms and typical early Cambrian fossils. For the sake of brevity and easy comprehension, palaeobiological remains are grouped under Archaean, Proterozoic, Ediacaran and early Cambrian time-frames. New evidence of life forms, documented from the sedimentary basins exposed in peninsular and extra-peninsular region of the country, have challenged the various paradigms, viz. evolutionary, biostratigraphic status of different lithounits and age of the encompassing sedimentary successions. Newly published geochronological and geochemical results are also considered in this review to understand the developments in these fields and their significance in the evolution of hydrosphere, biosphere, and moderating processes over the geological timescale. Recent Indian endeavors in the field of Astrobiology and inputs provided in the areas of astrobiological studies are also discussed.

A Mesoproterozoic carbonate platform (lower Belt Supergroup of western North America): Sediments, facies, tides, tsunamis and earthquakes in a tectonically active intracratonic basin

Article

Jun 2021
EARTH-SCI REV

Carbonate rocks of the lower Belt Supergroup (ca. 1.45 Ga) in west-central North America include the Haig Brook, Tombstone Mountain, Waterton and Altyn formations which crop out in the eastern Rocky Mountains of northwestern Montana, southwestern Alberta and southeastern British Columbia. They record the development on the present-day northeastern side of the Mesoproterozoic Belt Basin of a carbonate platform early in the basin history while it was still relatively deep. The Waterton–Altyn succession documents a shallowing-upward, westward-prograding, broadly ramp-style platform composed of mixed carbonate–siliciclastic sediments. Five main facies types are recognized: Laminite, Ribbon, Grainstone, Oolite and Stromatolite. The first two were formed of lime muds deposited on the ramp and outer platform under low-energy conditions. The Grainstone facies consists of sand-sized peloids, aggregates and intraclasts plus admixed quartz and feldspar sand, microspar grains, radial ooids and silicified oolite and anhydrite. The Oolite facies is dominated by ooids with a concentric cortex. These are allochthonous coarse-grained particles interpreted to have been transported westward to a marginal belt and outer platform mainly by tsunami off-surge from the platform interior, coastal sand shoals and tidal-flat sabkhas, outcrops of which are not preserved. Absence of hummocky or swaley cross-stratification suggests that the platform was not affected by strong storms. Instead, flat-pebble conglomerates on the ramp are ascribed to episodic, tsunami-induced wave action which caused localized rupture and imbrication of flat pebbles. Scouring by tsunami off-surge produced intraclasts in the outer platform. There, this sediment lay undisturbed, but in the shallower marginal belt it was reworked by strong tidal currents which generated variably directed cross-lamination from dune and ripple migration and, locally, large sand bars with northwest-dipping clinoforms. Deformation features caused by synsedimentary earthquakes are common, with the various seismite types reflecting the facies-specific rheology of the sediment. Seismites in the lower Altyn Formation appear not to be linked to individual tsunami-lain dolograinstone beds in the outer platform, suggesting that these two sets of features were not generated by the same faults. The carbonate factory shut down when the platform was suffocated by siliciclastic mud sourced from the west, and tidal activity diminished as the whole basin shallowed. The interpretation of the carbonate platform presented herein is radically different from previous views that considered the Waterton and Altyn formations to be predominantly of shallow-subtidal and tidal-flat origin. These rocks are relevant to the appreciation of other carbonate platforms, especially in that tsunamis may be an under-appreciated agent of erosion and sediment transport offshore.

Life in the aftermath of Marinoan glaciation: The giant stromatolite evolution in the Puga cap carbonate, southern Amazon Craton, Brazil

Article

Mar 2021
PRECAMBRIAN RES

Giant domal stromatolites associated with vertical tubestone are reported for the first time in the Marinoan (635 Ma) Puga cap dolostone on the southern Amazon Craton, Brazil. The well-preserved giant domal mounds reach 12 m in diameter and 10 m in height, forming laterally continuous biostromes of at least 200 m. The giant mounds occur mainly on the top cap carbonate succession overlying stratiform stromatolites directly underlying glacial diamictites marking the Cryogenian-Ediacaran boundary. We discuss the sedimentological and biological conditions during the greenhouse period that led to the development of these exceptional stromatolitic features. The microbial communities primordially colonized the diamictite substrate, forming stratiform bioconstructions in a shallow platform under hypersaline calm water conditions. The transition from stratiform to domal morphology reflects the progressive deepening of the platform, concomitant with increases in microbial activity. The high production and pre-lithification of carbonate muds, that formed a rigid load-resistant framework, allowed for the accumulation by vertical accretion, increasing the potential for preservation of giant mounds. The progressive generation of accommodation space related to sea-level rises and glacial isostatic adjustment (GIA) caused subsidence in the coastal zone, as proposed by the Snowball Earth hypothesis. The maximum flooding was associated with an increase in siliciclastic inflow, causing burial of microbial communities and the demise of giant stromatolites. The succession was succeeded by the deposition of long-term transgressive lime mud in a CaCO3-oversaturated sea. The giant stromatolites consist of a new element of the Marinoan Puga cap carbonate and provide essential insights to enlarge the discussion about the biological and sedimentological interaction that occurred on post-glacial carbonate platforms worldwide.

Fragmentation of South China from greater India during the Rodinia-Gondwana transition

Article

Oct 2020
GEOLOGY

Late Tonian to Cambrian sedimentary sequences in northwestern India and South China provide vital evidence for modeling their paleogeographic linkage, including their juxtaposition and subsequent separation during the transition from the Rodinia to the Gondwana supercontinents. Similarities in lithostratigraphy and detrital zircon U-Pb-Hf-O isotopic characteristics in the late Tonian sedimentary units from both regions underline a common provenance. A substantial decrease in zircon δ18O values from super- to sub-mantle compositions and simultaneous increase in the zircon εHf(t) values in South China and northwestern India for the 800–700 Ma time window suggest a common Neoproterozoic extensional magmatic event, corresponding with the Rodinia breakup. A distinct change in sedimentary provenance is noted during the Cryogenian period. Sedimentation along the northwestern margin of India for the remainder of the Neoproterozoic encompasses large volumes of clastic detritus dominated by old zircon ages, derived inboard from the Indian craton. In contrast, contemporaneous sedimentary units in the Yangtze region of South China are dominated by Neoproterozoic zircons. The detrital zircon age data underline a close paleogeographic linkage between northwestern India and South China (Yangtze and Cathaysia regions) in the Rodinia supercontinent configuration and argue for their separation through continental rifting during the Cryogenian. Northwestern India developed into a passive margin, whereas the South China block partially rifted, rotated, and migrated dextrally along the Gondwana margin toward northeastern India and Western Australia, such that the Cathaysia block continued to receive detritus from Gondwana continental regions.

Stabilization of Cut Slopes along the Highway by Optimizing Geometry, NH-58, Lesser Himalaya

Article

Full-text available

Jul 2020

The Himalayan ranges are well known for active and incessant slope failures. The network of highways and local roads in the region is under sustained threat of potential fatal landslides. Urban sprawl and increased human intervention increased the vulnerability and probability of sliding to manifolds. Tampering the fragile conditions of the sensitive Himalayan orogen may generate new avenues of slope failures. Road widening and development projects have a close association with slope failures. However, such landslides can be coped with proper geotechnical planning and execution. The slopes along national highway-58 (NH-58) are being excavated for road widening. Large numbers of steep and unstable cut sections were visible soon after the excavation along the highway. The geotechnical assessment has been undertaken to determine stable slope geometry. The profile of existing slopes is modified by creating benches, and safety factor was determined at different overall angle. The optimization of slope geometry suggests that most of the unstable slope may have safety factor of 1.5, by creating benches of 10 m (height), 5 m (span), at an angle of 65°. It can also be inferred from the analysis that the safety factor may be improved from 11.5 to 32.2% for each 5° reduction in the overall slope angle. Certain adverse impacts and future projections of inadequately performed road widening were also discussed in the paper.

Microfacies, diagenesis and hydrocarbon potential of the Neoproterozoic cap carbonate of the southern Amazon Craton

Article

Aug 2020
SEDIMENT GEOL

The low-latitude Marinoan glaciation (635 Ma) record in the southern Amazon Craton has been one of the most important sites with strong evidence for the Neoproterozoic Snowball Earth hypothesis. In Central Brazil, the 50-m-thick carbonate succession of the post-Marinoan cap carbonate exposed in the Tangará da Serra region, Central Brazil, represents a complementary section of the classical occurrence of the Puga cap carbonate in Mirassol d'Oeste region, that overlies glaciogenic deposits marking the well-preserved Cryogenian-Ediacaran boundary in South America. The 8-m-thick cap dolostone that represents the Mirassol d'Oeste Formation consists of pinkish microcrystalline peloidal dolomudstone, exhibiting even parallel to low-angle lamination locally tubestone, and megaripple bedded dolopackstone. The cap dolostone is succeeded upsection by the cap cementstone limestone (basal Guia Formation), comprising laminated lime mudstone rich in silt-sized terrigenous grains (quartz), and subordinately bituminous shale, dolomitic marls, intraformational breccias, and abundant calcite crystal fans and crusts. Crystal fans occur in isolated and decimetre-scale arrays and are considered aragonite pseudomorphs. Primary precipitation of dolomite, crystal fans, and micrite and the formation of tubestone are related to depositional processes. The cap dolostone was affected by the neomorphism of the dolomicrite, development of vug and intercrystalline porosity, euhedral dolomite precipitation, synsedimentary fracturing, calcite and quartz precipitation, chemical compaction, Fe-oxide substitution, and precipitation. The diagenetic processes of the cap cementstone limestone were pyrite precipitation, calcite cementation, neomorphism of micrite and crystal fan, partial crystal fan dissolution, dolomitization, chemical compaction, fracturing, ferrous dolomite and calcite precipitation, pyrite (pseudomorph) and micrite replacement with Fe-oxide and replacement of rhombohedral dolomite by Mn-oxides. Hydrocarbons are rare and its migration represents the last diagenetic event being found: 1) in fractures and interpeloidal, vug and intercrystalline pores (~2%) with low permeability and connectivity of the cap dolostone; and 2) in intercrystalline pores, fractures, and rare pores in crystal fans of the cap cementstone limestone. During the post-Marinoan sea-level rise occurred the establishment of a shallow to moderately deep microbially induced dolomitic platform locally influenced by storm-action. The progressive transgression led the implantation of a deep anoxic and CaCO3-oversaturated aragonitic platform. The similarities of cap carbonate microfacies among the post-Marinoan succession worldwide reflect an extraordinary and non-recurring global precipitation event in geological history. In contrast, besides the majority of cap carbonate have the same textural and compositional framework, the diagenetic history seems to be regional and completely dependent on the tectonic, thermal, and burial history of the depositional basins where they were deposited.

