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Permian sedimentary record of the Turpan-Hami basin and adjacent regions, northwest China: Constraints on postamalgamation tectonic evolution
Abstract and Figures
The Permian marks an important, yet poorly understood, tectonic transition in the Tian Shan region of northwestern China between Devonian-Carboniferous continental amalgamation and recurrent Mesozoic-Cenozoic intracontinental orogenic reactivation. The Turpan-Hami basin accommodated up to 3000 m of sediment and is ideally positioned to provide constraints on this transition. New stratigraphic data and mapping indicate that extension dominated Early Permian tectonics in the region, whereas flexural, foreland subsidence controlled Late Permian basin evolution. Lower Permian strata in the northwestern Turpan-Hami basin consist of coarse-grained debris-flow and alluvial-fan deposits interbedded with mafic to intermediate volcanic sills and flows. In contrast, Lower Permian rocks in the north-central and northeastern Turpan-Hami basin unconformably overlie a Late Carboniferous volcanic arc sequence. These Lower Permian strata include possible shallow-marine carbonate rocks and thick volcanic and volcaniclastic rocks that are in turn overlain by littoral- to profundal-lacustrine facies. Above a regional Lower Permian/Upper Permian unconformity, regional sedimentation patterns record the development of a more integrated sedimentary basin. The Upper Permian is entirely nonmarine and can be correlated east-west along the depositional strike of the basin. The lower Upper Permian consists of a broad belt of braided fluvial deposits shed northward. These strata are overlain by fluctuating littoral-and profundal-lacustrine facies and associated fluvial facies. The uppermost Permian is characterized by shallow lake-plain and fluvial environments. The Early Permian association of diffuse volcanism and partitioning of subbasins by normal faulting is consistent with an early phase of lithospheric extension. Local relationships indicate west-east extension in the Turpan-Hami basin along faults oriented normal to Late Devonian-Carboniferous collisional sutures within the Tian Shan. The cause of extension in the wake of Carboniferous orogenesis remains enigmatic. However, the temporal and spatial relationships of the two strain regimes suggest that they are genetically related. Upper Permian stratigraphy and unconformities and local Late Permian-Triassic contractional deformation record foreland-basin development when the Turpan-Hami region became a wedge-top basin with respect to the north Tian Shan fold-and-thrust belt. Flexurally induced Late Permian subsidence is also manifested in the larger Junggar basin to the north, where > 4000 m of strata are preserved in the foredeep region. The Turpan-Hami and Junggar basins were depositionally connected for much of the Late Permian when a vast lacustrine system developed across northwestern China. This lacustrine paleogeography was only occasionally interrupted, possibly by structural damming during uplift of the orogenic wedge.
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... Dur- ing the late Paleozoic, the Chinese Tianshan experienced consistent N-S compression, which is indicated by several lines of evidence. (1) Early Permian shallow-marine carbonate rocks and volcaniclastic rocks in the North Tianshan unconformably overlie a late Carboniferous volcanic arc succession (Wartes et al., 2002). ...
... The Central Tianshan, which is located between the North Tianshan fault and the Main Tianshan shear zone to the north and the Naltai and Kawabulake faults to the south (Fig. 1B), is dominated by a Precambrian basement overlain by Ordovician-Silurian volcano-sedimentary rocks and Carboniferous-Permian sedimentary rocks (Lei et al., 2013Xiao et al., 2013). The North Tianshan mainly consists of Ordovician to Carboniferous volcanic and pyroclastic rocks, turbidites, basalts, cherts, and ultramafic to felsic plutons ( Fig. 1B; Wartes et al., 2002;Xiao et al., 2004;Zhang et al., 2018;Chen et al., 2019). From north to south, the North Tianshan is composed of the Bogda-Harlik arc (Devonian-late Carboniferous), the Turpan-Hami Basin (unknown basement), and the Qoltag arc (also referred to as the Juelotage, which is Ordovician-Carboniferous in age) (Figs. ...
... (3) There is an absence of subduction-related porphyry Cu (-Au) mineralization with ages younger than 310 Ma throughout the Tianshan (Seltmann et al., 2011;Wang et al., 2018;Muhtar et al., 2021). (4) Early Permian shallow-marine carbonate rocks and volcaniclastic rocks occur in the North Tianshan unconformably overlying a late Carboniferous volcanic arc sequence (Wartes et al., 2002). (5) The South Tianshan received at least 2000 m of fluvial and marine sediments derived from the interior of Tarim craton during the Carboniferous to early Permian (Carroll et al., 1995). ...
The Chinese Tianshan experienced large-scale transcurrent tectonics, synkinematic emplacement of ultramafic to felsic intrusions, and the formation of various mineral deposits during late Paleozoic accretionary orogenesis. The relationships among the spatial variation of deformation, the distribution of Permian orogenic Au and magmatic Ni-Cu sulfide deposits, and the kinematic evolution of crustal-scale shear zones, however, remain ambiguous. To address these ambiguities, the spatial variation in the degree of deformation in the Kanggur-Huangshan shear zone in the Chinese Tianshan was characterized using detailed structural measurements and zircon U-Pb and muscovite 40Ar/39Ar age data. The new structural data indicate that a prominent spatial variation exists in the style of deformation throughout the Kanggur-Huangshan shear zone; intense ductile deformation structures are dominant in the east, while brittle structures become progressively more dominant toward to the west. Zircon U-Pb and muscovite 40Ar/39Ar age data for syn- and postkinematic intrusions along the Kanggur-Huangshan shear zone indicate that dextral strike-slip shearing occurred between 279 Ma and 249 Ma. The spatial variation in the degree of deformation and exhumation along the Kanggur-Huangshan shear zone was potentially caused by regional differential uplift induced by the collision of the Tianshan and Beishan regions; this was likely responsible for the predominant occurrence of magmatic Ni-Cu sulfide deposits in the eastern portion of the Kanggur-Huangshan shear zone and orogenic Au deposits in the western portion. The identified spatio-temporal relationship between deformation and distribution of orogenic Au and magmatic Ni-Cu sulfide deposits is crucial to the future success of mineral exploration in the Central Asian orogenic belt.
... The BOB was formed during the Devonian to Carboniferous period as an island arc due to the subduction of the Kelameili ocean (Xiao et al., 2004a). The Carboniferous strata with 4,500 m thickness are exposed in the BOB (Wartes et al., 2002), as shown in Figure 2A. The Lower Carboniferous is primarily comprised of tuffaceous sandstones, marine volcanic tuffs, and volcanic lavas (Wali et al., 2018). ...
... The Middle Permian, comprising mudstones, siltstones, and conglomerates, is considered evidence of a shallow lake plain environment. The thick red conglomerates of the Upper Permian indicate the presence of a terrestrial environment (Wartes et al., 2002). The study area belongs to Hami in the southern Junggar and is the easternmost section of the BOB. ...
The Late Carboniferous volcanic magmatic evolution in the Bogda Orogenic Belt is considerably important for understanding the evolution history of the Eastern Tianshan in the Central Asian Orogenic Belt. Our study focuses on the Upper Carboniferous Liushugou Formation in the Liudaogou and Qidaogou sections of Xishan Township in eastern Bogda. By analyzing the volcanics and sedimentary sequences, we present paleontological evidence, new zircon U–Pb ages, and geochemical data of the volcanics. The lithological composition of volcanics ranges from basic to acidic. The rhyolite has an age of 311.2 ± 1.7 Ma, which, when combined with guide fossils Plerophyllum sp., Zaphrentoides sp., and Zaphrentites sp., indicates its formation in the Late Carboniferous. The geochemical and zircon Lu–Hf isotopic data (εHf(t) = 8.0–11.9) indicate that the basalts originated from a metasomatized subcontinental lithospheric mantle, while andesites and rhyolites were products of crystallization differentiation of the basalts that underwent assimilative mixing. Based on the published data, we propose that the tectonic evolution, transitioning from island arc magmatic systems to post-collisional orogenic belts, commenced in the Bogda Orogenic Belt toward the end of the Late Carboniferous.
... 400 km (Wang et al., 2007). At both the southern and western margin of the Junggar Basin, stratigraphic patterns, bounding faults and magmatism reveal Early Permian extensional rifting (Carroll et al., 1995;Tang et al., 2021b;Wartes et al., 2002;Yang et al., 2013), followed by Middle Permian thermal sag subsidence and peneplanation (Shi et al., 2020;Tang et al., 2021a). Late Permian-Early Triassic Palaeomagnetic results show that West Junggar and Junggar Basin rotated counterclockwise (CCW) (80°-90°) with respect to the western part of the Chingiz Arc in the Late Permian-Early Triassic (Yi et al., 2015), with negligible rotation and latitudinal displacement since that time (Choulet et al., 2011). ...