Applicability of Q-slope Method in the Himalayan Road Cut Rock Slopes and Its Comparison with CSMR

Article

Full-text available

Oct 2020
ROCK MECH ROCK ENG

The engineered slopes along transportation routes in the Himalayan region are manifested by multiple sets of discontinuities and are vulnerable to structurally controlled failures. The hazard potential along one such route was assessed by an empirical Q-slope method. The investigated engineered slopes along the route are unstable, and the Q-slope value ranges from 0.0023 to 0.7701. By considering the ongoing road development and excavation works, optimal angles without any requirements of reinforcement measures (β) were determined for the probability of failure (PoF) as 1%, 15%, 30%, and 50%. The outcomes obtained by Q-slope were also correlated with the continuous slope mass rating (CSMR) values. A linear relationship was obtained between Q-slope and CSMR values, having the coefficient of determination as 0.67. The study also aims to assess the applicability of the existing Q-slope method for toppling mode of failure. A modified Q-slope method for toppling failure is proposed by considering the basic principles of toppling failure. The role of joint roughness parameter in topple mechanism is highlighted. Consequently, chart based modifications along with proper guidelines for the newly introduced parameter has also been proposed. It is also pointed out that the Q-slope chart is slightly conservative in nature particularly for β corresponding to low Q-slope values. The optimal slope angle for the PoF 1% was correlated with the slope angle at factor of safety as 2, and an extended stable zone is proposed with further scope of development. The modifications and suggestions for the refinement of the system will enhance the applicability of the method for a much broader range of initial ground conditions. The outcomes may be advantageous for the progressing road widening and development work along a section of the transportation route in Uttarakhand Himalaya.

Stability assessment of landslide-prone road cut rock slopes in Himalayan terrain: A finite element method based approach

Article

Full-text available

Aug 2019

Large-scale slope destabilization could be aggravated due to swift urbanization and ever-rising demands of geoengineering projects such as dams, tunnels, bridges and widening roads. National Highway-58 connects Delhi to Badrinath in India, which passes through complex geomorphological and geological terrain and often encounters cut slopes susceptible to slope failures. In the present investigation, a detailed geotechnical appraisal is conducted along the road cut slopes from Rishikesh to Devprayag in the Himalayas. Twenty vulnerable road cut slopes were demarcated for detailed slope stability analysis using Phase2D finite element modeling simulator. Nonlinear generalized Hoek-Brown (GHB) criterion was adopted for stability analyses. Out of 20 slopes, five slopes (S6, S7, S18, S19 and S20) are unstable with factor of safety (FoS) less than or equal to 1, and thus needs immediate attention. The FoS values of four slopes (S2, S9, S13 and S17) lie between 1 and 1.3, i.e. marginally stable, and slopes S1, S3, S4, S5, S8, S10, S11, S12, S14, S15 and S16 are stable. Mohr-Coulomb (MC) criterion was also adopted to compare the slope stability analysis with GHB criterion. The FoS calculated from GHB criterion is close to that using MC criterion for lower values of FoS whereas for higher values, the difference is marked. For the jointed rock in the Himalayan region, the nonlinear GHB criterion gives better results as compared to MC criterion and matches with the prevailing field conditions. Accordingly, some suggestions are proposed to strengthen the stability of cut slopes.

Neogene Kinematic Evolution and Exhumation of the NW India Himalaya: Zircon Geo‐ and Thermochronometric Insights From the Fold‐Thrust Belt and Foreland Basin

Article

Full-text available

Jun 2019
TECTONICS

The kinematic and exhumational evolution of the Lesser Himalaya (LH) remains a topic of debate. In NW India, the stratigraphically diverse LH is separated into the inner LH (iLH) of late Paleo‐Mesoproterozoic rocks and the outer LH (oLH) of Cryogenian to Cambrian rocks. Contradictory models regarding the age and structural affinity of the Tons thrust—a prominent structure bounding the oLH and iLH—are grounded in conflicting positions of the oLH prior to Himalayan orogenesis. This study presents new zircon (U‐Th)/He and U‐Pb ages from the thrust belt and foreland basin of NW India that refine the kinematic and exhumational evolution of the LH. Combined cooling ages and foreland provenance data support emplacement and unroofing of the oLH via southward in‐sequence propagation of the Tons thrust by middle Miocene time. This requires that, before India–Asia collision, the oLH was positioned as the southernmost succession of Neoproterozoic–Cambrian strata along the north Indian margin. This is further supported by detrital zircon U‐Pb ages from Cretaceous–Paleogene strata (Singtali Formation) unconformably overlying the oLH, which yield diagnostic Cretaceous detrital zircons correlative with coeval strata in the frontal Himalaya of Nepal. A pulse of rapid exhumation along the Tons thrust front at ~16 Ma was followed by east‐to‐west development of a midcrustal ramp at ~12 Ma which facilitated diachronous iLH duplexing. This duplexing shifted the locus of maximum exhumation northward, eroding away Main Central Thrust hanging wall rocks until the iLH breached the surface at ~9–11 Ma near Nepal and by ~3–7 Ma within the Kullu‐Rampur window.

PROVENANCE OF CRETACEOUS–PALEOGENE STRATA OF NORTHWEST INDIA: DETRITAL ZIRCON GEOCHRONOLOGIC AND HF ISOTOPIC INSIGHTS INTO THE TIMING OF INDIA–ASIA COLLISION

Conference Paper

Jan 2018

Sequence-Stratigraphic Position of Oolitic Bank of Cambrian in North China Platform: Example from the Kelan Section of Shanxi Province

Article

Jun 2018

According to the recent chronostratigraphic division of Cambrian, the Cambrian strata in the Kelan section can be subdivided into seven third-order sequences (DS1−DS7) based on cyclicity in sedimentary facies stacking patterns. The calcareous mudstone forming condensed section, micritic limestone comprising deep to middle ramp facies and the oolitic-grain bank facies in Series 3 and Furongian series represent the basic depositional fabric of Cambrian in northwestern part of Shanxi Province, North China Platform. These massive oolitic grainstones demonstrate that the oolitic-grain bank facies constitute the late-highstand systems tract or forced-regressive systems tract. The grains occupy upper parts of the third-order depositional sequences in response to relative sea-level fall. Furthermore, this forming pattern of oolitic-grain bank facies does not follow the standard model of sequence stratigraphy, in which deposition is believed to occur principally during sea-level rise, rather a continuous erosional unconformity develops during sea-level fall. Moreover, the microscopic analysis of oolitic grainstone shows the development of concentric and radial, rounded or elliptical, with or without nuclei, monocrystalline or polycrystalline, Girvanella or micritic ooids. The occurrence of diversified ooids in varying proportions provides a new dimension for studying evolution of the oolitic-grain bank in the North China Platform. The forming pattern of oolitic-grain bank controlled by their sequence-stratigraphic position in the Kelan section in the Shanxi province provides an important clue and a research direction for the regional correlation, as well as the paleogeographical reconstruction of the Cambrian Series 3 and Furongian series.

A dominant autocyclic signal on Supergreenhouse carbonates

Conference Paper

Full-text available

Jan 2011

Adam Zeiza

The nature of carbonate cycles and their stacking patterns have been interpreted to be related with global climate conditions through geologic history. Carbonate cycles formed during icehouse times (e.g., late Devonian – early Permian and Neogene) are laterally variable due to allocyclic controls in response to high amplitude, high frequency (fifth- and fourth-order) sea-level fluctuations superposed on lower amplitude, low frequency (third-order) eustatic sea-level changes. Whereas cycles formed during greenhouse times (e.g., Mesoproterozoic, Ordovician-early Devonian and late Permian-early Tertiary) are commonly more laterally continuous due to the domination of lower frequency (thirdorder) eustatic sea-level fluctuations. During supergreenhouse climate conditions such as in the Furongian, carbonate cycles were less likely controlled by eustatic sea-level fluctuations. High levels of atmospheric CO2 (>4000 ppm) during this epoch would have prevented the development of polar ice sheet, thus limiting and/or minimizing glacio-eustatic sea-level changes. Tracing the cycles and cycle boundaries in continuous outcrop in the Great Basin, western United States reveals significant lateral variability of cycles. Cycle boundaries disappear within tens to hundreds of meters and component facies of individual cycles pinch out or interfinger with other component facies. Within a cycle set (i.e., cycle stacking patterns bounded by key surfaces), cycle numbers and thickness vary locally and regionally, suggesting dominant autogenic formation of cycles and stacking patterns in the warm Furongian carbonate platforms. Understanding the style of cycles and stacking patterns formation through time has implications to carbonate exploration activities in Southeast Asia and elsewhere.

Zircon (U‐Th)/He Thermochronometric Constraints on Himalayan Thrust Belt Exhumation, Bedrock Weathering, and Cenozoic Seawater Chemistry

Article

Full-text available

Jan 2018
GEOCHEM GEOPHY GEOSY

Shifts in global seawater 187Os/188Os and 87Sr/86Sr are often utilized as proxies to track global weathering processes responsible CO2 fluctuations in Earth history, particularly climatic cooling during the Cenozoic. It has been proposed, however, that these isotopic records instead reflect the weathering of chemically distinctive Himalayan lithologies exposed at the surface. We present new zircon (U-Th)/He thermochronometric and detrital zircon U-Pb geochronologic evidence from the Himalaya of northwest India to explore these contrasting interpretations concerning the driving mechanisms responsible for these seawater records. Our data demonstrate in-sequence southward thrust propagation with rapid exhumation of Lesser Himalayan strata enriched in labile 187Os and relatively less in radiogenic 87Sr at ∼16 Ma, which directly corresponds with coeval shifts in seawater 187Os/188Os and 87Sr/86Sr. Results presented here provide substantial evidence that the onset of exhumation of 187Os-enriched Lesser Himalayan strata could have significantly impacted the marine 187Os/188Os record at 16 Ma. These results support the hypothesis that regional weathering of isotopically unique source rocks can drive seawater records independently from shifts in global-scale weathering rates, hindering the utility of these records as reliable proxies to track global weathering processes and climate in deep geologic time.

Proterozoic excluding basal ∼2500–2350 ma Paleoproterozoic and ∼635–541 ma Vendian

Chapter

Jan 2024

Jai Krishna

Giga-sequence GS-V

Chapter

Jan 2024

Jai Krishna

Terminal Ediacaran microbialite lithofacies associations with paleo-environmental constraints in a high-frequency sequence stratigraphic framework of Sichuan Basin, SW China

Article

Full-text available

Feb 2023

The terminal Ediacaran shallow marine microbialites are significant for paleo-environment reconstruction and petroleum exploration. Their depositional and paleo-environmental characteristics during high-frequency sea-level fluctuations were paid less attention, which limited the understanding of their evolution and related reservoir characterization. Microbialites from the fourth member of the upper Ediacaran Dengying Formation, Gaoshiti-Moxi area, Sichuan Basin, SW China are important records for studying their lithofacies associations and paleo-environmental constraints in high-frequency depositional sequences. Petrographic and geochemical analyses (in-situ major and trace elements and carbon and oxygen isotopes) indicate microbialites depositional and paleo-environmental evolution on a rimmed platform within a fourth-order depositional sequence. From late TST (transgressive systems tract) to early HST (highstand systems tract), thick-bedded dark-grey dolomudstone-thin-bedded spotted thrombolite dominates the platform margin; while thick-bedded dark-grey dolomudstone-thin bedded dark-grey wavy stromatolite dominates the platform interior. From late HST to early TST of the next fourth-order sequence, the platform margin is dominated by thick-bedded spotted thrombolite—dolograinstone, while the carbonate interior is dominated by thin-medium-bedded layered thrombolite—light-grey wavy stromatolite. Salinity correlates well with microbialite lithofacies associations: the platform interior layered thrombolite and wavy stromatolite enriched lithofacies associations are characterized by higher salinity than that of the platform margin spotted thrombolite enriched lithofacies associations. The microbialite lithofacies associations are less constrained by redox: platform interior layered thrombolite and wavy stromatolite enriched lithofacies associations are slightly more reduced than the platform interior spotted thrombolite enriched lithofacies associations. The redox analysis of the microbialites also indicates low oxygen levels of shallow marine in the study area during terminal Ediacaran, with possible oxygenation at the Ediacaran-Cambrian boundary. The study suggests that the terminal Ediacaran microbialite development was controlled by both high-frequency sea-level fluctuations and paleo-environmental factors such as salinity and redox. Platform margin spotted thrombolitic reefs from late fourth-order HST to next early fourth-order TST serve as favorable microbialite reservoirs. Platform interior layered thrombolites and wavy stromatolites are regarded as potential reservoirs, which deserve more detailed depositional-paleo-environmental research.

Temporal and spatial distribution of Precambrian red beds and their formation mechanisms

Article

Full-text available

Jun 2022

Red beds are irrefutable evidence of important geological events; understanding their formation mechanism and sedimentary environment provide insights about Earth's history and the evolution of life. In this paper, Precambrian red beds from over 70 outcrops in nearly 30 regions in different countries are statistically assessed using their lithology, sedimentary environment, and spatio-temporal distribution. Twelve sets of red beds are identified, five of which are distributed globally; the other sets are regional or local. The depositional environments for red beds are varied. They include terrestrial (aeolian, alluvial, and fluvial) and shallow marine environments (dominantly, tidal and glacial marine), as well as the transition areas from shallow to deep-water environments. While the former two are dominant environments, the latter has fewer occurrences – most of which are concentrated in strata of the Ediacaran Period. Red beds are lithologically diverse; terrestrial red beds are mainly composed of conglomerate and sandstone, while shallow marine red beds primarily consist of siltstone, mudstone, and carbonate rocks. The formation of red beds is directly controlled by the concentration of dissolved Fe and O2 content, and penecontemporaneous geological events. Here, they are sorted into two groups based on their chronological coupling with glaciations, the assembly and breakup of supercontinents, eustatic sea-level changes, the flourishing of photoautotrophs like cyanobacteria, as well as δ¹³C and ⁸⁷Sr/⁸⁶Sr isotope anomalies. The formation of the first group of red beds is closely related to the Paleoproterozoic and Neoproterozoic glaciations, the Great Oxidation Event, and the Neoproterozoic Oxygenation Event. Much of the dissolved Fe scavenged from seawater during oxygenation events precipitated into banded iron formations (BIF). The transmittance of light through water reduced during glaciations. It prevented the production of O2 and limited Fe²⁺ oxidation – which worked as a protector for oceanic Fe re-accumulation. The melting of glaciers improved the availability of light which restarted the production of photosynthetic O2 that aided the formation of red beds. The formation of the second group of red beds was aided by supercontinent fragmentation that created many shallow margin habitats for photoautotrophs, facilitating O2 production. The appearance of this group of red beds coincides with and is geochemically supported by pulses of O2 increase recorded by Cr and C isotopes, as well as by iron speciation data. Future research should examine how Precambrian red beds can be used to systematically decode paleoenvironment changes.

An integrated assessment of the geomorphic evolution of the Garhwal synform: Implications for the relative tectonic activity in the southern part of the Garhwal Himalaya

Article

Full-text available

Mar 2022

The geomorphic changes over the earth’s crust are influenced by tectonic activities. These geomorphic changes are remnants of deformation that occurred in the recent geological past. Geomorphic features can be quantified to assess relative tectonic activity and response of landscape to active tectonics, regional structures, lithology and climate. To achieve the objectives, we evaluated the relative tectonic activity of the Garhwal synform, for which six major river basins were selected. The relative tectonic activity of all the basins is computed based on quantitative analysis of geomorphic indices. Quantitative analysis of each geomorphic parameter has been carried out, and a combined product of relative tectonic activity index (TAI) was derived for each basin. The TAI is classified into three classes based on their relative tectonic activity; basins having TAI value ≤1.75 (basins I, II and III) are placed in very high tectonic activity class, basin with a value ranging >1.75 to <2.0 are categorised as moderately active basins (basin ‘IV’), while basins having values >2.0 are less active (basins V and VI). A relative tectonic activity map of the area suffices for the prioritisation of each basin based upon their TAI. Furthermore, analysis of the longitudinal profile of rivers for knickpoint, precipitation and temperature variability over the last 100 years and seismic events since the last 100 years have been studied to interpret the tectonic regime and their influence on landscape evolution. The regional seismicity data suggest that the area falls in a seismic gap and has not experienced a great earthquake in recent history but have received seismic events of moderate intensity in the past. We opine that the Garhwal synform is tectonically active, and thus, significant steps should be taken for seismic risk assessment along with preventive measures. We also suggest that the influence of tectonic activities in the southeastern part of the Garhwal synform comprised by basins V and VI is relatively less than the rest of the basins. Finally, the six basins were prioritised based on their relative tectonic activity. Assessment of geomorphic indices from 30 m shuttle radar topography mission-digital elevation model (SRTM-DEM) in six drainage basins of the Garhwal Himalaya.Six river basins categorised under relative tectonic classes based on the calculation of geomorphic indices.Correlation of the large-scale geological setting and drainage basin dynamics contemplated with field evidence and regional seismicity.Quantification of relative tectonic activity index (TAI) of six river basins in the Lesser Himalaya of the Garhwal Himalaya. Assessment of geomorphic indices from 30 m shuttle radar topography mission-digital elevation model (SRTM-DEM) in six drainage basins of the Garhwal Himalaya. Six river basins categorised under relative tectonic classes based on the calculation of geomorphic indices. Correlation of the large-scale geological setting and drainage basin dynamics contemplated with field evidence and regional seismicity. Quantification of relative tectonic activity index (TAI) of six river basins in the Lesser Himalaya of the Garhwal Himalaya.

Geochemistry of Neoproterozoic Chhaosa shales, Simla Group, Lesser Himalaya: Its implications on provenance and tectonics

Article

Sep 2021

Research highlights: This is the first geochemical work ever done for the shales of Neoproterozoic Chhaosa Formation of Simla Group, Lesser Himalaya.K2O/Na2O vs. SiO2/Al2O3, TiO2 vs. Al2O3, and Zr vs. TiO2 binary diagrams of the studied shales indicate felsic source rock.High CIA, CIW, A–CN–K and Al2O3 values of Chhaosa shales support intermediate to strong weathering of source rock.K2O/Na2O vs. SiO2 plot, K2O vs. Na2O, plot and CaO–K2O–Na2O ternary plot represent deposition of Chhaosa deposits in a passive margin setting.Values of Ni/Co, Cu/Zn, V/Cr, authigenic U and U/Th, represent a well-oxygenated condition during the deposition of the Chhaosa delta system. This is consistent with the concept of an increase in oxygen levels during the Neoproterozoic era is favoured by many workers. Abstract: Neoproterozoic Chhaosa shales of Simla Group, Lesser Himalaya, were considered for major and trace element analysis to delineate palaeo-weathering, palaeo-oxygenation, tectonic setting, sediment maturity, and provenance. The shales exhibit a significant proportion of SiO2, Al2O3, K2O, MgO, Fe2O3, Zr, Zn, Rb, V, Cr, Sr, Y, Co, Ga, Th, Nb, Sc, and U. Source rocks are primarily of granitic (acidic) origin, as indicated by Al2O3 wt.% vs. TiO2 wt.% and Cr (ppm) vs. Ni (ppm) bivariate plots. Th/Cr, Cr/Th, Th/Sc, and Th/Co values of studied samples have been correlated with post-Archaean Australian average shale (PAAS) and upper continental crust (UCC) values, which indicate that the source rock is felsic. Palaeo-weathering condition of the source is delineated by average CIA, chemical index of weathering (CIW), and A–CN–K triangular plot values. The shales depict intermediate to strong chemical weathering and semi-arid to humid climatic conditions. V/Cr, Cu/Zn, Ni/Co, and U/Th values reveal sedimentation of Chhaosa Formation under oxic atmospheric conditions, which corroborates with the inferred oxic conditions of the Neoproterozoic. SiO2 vs. K2O/Na2O discriminant diagram exhibits deposition along a passive margin field. ICV values suggest deposition of compositionally mature sediments under tectonically stable conditions. Deposition of Chhaosa delta system occurred during a relatively quiescent stage of tectonism between intermittent phases of rifting related to the Rodinia Supercontinent.

Tectonics of the Himalaya

Chapter

Apr 2020

This chapter provides summarized geological and geophysical account of the youngest and highest mountain belt to elucidate the role of the continental lithospheric subduction of the Indian Plate in the evolution of the Himalaya during the Cenozoic.