... 2D seismic profiles reveal that multiple graben and half-graben structures formed in the Early Permian Junggar Basin , and the deposits of the lacustrine transgression sequence were derived from the hanging wall of the basin-bounding normal fault, showing that the Junggar Basin was a typical post-accretionary successor rift basin at that time (Tang et al., , 2021bYang et al., 2013). The regional stratigraphic pattern, bimodal volcanic rocks and regional geodynamic setting all support this interpretation (Carroll et al., 1995;Wartes et al., 2002). The Early Permian syn-rift mechanical subsidence gave way to Middle Permian post-rift thermal sag in the Junggar Basin, and lacustrine successions developed . ...
The Junggar Basin is located on the southwestern margin of the Central Asian Orogenic Belt (CAOB). Whether the Late Permian-Early Triassic tectonic inversion there recorded the final closure of the North Tianshan Ocean or post-accretionary intracontinental deformation remains controversial. Linking the structural style and provenance analysis of the western and northern margins of the Junggar Basin can provide a better understanding of this tectonic event and its geodynamic mechanisms. Seismic reflection profiles show that Early Permian syn-rift half-grabens were followed by the Middle Permian thermal sag, which is characterized by regional onlap and the migration of the depocenter to the center of the basin. Together with the published isopach and paleogeography maps in the western margin of the Junggar Basin, the seismic profiles demonstrate that the reactivation of the Ke-Bai and Wu-Xia dextral transpressive fault zones between the West Junggar terrane and the Mahu sag controlled the tilting and deformation of pre-Permian strata and the distribution of Late Permian-Early Triassic fan deltas. The reported igneous and sedimentological evidence indicates that the southern margin of the Junggar Basin was a rift basin controlled by transtensional strike-slip faults in the Early Permian, and also was followed by a Middle Permian thermal sag. Quantitative provenance analysis using detrital zircon geochronology and the DZmix program shows that the West Junggar terrane and Tianshan orogenic belts experienced varied uplift, indicative of a transition from the Middle Permian thermal sag peneplanation to the Late Permian-Early Triassic tectonic inversion involving reactivation of Early Permian normal faults. This intracontinental deformation event in the Junggar Basin was taken up by block counterclockwise rotation during the final amalgamation of the Pangea, which may be the long-range effect of the final closure of Paleo-Asia Ocean in the eastern part of the CAOB.
... The Chinese Tianshan lies in the southmost part of CAOB and records the final amalgamation of the Kazakhstan and Tuva-Mongol reclines, as well as the collision of the Tarim and North China cratons ( Figure 1; [1,2,4]). The Late Carboniferous to Early Permian was thought to be a crucial period in the evolutionary history of the Chinese Tianshan, marked by the closure of the paleo-Tianshan Ocean, the transition from subduction/accretion to post-collisional extension tectonism and the occurrence of extensive mantle-derived magmatism [4][5][6][7]. However, the nature and timing of this tectonic transition remain open questions. ...
... Late Carboniferous to Early Permian is a crucial tectonic period for the NTB during which various tectonic blocks were finally consolidated following the closure of branches of the Paleo-Asian Ocean (i.e., the North Tianshan Ocean in the south and Kalamaili ocean in the north), thus constituting the tectonic transition from subduction to post-collisional extension (Figure 1b; [4][5][6][7]). Unfortunately, the geological records, i.e., synorogenic deformation, sedimentation, and even magmatism during this tectonic transition, were generally destroyed or obscured by later geological events, making it hard to reveal such tectonic transitions, and the nature and timing of this tectonic transition remain controversial [4,7]. ...
The Late Carboniferous to Early Permian is a critical period of the Chinese Tianshan, witnessing the tectonic transition from subduction to post-collisional extension during the final amalgamation of the Central Asian Orogenic Belt (CAOB). The late Carboniferous Mozbaysay mafic–ultramafic complex in the Qijiaojing–Balikun area, eastern North Tianshan, provides important clues for revealing the nature and timing of this tectonic transition. The Mozbaysay complex comprises mainly hornblende gabbros and lherzolites. LA-ICP-MS U-Pb zircon ages of hornblende gabbro yielded a weighted mean age of 306 ± 1.9 Ma for this complex. These mafic–ultramafic rocks have high contents of MgO (up to 30 wt.%), Cr (up to 2493 ppm), and Ni (up to 1041 ppm), but low contents of SiO2 (40.34–47.70 wt.%). They are enriched in LREE and show characteristics of enriched mid-ocean ridge basalts (E-MORB). The relatively high Th/Yb and Ba/Nb ratios imply the mantle sources could have been metasomatized by slab–mantle interaction with aqueous fluids from dehydration of the subducted slab. Thus, these mafic–ultramafic rocks were most likely produced by partial melting of the asthenospheric and lithospheric mantle with a slight influence of slab-derived fluids. Therefore, we suggest that the formation of these Late Carboniferous mafic–ultramafic rocks was triggered by the decompression-induced influx of asthenospheric heat and melting through a slab window during post-collisional slab breakoff. Combined with geological data, the petrogenetic links of the Late Carboniferous mafic–ultramafic rocks in eastern North Tianshan to slab breakoff suggest that the tectonic transition from convergence to post-collision most likely initiated in situ at ca. 306 Ma and lasted to ca. 300 Ma.