Carbonates before skeletons: A database approach

Article

Dec 2019
EARTH-SCI REV

Carbonate minerals have precipitated from seawater for at least the last 3.8 billion years, but changes in the physical and chemical properties of carbonate rocks demonstrate that the nature, loci, and causes of this precipitation have varied markedly through time. Biomineralization is perhaps the most obvious time-bounded driver. However, other changes in the sedimentological character of Archean and Proterozoic carbonates offer an under-considered record of fluctuations in Earth’s chemical, biological, and physical systems before and during the rise of animal life. Geobiologists and geochemists have successfully used large geochemical datasets to track changes in Earth’s surface chemistry through time, but inconsistencies between proxies make the recognition of clear spatial-temporal patterns challenging. In order to place new constraints on the environmental history of Archean and Proterozoic oceans, especially with regard to redox state, we present for the first time a high-resolution database of global Precambrian and Cambrian carbonate sedimentation. Our initial datasbase, named Microstrat, comprises carbonate-bearing successions dating from c. 3.4 billion to ~500 million years old; these include more than 130 carbonate-bearing formations, digitized at the highest stratigraphic resolution possible and classified by environment of deposition. Lithofacies details are recorded at the finest scale available, including a range of microbial and other depositional fabrics, mineralogy, environment of deposition, age and location. We interrogate the Archean, Proterozoic, and Cambrian carbonate records, testing both long-standing and novel hypotheses about changes in Earth’s carbonate system through time. Changes in carbonate sedimentology illuminate global changes in bioturbation and microbial community structure; the depth-dependent timing of marine oxygenation and the transition from anaerobic to aerobic respiration at and within the seafloor; and temporal changes in patterns of dolomitization. Shifts in both the loci of carbonate precipitation and in markers of animal activity are consistent with time- and depth-dependent trends in ocean oxygenation. We also note changes through time in the morphologies of stromatolites and thrombolites, which we suggest reflect secular shifts in carbonate precipitation, clastic sediment transport, and eukaryotic evolution. Observations from the database and petrographic studies confirm that the nature and extent of Proterozoic dolomite contrasts with that of dolomite in the Phanerozoic, consistent with dolomite being an early or even, in some cases, primary precipitate prior to the Cambrian Period. Because this database preserves environmental context and the stratigraphic relationships of carbonate features rather than tracking the occurrence of single features through time, it permits new and detailed analyses of the physical sedimentology of Precambrian carbonates and correlations in time and space among different features. We also consider how to build upon this initial dataset. Capturing and analyzing sedimentological data is difficult. Analysis becomes more challenging when data have been collected by multiple researchers working across Earth’s entire history and surface. Field observations have a great deal of power to describe and explain changes in Earth’s surface environments, yet they are often reported in idiosyncratic ways that make it difficult to compare them, obscuring their true explanatory power. Microstrat is an effort to pool idiosyncratic sedimentological data in hopes of illuminating Archaean, Proterozoic, and Cambrian carbonate depositional environments, yet it is unavoidably limited. We describe how other researchers can help to build Microstrat into a community-curated database by reporting and sharing data.

Reconstructing the Himalayan margin prior to collision with Asia: Proterozoic and lower Paleozoic geology and its implications for Cenozoic tectonics

Article

Nov 2018

Reconstructing the stratigraphic architecture of deposits prior to Cenozoic Himalayan uplift is critical for unravelling the structural, metamorphic, depositional and erosional history of the orogen. The nature and distribution of Proterozoic and lower Paleozoic strata have helped elucidate the relationship between lithotectonic zones, as well as the geometries of major bounding faults. Stratigraphic and geochronological work has revealed a uniform and widespread pattern of Paleoproterozoic strata >1.6 Ga that are unconformably overlain by <1.1 Ga rocks. The overlying Neoproterozoic strata record marine sedimentation, including a Cryogenian diamictite, a well-developed carbonate platform succession and condensed fossiliferous Precambrian–Cambrian boundary strata. Palaeontological study of Cambrian units permits correlation from the Indian craton through three Himalayan lithotectonic zones to a precision of within a few million years. Detailed sedimentological and stratigraphic analysis shows the differentiation of a proximal realm of relatively condensed, nearshore, evaporite-rich units to the south and a distal realm of thick, deltaic deposits to the north. Thus, Neoproterozoic and Cambrian strata blanketed the northern Indian craton with an extensive, northward-deepening, succession. Today, these rocks are absent from parts of the inner Lesser Himalaya, and the uplift and erosion of these proximal facies explains a marked change in global seawater isotopic chemistry at 16 Ma.

Sequence-Stratigraphic Position of Oolitic Bank of Cambrian in North China Platform: Example from the Kelan Section of Shanxi Province

Article

Full-text available

Jun 2018

According to the recent chronostratigraphic division of Cambrian, the Cambrian strata in the Kelan section can be subdivided into seven third-order sequences (\(\hbox {DS}_{1}{-}\hbox {DS}_{7})\) based on cyclicity in sedimentary facies stacking patterns. The calcareous mudstone forming condensed section, micritic limestone comprising deep to middle ramp facies and the oolitic-grain bank facies in Series 3 and Furongian series represent the basic depositional fabric of Cambrian in northwestern part of Shanxi Province, North China Platform. These massive oolitic grainstones demonstrate that the oolitic-grain bank facies constitute the late-highstand systems tract or forced-regressive systems tract. The grains occupy upper parts of the third-order depositional sequences in response to relative sea-level fall. Furthermore, this forming pattern of oolitic-grain bank facies does not follow the standard model of sequence stratigraphy, in which deposition is believed to occur principally during sea-level rise, rather a continuous erosional unconformity develops during sea-level fall. Moreover, the microscopic analysis of oolitic grainstone shows the development of concentric and radial, rounded or elliptical, with or without nuclei, monocrystalline or polycrystalline, Girvanella or micritic ooids. The occurrence of diversified ooids in varying proportions provides a new dimension for studying evolution of the oolitic-grain bank in the North China Platform. The forming pattern of oolitic-grain bank controlled by their sequence-stratigraphic position in the Kelan section in the Shanxi province provides an important clue and a research direction for the regional correlation, as well as the paleogeographical reconstruction of the Cambrian Series 3 and Furongian series.

Lithostratigraphic classification of the Blaini, Infra-Krol, Krol and Tal formations: A review

Article

Full-text available

Jan 1979

Om N. Bhargava

Tertonirs of the Himalaya and southern Tibet iiom two perspertives

Article

Full-text available

Jan 2000

Kip Hodges

The Himalaya and Tibet provide an unparalleled opportunity to examine the complex ways in which continents respond to collisional orogenesis. This paper is an attempt to synthesize the known geology of this orogenic system, with special attention paid to the tectonic evolution of the Himalaya and southernmost Tibet since India-Eurasia collision at ca. 50 Ma. Two alternative perspectives are developed. The first is largely historical. It includes brief (and necessarily subjective) reviews of the tectonic stratigraphy, the structural geology, and metamorphic geology of the Himalaya. The second focuses on the processes that dictate the behavior of the orogenic system today. It is argued that these processes have not changed substantially over the Miocene–Holocene interval, which suggests that the orogen has achieved a quasi–steady state. This condition implies a rough balance between plate-tectonic processes that lead to the accumulation of energy in the orogen and many other processes (e.g., erosion of the Himalayan front and the lateral flow of the middle and lower crust of Tibet) that lead to the dissipation of energy. The tectonics of the Himalaya and Tibet are thus intimately related; the Himalaya might have evolved very differently had the Tibetan Plateau never have formed.

Microbialites: Organosedimentary Deposits of Benthic Microbial Communities

Article

Full-text available

Jan 1987

Microbialites are organosedimentary deposits formed from interaction between benthic microbial communities (BMCs) and detrital or chemical sediments. Processes involved in the formation of calcareous microbialites include trapping and binding of detrital sediment (forming microbial boundstones), inorganic calcification (forming microbial tufa), and biologically influenced calcification (forming microbial framestones). Microbialites contrast with other biological sediments in that they are generally not composed of skeletal remains. -from Authors

Two or four Neoproterozoic glaciations?

Article

Full-text available

Dec 1998
GEOLOGY

A thick Neoproterozoic carbonate and glaciogenic succession of the southern Congo craton has yielded delta13C and 87Sr/86Sr records through the later Cryogenian (ca. 750 600 Ma) and earlier part of the Terminal Proterozoic (ca. 600 570 Ma). Sizeable negative delta13C excursions (to less than 50/00) occur above each of two glacial intervals and the 87Sr/86Sr values of marine carbonates shift from ˜0.7072 to ˜0.7079 at the upper glacial level. These geochemical constraints provide a Marinoan (younger Varanger) age for the upper glacial interval, previously regarded as a second phase of the Sturtian glaciation. The delta13C record from the Congo craton is therefore incompatible with recent global delta13C syntheses that have identified four or more separate ice ages during the Neoproterozoic. A cladistic analysis of geologic and geochemical characters of 12 Neoproterozoic glacial deposits identifies two distinct groups that are found in a consistent stratigraphic order whenever two glacial units occur within a single succession. We use delta13C and 87Sr/86Sr records from the Congo craton and other key successions to test the null hypothesis that there were only two global glaciations (Sturtian and Marinoan) during the Neoproterozoic. Placing the GSSP (global stratotype section and point) for the base of the Terminal Proterozoic within or just above a cap carbonate of the younger (Marinoan) glaciogenic succession would confine all known Neoproterozoic glaciations to the Cryogenian. The rapid shift in marine 87Sr/86Sr to more radiogenic values during the Marinoan glaciation is opposite that predicted by the snowball Earth scenario which calls for continental runoff to cease during glaciation, resulting in a shift to less radiogenic values.

Structure and shortening of the Kangra and Dehra Dun reentrants, Sub-Himalaya, India

Article

Full-text available

Aug 1998

Surface geology, oil-well, seismic-reflection, and magnetostratigraphic data are integrated to evaluate the structural style and the shortening rate at the Himalayan front(Sub-Himalaya) of northwest India. The Sub-Himalaya, between the Main Boundary thrust and the Himalayan Frontal fault, is the primary surface expression of shortening between the Himalaya and the Indian plate. At certain locations, the Himalayan Frontal fault is a blind thrust beneath anticlines of Siwalik (Tertiary) molasse, parallel to the Himalayan orogen. The Main Boundary thrust is sinuous, so the width of the Sub-Himalaya ranges from 30 to 80 km. Where the Sub-Himalaya is narrow (Nahan salient), Tertiary rocks are exposed in imbricate thrust sheets; where the Sub-Himalaya is broad (Kangra and Dehra Dun reentrants), alluvium fills wide synclinal valleys (duns). Seismic-reflection data reveal that surface anticlines form in association with south-vergent thrusts that root in a decollement at the base of the Tertiary section. Reflection profiles and well data also indicate that the basement lithology changes northward from Precambrian crystalline rocks beneath the Indo-Gangetic plains to Precambrian and Cambrian metasedimentary rocks beneath the Sub-Himalaya. The Sub-Himalayan decollement dips 2.5° northward beneath the Kangra reentrant, but it is steeper, 6°, beneath the Dehra Dun reentrant. The Kangra and Dehra Dun reentrants display fault-propagation folds having steep limbs in the north, and fault-propagation and fault-bend folds that have gently north-dipping limbs in the south. A balanced cross section of the Kangra reentrant shows that a minimum of 23 km shortening has occurred since 1.9-1.5 Ma, yielding a shortening rate of 14±2 mm/yr. Shortening has occurred at a rate of 6-16 mm/yr across the Dehra Dun reentrant. These data are similar to other published shortening rates and indicate that approximately 25% of the total India-Eurasia convergence at this longitude is accommodated within the Sub-Himalaya. Given continued convergence and the presence of over-pressured wells in the Kangra reentrant, the region is likely at risk from moderate and/or great earthquakes in the future.

Pre-Mesozoic Ice Ages: Their Bearing on Understanding the Climate System

Book

Jan 1999

John C. Crowell

Some observations on the sedimentology of the Krol succession of the Mussoorie area, Uttar Pradesh, India.

Article

Jan 1980

Study of sedimentary structures indicate that Krol carbonates represent deposits of a shallow tidal sea: 5 facies are distinguished: A - open sea tidal flats in intertidal-subtidal zone: B - protected embayment or lagoon; C - open tidal flats to intertidal zone of restricted tidal flat, D - algal mat facies of inner intertidal to supratidal zone with increased salinity, E - intertidal zone. The algal mat facies of E shows dominantly Loferite, fenestral dolomite or bird's eye dolomite, showing different types of shrinkage cracks. The supratidal facies is quite abundant, represented by vuggy dolomite and bird's eye pisolitic dolomite. -Authors

Distribution and deposition of mudstone facies in the upper devonian sonyea group of new york

Article

Jan 1999

J. Schieber

Petrographic and small-scale sedimentary features of the mudstone-dominated Upper Devonian Sonyea Group of New York were examined in order to (1) improve our understanding of mudstone facies, (2) examine coeval mudstone facies within a broader depositional context (3) promote a simple methodology for the study of mudstones. Six mudstone facies are distinguished, each characterized by several component mudstone types or subfacies. Sedimentary conditions and environments are reconstructed from primary sedimentary structures and bioturbation characteristics. Mudstone facies are characterized by soils and flood deposits in the coastal-plain region, by rapid sediment deposition and frequent reworking in nearshore areas, by storm-dominated offshore transport on a wide, basin-margin platform, by turbidite slope deposits below storm wave base by bottom currents and slow settling in the distal, "deep"-basin black mudstones. The latter, although commonly thought of as deposits of a stratified anoxic basin, contain indications of benthic life, such as burrows, disrupted laminae clay/silt fecal pellets. These observations are incompatible with a stratified-basin model indicate the need to search for alternative models of black-shale accumulation.

Additional Ediacaran biota from the Krol Group, Lesser Himalaya, India and their significance

Article

Jan 1997

Some observations on the depositional environment of the Krol Formation in Nainital area

Article

Jan 1978

Construction of an Early Proterozoic (1‐9 Ga) barrier reef complex, Rocknest platform, Northwest Territories

Article

Jan 1989

J.P. Grotzinger

Geological evolution of the Krol Belt in Simla Hills, H.P

Article

Jan 1971
HIMAL GEOL

Fauna and biogenic structures in Krol-Tal succession (Vendian-Early Cambrian), Lesser Himalaya: Their biostratigraphic and palaeoecological significance

Article

Jan 1983

Precambrian-Cambrian sequence in Krol belt and additional Ediacaran fossils

Article

Jan 1992

Detailed lithostratigraphic and palaeontologic studies, including the elements of Ediacaran fossils, suggest that Blaini-Krol-Tal succession of Krol belt in Lesser Himalaya represents a part of Precambrian-Cambrian sequence in the basin. The Chert Member of the Tal Formation contains fossils of Meishucunian Zone I(uppermost Precambrian) and the overlying lower part of Arenaceous Member has yielded the fossils of Meishucunian Zone III(early Cambrian). The Precambrian-Cambrian boundary, thus may lie between Chert and Arenaceous members. There is, therefore, a scope of locating fauna of Meishucunian Zone II in the intervening Argillaceous Member and delineating the boundary with precision. -Authors

Stochastic modeling of incised valley geometries

Article

Jun 1998
AAPG BULL

A stochastic modeling procedure, designed to capture sequence stratigraphic principles, has been developed for modeling fluvial reservoirs where high-frequency base-level fluctuations have exerted a strong influence on reservoir architecture. Reservoir stratigraphy and architecture are defined by the successive simulatjon of two types of surfaces: base-level rise (flooding) surfacesa and base-level fall surfaces (sequence boundaries). The flooding surfaces are modeled as standard two-dimensional Gaussian fields. Modeling sequence boundaries with incised valleys is more complex and required the development of a novel 'object' modeling technique. This new model can be used to generate realistic valley geometries and is flexible enough to allow for complex multiwell conditioning. The modeling procedure is illustrated using a test data set based on well interpretations from the fluvial Statfjord Formation in the Stratjord field of the Norwegian North Sea. The main reservoir sandstones were deposited in valleys defined by a sequence boundary at the base and a flooding surface at the top, whereas the main barries to flow are mudstone-rich intervals deposited on unconfined alluvial plains. Five sequence boundaries and four flooding surfaces have been interpreted within an approximately 60-m-thick reservoir interval. Simulation of these surfaces using the new modelling procedure defines the three-dimensional distribution of reservoir units and barriers. The modelling procedure allows the simulation of realistic geometries that are in accordance with the geologist's conceptual model for the reservoir. The models also provide an enhanced description of reservoir distribution and connectivity, and can serve as an improved basis for reservoir management, well placement, and predictions of reservoir performance in complex fluvial reservoirs.

Stratigraphy of Blaini, Infra Krol, Krol and Tal succession, Krol Belt, Lesser Himalaya

Article

Jan 1993

The lithostratigraphic classification of the Blaini-Infra Krol-Krol-Tal succession of the Lesser Himalaya, ranging in age from Terminal Proterozoic to Early Cambrian, has been standardised. The succession rests unconformably over Jaunsar/Simla Group and in turn is unconformably overlain by younger sequences belonging either to Early Permian (Boulder Slate Formation) or (as generally is the case) by Late Cretaceous-Eocene sediments represented by Shell Limestone (Manikot) and Subathu formations, respectively. Reviewing the lithostratigraphy, the status of each formation has either been raised to that of a group or redefined. -from Authors

Dolomitized Middle Proterozoic Calcretes, Bathurst Inlet, Northwest Territories, Canada

Article

Jan 1991

Dolostones within the terrigenous clastic middle Proterozoic Kanuyak Formation are interpreted to be dolomitized calcretes (dolocretes) which formed on a landscape largely devoid of macroscopic organic life. While missing the features commonly associated with higher plants in post-Silurian calcretes (rhizoconcretions, root-mouldic porosity, etc.), the presence of an array of other common calcrete fabrics (eg, cracks, cements, accretionary grains and deformation structures) suggests that physical and chemical processes were probably largely responsible for the carbonate precipitates within Kanuyak profiles. Microbiota most likely also contributed to calcretization; however, if not significantly altered by dolomitization, stable carbon and oxygen isotope values do not indicate any significant influence by organic matter. -from Authors

First record of Ediacaran fossils from the Krol Formation of Naini Tal Syncline

Article

Jan 1989

The paper records the discovery of Ediacaran fossils, viz. Beltaneliformis sp.cf. B. brunsae Menner, Pteridinium sp.cf. P. simplex Gurich and Ichnogenus Gordia sp.cf. G. marina Emmons from the upper part of the Krol Formation in the Naini Tal Syncline, Lesser Himalaya. These taxa are comparable with the fossils present in the late Precambrian Wernecke assemblage of Canada and Ediacaran assemblage of other areas, particularly Australia and Eurasia. -Authors

Early Diagenesis of Carbonate Sediments

Chapter

Jan 1983

R. G. C. Bathurst

The realm of early diagenesis will be regarded here as synonymous with eogenetic environment of Choquette and Pray (1970). In this the various diagenetic processes act during that stage between accumulation of the sediment on the Earth’s surface and its burial below the depth of significant influence by processes related to that surface. As most limestones are marine in origin, attention will be concentrated on this group of marine carbonate sediments.

Precambrian carbonates: evolution of understanding

Chapter

Jan 2000

Evolution of Early Proterozoic Passive-Margin Carbonate Platform, Rocknest Formation, Wopmay Orogen, Northwest Territories, Canada

Article

Jan 1986

John P. Grotzinger

This early Proterozoic (1.89Ga) carbonate platform found along the E passive margin of Slave craton (Archean). The palinspastically restored shelf sequence is an E-thinning prism, 0-1100m thick, extending for over 220km parallel to strike, and over 200km perpendicular to strike. It contains, from W to E, slope, outer-shelf, shoal-complex, and inner-shelf facies. The Rocknest carbonate shelf developed on an underlying siliciclastic shelf. An initial ramp rapidly evolved into a progradational accretionary rim and shoal complex, followed by vertical aggradation of the rim. Concurrently, the shoal complex underwent numerous W to E progradational events to produce many shallowing-upward cycles in the shelf interior. -from Author

Palaeoenvironment of Blaini Formation, Lesser Himalaya

Article

Jan 1982

Modern carbonate tidal flats: Their diagnostic features

Article

Jan 1986

E.A. Shinn

Large acanthomorphic acritarchs from the Infrakrol Formation of the Lesser Himalaya and their stratigraphic significance

Article

Jan 1994

Introduction

Chapter

Jan 1988

Karst is as dramatic a feature of the earth’s surface as it is unique and complex. The manifold and convolute landforms, the complicated and delicately adorned caves, the bizarre drainage systems and collapse structures have few anologs in other kinds of terrains. But these features are only the most obvious in an array of surface and subsurface structures which range in size down to the submicroscopic and comprise systems that are only partly understood—systems that, quite uniquely, form almost entirely by dissolution.

Stratigraphic and structural evolution of the Kumaun Lesser Himalaya

Article

Jan 1974
SEDIMENT GEOL

The geology of the Krol belt

Article

J.B. Auden

Peritidal Carbonate Platform Growth and Cyclicity in an Early Proterozoic Foreland Basin, Upper Pethei Group, Northwest Canada

Article

Jan 1994

Stratigraphy and sedimentation in Himalaya: a reappraisal

Article

Jan 1989

Parasequence stacking patterns, third-order accommodation events, and sequence stratigraphy of Middle to Upper Cambrian platform carbonates, Bonanza King Formation, Southern Great Basin

Article

Jan 1993

Tidal Channel, Levee, and Crevasse-Splay Deposits from a Cambrian Tidal Channel System: A New Mechanism to Produce Shallowing-Upward Sequences

Article

Jan 1990

The Waterfowl Formation comprises 5 rock types: intraformational conglomerate; cross-stratified grainstone; wavy-bedded grainstone-mudstone; massive dolomite mudstone; and prism-cracked, crypt-microbial-laminated mudstone. These rock types form a grainstone body in the upper 7 m of the outcrops and 4 fining-upwards sequences in the lower 3 m. The fining-upwards sequences may be shallowing-upwards sequences in the sense of James (1984) and represent aggrading-prograding tidal flat deposits. Alternatively, these sequences may be crevasse-splay and levee deposits analogous to fluvial overbank flood sequences. -from Authors

The Recognition and Interpretation of Paleokarsts: Two Examples from the Lower Carboniferous of South Wales

Article

Jan 1982

V.P. Wright

Criteria for the identification and interpretation of paleokarstic surfaces in ancient carbonate sequences are discussed and two examples are described from the Lower Carboniferous of South Wales. One type is compared to Kavornossen karren, a type of tropical karst well developed today on Puerto Rico. The second example, a mammillated paleokarstic surface is interpreted as a deckenkarren, formed beneath a vegetation cover and similar to the Makondo karsts of South Africa. Paleokarsts can provide information on the paleoclimates, paleohydrology, and vegetation cover existing at the time of their formation. -from Author

Ediacaran medusoids from the Krol Formation, Naini Tal Syncline, Lesser Himalaya

Article

Jan 1990
J GEOL SOC INDIA

Late Precambrian Ediacaran medusoid coelenterates, viz., Tirasiana sp., Medusinites sp. and Beltanella sp. are reported from the upper part of the Krol Formation in the Naini Tal Syncline, Lesser Himalaya in association with older Ediacaran fossils, viz., Beltanelliformis sp., Pteridinium sp. and Ichnogenus Gordia sp. -Authors

Proterozoic Paleokarst Profile, Dismal Lakes Group, N.W.T., Canada1

Chapter

Jan 1988

The Middle Proterozoic Dismal Lakes Group provides a rare example of Precambrian paleokarst which contains a remarkably complete spectrum of solutional, precipitative, and depositional features. The Dismal Lakes Group is a 1.5-km siliciclastic to carbonate succession that records dominantly stable peritidal carbonate sedimentation along the northwestern margin of the Canadian Shield. Karst features are widespread along a regionally extensive subaerial exposure surface associated with regression near the end of Dismal Lakes deposition, and are especially abundant near a depositional high developed earlier near September Lake. An east-to-west transect down depositional dip from the crest of the profile to the flanking shoreline displays the following karst features: 30 m-thick collapse breccias with preserved roof pendants and cross-stratified chert granulestone cave-floor sediments, grike systems filled by transgressive marine sand, fibrous flowstone, cave popcorn, regolith, and finally, a sharp erosional surface overlain by mixed siliciclastic-carbonate facies of the succeeding marine transgressive phase. Evolution of the karst profile occurred in three stages: (1) initial subaerial exposure and phreatic dissolution leading to formation of subsurface karst, (2) continued regression and vadose infill by autochthonous and allochthonous clastic sediments and flowstone precipitation, and (3) cave collapse prior to marine transgression. The common development of grikes and the concomitant paucity of small- or medium-scale karren indicate either a temperate climate during subaerial exposure, or karst dissolution beneath a soil mantle now only locally preserved. The eustatic versus local tectonic nature of the regressive event leading to karst formation is speculative; both factors appear to have had an influence.

Surface and Subsurface Paleokarst, Silurian Lockport, and Peebles Dolomites, Western Ohio

Chapter

Jan 1988

Charles F. Kahle

The Lockport Dolomite in northwestern and west- central Ohio and the stratigraphically equivalent Peebles Dolomite in southwestern Ohio collectively display a variety of previously unrecognized subsurface and surface paleokarst features. The former include molds, vugs, in situ breccia with corroded clasts, collapsed strata, solution-enlarged joints, internal sediment derived partly from soil, boxwork, and caves. Surface paleokarst features at the top of these units are represented by locally developed pa- leosol and sinks. Erosional relief at the top of the Lockport/Peebles ranges from fractions of an inch (typical) up to 9 feet (2.7 m) (rare). The contact between the Lockport/Peebles and the overlying Greenfield Dolomite is typically a paraconformity in the form of a paleokarst planar erosion surface. As such, the contact can be difficult to recognize, but it is not a fades contact nor is it gradational.

Facies and evolution of Precambrian carbonate depositional systems: emergence of the modern platform archetype

Chapter

Jan 1989

JOHN P. GROTZINGER

Precambrian carbonates generally have not been examined from the perspective of platform evolution. The major facies and stratigraphic relations of several carbonate sequences ranging from the early Archean through late Proterozoic are examined and discussed in terms of platform construction and the dominant factors that influenced patterns of sedimentation and the production of carbonate. Eustasy apparently influenced the stratigraphic architecture of many platforms, causing rim backstepping, incipient shelf drowning, and the development of third-order sequences of "Grand Cycles'. It also was the likely cause of parasequences or "small-scale cycles' that characterize peritidal environments in late Archean through late Proterozoic sequences. -from Author

Airborne Silts and Dune-Derived Sands in The Permian of the Delaware Basin

Article

Jan 1988

Michael Sarnthei Alfred G. Fischer

The sandstones and siltstones which dominate the mid-Permian in the Delaware Basin (Texas, New Mexico) are reinterpreted by comparison with the Pleistocene eolo-marine sediments off the Saharan west coast. We suggest that the separation of sands and silts occurred during eolian transport across deserts. Dust was directly contributed to the basin by fallout, yielding topography-mantling, laminated sandy siltstones. Dunes reached the shelf break during a low sea-level stand (peak glaciation) and gave rise to turbidites that deposited the massive channel sands of Brushy Canyon time. Subsequently (Cherry Canyon and Bell Canyon times), dune sands and eolian silts were blended on the platform before aqueous delivery to the shelf break, to yield silty and micaceous channel sands and overbank turbidites interbedded with eolo-marine silt beds. The black-white laminated eolo-marine silstones may record seasonal fluctuations in organic productivity related to the dustborne nutrient supply and to upwelling in a strong seasonal wind regime such as that recorded in the Sahara.

Relation of Eustasy to Stacking Patterns of Meter-Scale Carbonate Cycles, Late Cambrian, U.S.A.

Article

Jan 1991

J. Fred Read David Osleger

An interbasinal study of Late Cambrian cyclic carbonate successions in the Appalachian and Cordilleran passive margins suggests that superimposed orders of eustasy controlled the development of large-scale depositional sequences and the component peritidal to subtidal meter-scale cycles that comprise them. The focus of this paper is on the small-scale cyclicity, its probable control by Milankovitch-forced sea-level oscillations, and how stacking patterns of meter-scale cycles can be used to define internal components of larger-scale sequences and estimate variations in relative sea level. Fining-upward peritidal cycles showing evidence of episodic emergence grade seaward into coarsening-upward subtidal cycles which lack evidence of emergence and form a continuum across the Cambrian carbonate platforms. Eustasy appears to exert the dominant control on the simultaneous development of peritidal and subtidal cycles on Late Cambrian carbonate platforms. Evidence for Milankovitch forcing of glacio-eustatic sea-level oscillations is provided by a 4:1 bundling of fifth-order meter-scale cycles ( 96 ky) within fourth-order cycles spanning tens of meters ( 440 ky) within the Big Horse Member of the Orr Formation in the House Range of Utah. The 4:1 bundling may manifest the short e centricity to long eccentricity ratio of the Milankovitch astronomical rhythms. Systematic changes in the stacking patterns of meter-scale cycles can be used in conjunction with Fischer plots to define long-term sea-level cycles. On Fischer plots of peritidal cyclic successions, long-term relative sea-level rises are characterized by thick, subtidal-dominated cycles with thin laminite caps. Long-term relative sea-level falls are defined by stacks of thin, laminite-dominated cycles that show brecciated cycle caps and quartz sands toward the relative sea level lowstand. On Fischer plots of dominantly subtidal cyclic successions, long-term sea-level rise is characterized by storm-dominated, open marine carbonate cycles or thick, deep ramp, shale-based cycles. Falling segments of the Fischer plot are characterized by thin, shallow subtidal cycles compos d of restricted lithofacies. Cycle stacking patterns (parasequence sets) provide the crucial link between the meter-scale cycles (parasequences) and the larger scale sequences and their component systems tracts. One- and two-dimensional models of peritidal and subtidal cycle development indicate that, whereas peritidal cycle thickness is primarily controlled by accommodation space, deeper subtidal cycle thickness is primarily controlled by sedimentation rate. Lithofacies within peritidal cycles reflect the sedimentologic response to fluctuations in sea level; lithofacies within subtidal cycles may be related to fluctuations in the zones of fairweather and storm-wave reworking that oscillated in harmony with sea-level fluctuations and may have acted as a subtidal limit to upward aggradation. The 2-D modelling illustrates how stacked peritidal to deep subtidal carbonate cycles with thicknesses, compositions and stacking patterns similar to the Late Cambrian of North America can be generated by ilankovitch-driven composite eustasy.

Stromatolite-Thrombolite Associations in a Modern Environment, Lee Stocking Island, Bahamas

Article

Apr 1998

Columnar buildups found in a tidal channel off Lee Stocking Island, Exuma Cays, Bahamas, have been interpreted as modern giant stromatolites growing in a subtidal normal marine environment. However, these organically-formed columns reveal three discernible internal structures: (1) prokaryotic stromatolites comprised of alternating layers of coarse-grained ooids and peloids, and fine micrite that formed exclusively by microbial activity; (2) eukaryotic stromatolites comprised of microbially-induced micritic layers alternating with detrital layers accumulated, bound, and cemented by eukaryotic algae; and (3) thrombolites displaying irregular, clotted fabrics and formed by microbes, algae, and metazoans. Phanerozoic thrombolites, in contrast, have been interpreted as unlaminated stromatolites constructed by cyanobacteria. Eukaryotic organisms overgrow all of the columns at present. Thus, the contemporaneous formation of prokaryotic stromatolites, eukaryotic stromatolites, and thrombolites under identical conditions within the present environment appears unlikely. We suggest that the prokaryotic stromatolites represent forms that began to develop in an intertidal setting with the Holocene flooding of the Great Bahama Bank. The thrombolites, however, began to form under the present, normal-marine subtidal conditions. The eukaryotic stromatolites represent intermediate forms between prokaryotic stromatolites and thrombolites. There is evidence for a gradual change from stromatolite to thrombolite reefs associated with rising sea-level. With the deepening, there would have been a decrease in salinity, an increase in energy, and possibly an increase in nutrient supply; all factors that favor thrombolite growth. We propose that the co-existing stromatolites and thrombolites found off Lee Stocking Island did not grow contemporaneously, but reflect a response to changing environmental controls with changing sea-level.

Sequence stratigraphy and relative sea-level history of the Middle and Upper Ordovician of the Nashville Dome, Tennessee

Article

Jul 1998

Eleven depositional sequences are present in the Middle and Upper Ordovician of the Nashville Dome. AH but one lack lowstand systems tracts, and many display evidence of subaerial exposure such as paleokarst or regional erosional truncation at their sequence boundaries, which are combined with transgressée surfaces. Features formed during the transgression, such as hardgrounds, are more prominent at most sequence boundaries than are those features related to subaerial exposure. All sequences have maximum flooding zones with aggradational stacking rather than discrete maximum flooding surfaces. Condensation features are much more common at transgressive surfaces than within the maximum flooding zone. The expression of these parasequences and sequences changes with the onset of the Taconic orogeny. Although parasequences from equivalent depositional environments can be recognized before and after the onset of the orogeny, their faciès composition changes completely. Sequences following the orogeny display fewer paleokarst features, more pyritized and ironstained hardgrounds at flooding surfaces and transgressive surfaces, and more phosphatic lags and mineralization at transgressive surfaces than those preceding the orogeny. A relative sea-level curve for the Nashville Dome has a long-term (> 5 m.y.) component presumably dominated by subsidence and a short-term (1-3 m.y.) component consisting of 10-50 m fluctuations that is primarily the result of eustasy.

Eustatic control on early dolomitization of cyclic peritidal carbonates: Evidence from the Early Ordovician Upper Knox Group, Appalachians

Article

Jul 1992
GEOL SOC AM BULL, GSA Bulletin

Describes stacked meter-scale peritidal cycles that repeat at high frequencies (104 to 105 yr). Stacking patterns (stratigraphic trends in lithofacies and cycle thickness) of meter-scale cycles define five depositional sequences that, in conjunction with Fischer plots, delineate five long-term relative sea-level fluctuations during Upper Knox deposition. Intrabasinal and interbasinal correlation of Upper Knox Fischer plots suggests that the third-order sea-level events were eustatic. Meter-scale peritidal cycles likely formed in response to high-frequency, fourth- and fifth-order, eustatic sea-level fluctuations superimposed on these third-order sea-level events. -from Authors

Pre-Mesozoic Ice Ages: Their Bearing on Understanding the Climate System

Book

Jan 1999

John C Crowell

Pre-Mesozoic glaciations on Earth are documented by identifiable glacial debris preserved within the stratal record, by rare glacial pavements and landforms, and by geo-chemical proxies indicating times of climate coolness. Although much of the record of ancient cool intervals has been worn away following uplift and erosion, documentation remains in sequences laid down in basins where strata have been preserved through long periods of geologic time, and then later uplifted and exposed for our examination. Crustal tectonic mobility, similar to plate-tectonic activity today, is deemed to have operated as far back as the icy record has been identified, to nearly 3 b.y. ago. Glaciers are viewed as always having occupied some continental areas, usually mountains, throughout this long duration. From time to time glaciers have expanded to reach the sea and have left a stratal mark showing times of coolness that interrupt irregularly times of relative warmth. Studies of the Late Cenozoic Ice Age, which began in late Eocene time, disclose complex interrelations between continental positioning and topography, oceanic bathymetry and location of gateways between land masses, organizations of both air and ocean currents, and concurrent effects of evolving life and its biogeochemical products. Research on the Cenozoic record is revealing processes and products to guide interpretations of cool climate intervals during the past nearly 3 b.y. of Earth history. Iciness on Earth is reviewed here by working back into time from the present because the record becomes dimmer and is less securely dated the older it becomes. The Cenozoic Ice Age, within which we are now living, began at ca. 43 Ma and was preceded by a warmer interval of ca. 70 m.y. back into mid-Cretaceous time. The next older Mesozoic icy intervals are Early Cretaceous (ca. 105-140 Ma) and Jurassic (ca. 160-175 Ma and ca. 188-195 Ma) and were preceded by relative warmth for ca. 60-5 m.y. before the end of the Permian period. The Late Paleozoic Ice Age waxed and waned over some 82 m.y. between ca. 256 Ma and 338 Ma. It was preceded by a warmer interval lasting 15 m.y. Iciness expanded during early Carboniferous and Late Devonian times for ca. 15 m.y. between 353 Ma and 363 Ma. Relative warmth prevailed for the next older interval for nearly 61 m.y. and followed the Ordovician-Silurian strong and short ice age of some 16 m.y. between ca. 429 Ma and 445 Ma. For the next 66 m.y., no ice is recorded. During the 430 m.y. of the Late Proterozoic, and lasting into the Cambrian, there were three or four ice ages (ca. 520-950 Ma). At some localities glaciation occurred at low latitudes. Previously, for about 1250 m.y. from ca. 950 Ma back to nearly 2200 Ma, no record of glaciation has yet been confidently documented, posing a paleoclimatological problem. Earlier glaciation during the Paleoproterozoic (ca. 2200-2400 Ma) is demonstrated in three or four far-distant regions. The oldest glaciation so far recognized on Earth (ca. 2914-2990 Ma) is recorded in strata of the late Middle Archean, nearly 3 b.y. ago. These ice ages occur with irregular repetitions rather than at time-cyclic intervals. Their causes are viewed as rooted in nonperiodic thermodynamic overturns deep within the Earth that result in a mobile crust and changing fluid fluxes to the air and oceans, including carbon dioxide. Climate, including times of unusual iciness, has responded to tectonic plate rearrangements, continental drift and fragmentation, sea-level changes, and especially biogeochemical changes resulting from the winding pathways of life's evolution. Climate has also been modulated by orbital variations and perhaps by variations in the tip of the spin axis and other extraterrestrial events. It has been drastically affected temporarily by bolide impacts from time to time. Climate upon the Earth over the past 3 b.y., however, is primarily the result of changing tectonobiogeochemical activities rooted within the complex earth-air-ocean system itself.

The Origin of High-Frequency Platform Carbonate Cycles and Third-Order Sequences (Lower Ordovician El Paso Gp, West Texas): Constraints from Outcrop Data and Stratigraphic Modeling

Article

Jan 1993
J Sediment Petrol

The passive-margin succession of the Diablo Platform is represented by the second-order Sauk C supersequence set, consisting of a basal transgressive clastic unit (The Bliss Sandstone) above the breakup unconformity, marking the second-order basal lowstand transgressive phase, overlain by 750 m of drift-related, shallow-marine platform carbonate (the El Paso Group) recording the second-order highstand. Due to late Paleozoic structuring of the Gondwanan passive margin, present exposures in Texas are in an updip shelf position and lack internal stratal geometries across depositional strike, so sequences and systems tracts are identified solely by the vertical stacking patterns of depositional subfacies and higher-frequency, fifth-order cycles. By analysis of outcrop data the authors develop a sequence-stratigraphic model for lower Paleozoic passive-margin, shallow-water, platform carbonates that de-emphasizes the physical expression of sequence boundaries and systems-tract boundaries and focuses on the vertical and lateral, meter-scale cyclic and subfacies architecture of carbonate shelf deposits in the framework of third-order depositional sequences. They rely on analyses of stratal stacking patterns to bridge the gap from cycle-scale stratigraphy to seismic-scale sequence stratigraphy. In many settings, like the Permian of west Texas, sequence boundaries are often obvious, but in others, especially shallow-dipping ramps or flat-topped platforms on passive margins, third-order sequence boundaries are less obvious, particularly in deformed terranes lacking two-dimensional dip continuity.

Pre-Mesozoic Ice Ages: Their Bearing on Understanding the Climate System

Article

Jan 2000
Eos Trans. AGU

Thomas J. Crowley

It is a pleasure to read a book that summarizes a well‐respected scientist's lifetime experience with a subject. In this particular case the subject involves pre‐Mesozoic glaciations, and the author, John Crowell, has played a key role over the last few decades in both fundamental research and synthesizing our understanding of this interesting and important topic. Crowell's book, which is a monograph in the true sense of the word, summarizes in considerable detail the various types of geologic evidence for glaciations in the Late Paleozoic (256–338 Ma), Late Devonian‐Early Carboniferous (353–363 Ma), Late Ordovician‐Early Silurian (429–445 Ma), Early Cambrian/Neoproterozoic (∼520–950 Ma), Early Proterozoic (2200–2400 Ma), and the earliest glaciation recognized so far, in the Archean of South Africa (∼2950 Ma). There is also a brief but valuable discussion of the nature of evidence for cooling in the Cretaceous.

Cyclostratigraphy of Middle Devonian Carbonates of the Eastern Great Basin

Article

Jan 1995

Maya Elrick

Middle Devonian carbonates (250-430 m thick) of the eastern Great Basin were deposited along a low energy, westward-thickening, distally steepened ramp. Four third-order sequences can be correlated across the ramp-to-basin transition and are composed of meter-scale, upward-shallowing carbonate cycles (or parasequences). Peritidal cycles (shallow subtidal facies capped by tidal-flat laminites) constitute 90% of all measured cycles and are present across the entire ramp. The peritidal cycles are regressive- and transgressive-prone (upward-deepening followed by upward-shallowing facies trends). Approximately 80% of the peritidal cycle caps show evidence of prolonged subaerial exposure including sediment-filled dissolution cavities, horizontal to vertical desiccation cracks, rubble and ka st breccias, and pedogenic alteration; locally these features are present down to 2 m below the cycle caps. Subtidal cycles (capped by shallow subtidal facies) are present along the middle-outer ramp and ramp margin and indicate incomplete shallowing. Submerged subtidal cycles (64% of all subtidal cycles) are composed of deeper subtidal facies overlain by shallow subtidal facies. Exposed subtidal cycles are composed of deeper subtidal facies overlain by shallow subtidal facies that are capped by features indicative of prolonged subaerial exposure (dissolution cavities and brecciation). Average peritidal and subtidal cycle durations are between approximately 50 and 130 k.y. (fourth- to fifth-order). The combined evidence of abundant exposure-capped peritidal and subtidal cycles, transgressive-prone cycles, and subtidal cycles correlative with updip peritidal cycles indicates that the cycles formed in response to fourth-to fifth-order, glacio-eustatic sea-level oscillations. Sea-level oscillations of relatively low magnitude (Exposure surfaces at the tops of peritidal and subtidal cycles represent one, or more likely several, missed sea-level oscillations when the platform lay above fluctuating sea level, but the amplitude of fourth- to fifth-order sea-level oscillation(s) were not high enough to flood the ramp. The large number of missed beats (exposure-capped cycles), specifically in Sequences 2 and 4, results in Fischer plots that show poorly developed rising and falling limbs (subdued wave-like patterns); consequently the Fischer plots are of limited use as a correlation tool for these particular depositional sequences. The abundance of missed beats also explains why Milankovitch-type cycle ratios ( 5:1 or 4:1) are not observed and why such ratios would not be expected along many peritidal-cycle-dominated carbonate platforms.

Low- and high-frequency sea-level changes control peritidal carbonate cycles, facies and dolomitization in the Rock of Gibraltar (Early Jurassic, Iberian Peninsula)

Article

Jan 2000

The Rock of Gibraltar comprises two tectonically separated limbs of an isolated klippe of Liassic Gibraltar Limestone Formation. Both limbs have similar, c. 400 m thick sequences of inner carbonate platform facies arranged in high‐frequency, metre‐scale, shallowing‐upward, peritidal cycles with emergent, caliche caps. Four cycle types are recognized on the basis of vertically repeated successions of different sedimentary structures, lithologies, facies and biota. When compared with other Liassic cycles from fault‐bound platforms of the western Mediterranean region all are found to be of similar scale, facies and cycle type. Likely common origins are through Milankovitch band allocyclicity, or autocyclic tidal flat progradation superimposed on regional subsidence. Within the Gibraltar Limestone high‐frequency cycles are superimposed on a low‐frequency (third order?) cyclicity that is revealed, through the use of Fischer plots, to control the occurrrence of facies, biota, high‐frequency cycle types and dolomitization. Falling sea‐level and lowstand phases, with reduced accommodation space, are typified by restricted, inner platform facies and cycles and by early reflux dolomitization. Transgressive and highstand phases, with more accommodation space, are characterized by the absence of early dolomites, the incoming of inner platform microfossils (i.e. foraminifera and calcareous algae) and by less restricted marine facies (i.e. oncoids, shelly rudstones, packstones and grainstones). Fischer plots have demonstrable value in the correlation and analysis of tectonically separated and geographically isolated cyclic sequences that lack prominent marker beds or stratigraphically useful biotas.

Reconstructing early Himalayan tectonic evolution and paleogeography from Tertiary foreland basin sedimentary rocks, northern India

Article

Mar 2000

The latest Paleocene-middle Eocene Subathu Formation and the Oligocene-Miocene Dagshai and Kasauli Formations of the Indian foreland basin record the early evolution of the Himalayan fold-thrust belt. Sandstone petrography of the Subathu Formation shows a predominantly recycled sedimentary source, with a distinct ophiolitic and volcanic influence that was drastically reduced by the time of deposition of the Dagshai Formation. Sandstones in the Dagshai and Kasauli Formations consist predominantly of metapelitic detritus. The metamorphic grade of metapelitic lithic grains increases with time, from dominantly very low grade at the base of the Dagshai Formation to dominantly low grade in the Kasauli sandstones. Mudstone geochemistry documents the presence of a mafic-ultramafic source during the time of deposition of the Subathu Formation that becomes significantly less important by the time of deposition of the Dagshai Formation. Compositions of Subathu Formation detrital spinels show they were either derived from both mid-ocean ridge basalt-type and arc-type ophiolites or from an ophiolite of composite origin, and that southerly derivation from the Deccan Trap continental flood basalts is unlikely. Kasauli Formation garnet compositions suggest derivation from medium-grade metamorphic rocks to amphibolite facies. Subathu Formation composition reflects provenance influence from the Indus suture zone during latest Paleocene-middle Eocene time, indicating initiation of continent-continent collision and development of the foreland basin by this time. Suture-zone influence was drastically reduced by the time of deposition of the Dagshai Formation, when the embryonic thrust belt provided a barrier sufficient to partially separate the suture zone from the Himalayan foreland basin. The first appearance of Himalayan metamorphic detritus occurs in the Dagshai Formation at the close of Oligocene time, whereas the Kasauli Formation records erosion to deeper metamorphic levels during earliest Miocene time. The occurrence of garnet and higher grade metamorphic lithic grains during early Neogene time is coincident with the timing of displacement along the Main Central thrust and South Tibetan detachment zone. Composition of early foreland basin sediments from Pakistan (Balakot, Murree and Kamlial Formations) to Nepal (Bhainskati and Dumri Formations) and Bangladesh (Kopili, Barail Formations; Surma Group) indicates diachronous arrival of ophiolitic to low-grade metamorphic detritus derived respectively from the Indus Suture and early Himalayan thrust sheets in the north. This is consistent with progressively later closure of Neotethys along the suture, from latest Paleocene time in the west to Eocene time or even later in the east.

Depositional cycles, composite sea-level changes, cycle stacking patterns, and the hierarchy of stratigraphic forcing: Examples from Alpine Triassic platform carbonates

Article

May 1990

Carbonate platform deposits record a complex interplay of numerous geodynamic variables, of which eustasy, subsidence, and sediment accumulation are prime factors in determining both the kilometer-scale (depositional sequence scale) and meter-scale (depositional cycle scale) stratigraphic packaging. In this study, we looked particularly at composite eustasy, that is, superimposed sea-level fluctuations with different frequencies (defined as orders) and different amplitudes, and the role it plays in the linkage between meter-scale cyclic stratigraphy and kilometer-scale sequence stratigraphy. Specifically, we have investigated the relationship between low-frequency, third-order (1-10 m.y. period) depositional cycles and their component high-frequency, fourth-(0.1-1 m.y.) and fifth-(0.01-0.1 m.y.) order cycles through detailed stratigraphic analyses of Alpine Triassic platforms, complemented by computer modeling. On the basis of our results, we suggest that in general, there exists a hierarchy of stratigraphic forcing driven by composite eustasy that results in organized stacking patterns (thickness, subfacies character, early diagenetic attributes) of high-frequency, typically fourth- and fifth-order, shallow-water carbonate cycles dictated by low-frequency, third-order relative sea-level effects. We suggest that systematic vertical changes in stacking patterns of high-frequency cycles across a larger depositional sequence are due to systematic and predictable differences in depositional space available during the rising and falling stages of a relative third-order sea-level change. We also suggest that these systematic variations in cycle stacking patterns will exist regardless of the mechanism responsible for generating the high-frequency cycles, be it an autocyclic or allocyclic mechanism. This approach has major implications for the use of high-frequency, fourth- and fifth-order cycle characteristics to identify third-order cycles in outcrops and cores of shallow-water carbonates, where stratigraphic control may be less than desirable. This would constitute a valuable bridge between cyclic stratigraphy at the meter scale and sequence stratigraphy at the seismic scale. We present two examples from Triassic buildups of the Alps (the Ladinian Latemar buildup and the Norian Dolomia Principale) where a systematic succession of high-frequency cycle stacking patterns and early diagenetic features exists within an overriding third-order cycle (sequence) reflecting the interplay of short-term, high-frequency (fourth, fifth order) eustasy and long-term, low-frequency (third order) eustasy in accordance with the hierarchy of stratigraphic forcing. Central to the interpretation of these examples is the demonstration that true eustatic rhythms are recorded in the high-frequency cyclicity, as verified by time-series analyses and the use of "Fischer plots." These examples can be modeled by computer under conditions of lag-depth-constrained sedimentation, uniform subsidence, and composite eustasy. We also present two examples, one from the Alpine Triassic (the Norian Lofer cyclothems) and one from the Pleistocene of south Florida, that lack both a systematic succession of high-frequency cycle stacking patterns and identifiable composite rhythms in the stratigraphic record, despite the existence of composite high-frequency eustasy. In these examples, we call upon (1) local tectonic forcing in the form of short-term deviations in subsidence via faulting (Lofer example) and (2) large differences in the relative amplitudes of different orders of high-frequency sea-level oscillations (Pleistocene example) to explain the lack of composite rhythms, and we present computer simulations to illustrate the concepts. An understanding of composite relative sea-level changes and the potential for a hierarchy of stratigraphic forcing provides the link between cyclostratigraphy and sequence stratigraphy and also has important implications regarding the stratigraphy of early diagenesis.

Cyclicity and paleoenvironmental dynamics, Rocknest platform, northwest Canada

Article

Jan 1986

JOHN P. GROTZINGER

Describes 140 to 160 shallowing-upward cycles (1 to 15 m thick) that contain distinctive marker beds and can be correlated for > 200 km parallel to strike and > 120 km perpendicular to strike. Cycles are grouped according to cycle base lithology, which reflects paleogeographic position on the platform.-from Author

Palaeomagnetic polarity reversals in Marinoan (ca. 600 Ma) glacial deposits of Australia: Implications for the duration of low-latitude glaciations in Neoproterozoic time

Article

Aug 1999
GEOL SOC AM BULL, GSA Bulletin

A paleomagnetic investigation of Marinoan glacial and preglacial deposits in Australia was conducted to reevaluate Australia’s paleo- geographic position at the time of glaciation (ca. 610–575 Ma). The paleomagnetic results from the Elatina Formation of the central Flinders Ranges yield the first positive regional- scale fold test (significant at the 99% level), as well as at least three magnetic polarity inter- vals. Stratigraphic discontinuities typical of glacial successions prevent the application of a magnetic polarity stratigraphy to regional cor- relation, but the positive fold test and multiple reversals confirm the previous low paleolati- tude interpretation of these rocks (mean D = 214.9°, I = –14.7°, α95 = 12.7°, paleolatitude = 7.5°). The underlying preglacial Yaltipena For- mation also carries low magnetic inclinations (mean D = 204.0°, I = –16.4°, α95 = 11.0°, paleo- latitude = 8.4°), suggesting that Australia was located at low paleolatitude at the onset of glaciation. The number of magnetic polarity in- tervals present within the Elatina Formation and the Elatina’s lithostratigraphic relation- ship to other Marinoan glacial deposits suggest that glaciation persisted at low latitudes in Aus- tralia for a minimum of several hundreds of thousands to millions of years.

Carbonate platform growth and cyclicity at a terminal Proterozoic passive margin, Infra Krol Formation and Krol Group, Lesser Himalaya, India

Abstract

No full-text available

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