... It is 660 km long from east to west and 130 km wide from north to south, with a total covered area of 5.35 × 104 km 2 . The Turpan-Hami Basin has undergone four stages: the extensional rift basin development stage; the compressional foreland basin development stage; the extensional faulted basin development stage; and the compressional regenerated foreland basin development stage, which finally formed the current pattern of the Mesozoic-Cenozoic superimposed composite inland basin (Zhu et al., 2009;Jiang et al., 2015;Wartes et al., 2002;Greene et al., 2005). According to the tectonic evolution characteristics of the Turpan-Hami Basin, the Turpan-Hami Basin can be divided into three primary tectonic units from east to west: the Hami Depression, the Liaodun Uplift, and the Turpan Depression (Miao et al., 2021;Fig. ...
Organic matter types in the Taodonggou Group mudstone exhibit significant differences with depth. In order to understand the formation mechanism of this special phenomenon, we analyzed the mineralogy and geochemistry of the mudstone, as well as the source rocks, depositional environment, and depositional processes of the Taodonggou Group. Based on this, we have gained the following insights. (1) The Taodonggou Group mudstone was deposited in an intermediate-depth or deep, dysoxic, freshwater–brackish lake environment under warm and humid paleoclimatic conditions. The input of terrestrial debris was stable, but the sedimentation rate was slow. In addition, the sedimentation in the middle stage was influenced by hydrothermal activities, and the changes in the depositional environment corresponded to variations in organic matter types. (2) The source rocks of the Taodonggou Group mudstone are mainly andesitic and feldspathic volcanic rocks. Sediment sorting and recycling were weak, and hydrocarbon source information was well preserved. The tectonic background of the source area was a continental island arc and an oceanic island arc. Furthermore, changes in the provenance of the Taodonggou Group also had a significant impact on the variations in organic matter types. (3) The sedimentation of the Taodonggou Group involved both traction and gravity flows. The variations in source area, depositional environment, and depositional processes during different depositional periods led to changes in the organic matter types of the Taodonggou mudstone. (4) Based on the depositional environment, provenance, and depositional processes, the sedimentation of the Taodonggou Group can be divided into three stages. In the early stages, the sedimentation center was in the Bogda area. At this time, the Bogda Mountain region was not exposed, and the depositional processes inherited the characteristics of early Permian gravity flow sedimentation, resulting in the widespread deposition of a series of high-quality Type III source rocks in the basin. In the middle stage of the Taodonggou Group sedimentation, the sedimentation center gradually migrated to the Taibei Sag. During this period, the Bogda Mountain region experienced uplift and hydrothermal activity, and the depositional processes gradually transitioned to traction flows, resulting in the widespread deposition of a series of Type II source rocks in the basin. In the late stage of the Taodonggou Group, the uplift of the Bogda Mountain region ceased, and the sedimentation center completely shifted to the Taibei Sag. Meanwhile, under the influence of gravity flows, the organic matter types of the Taodonggou mudstone changed to Type III.
... The Turpan-Hami Basin is one of the 10 super-large coal-accumulating basins in the world, with coal resources of more than 5000 × 10 8 t [13]. In recent years, the predecessors have studied the Jurassic coalbearing stratum in the exploration of the Turpan-Hami Basin and gained a great deal of understanding of the ancient structure, paleogeography, basin evolution, stratigraphic sequence, and coal accumulation evolution [13][14][15][16][17][18][19][20][21][22][23]. ...
The Turpan-Hami Basin is one of the three coal-accumulating basins in Xinjiang. There is coal, natural gas, petroleum, sandstone-type uranium ore, and other ore resources in the Jurassic strata developed inside. This study aims to gain a deeper understanding of the formation process of ore resources in the Turpan-Hami Basin by studying the provenance and depositional environment of No. 4 coal in the Sandaoling Mine. The results show that No. 4 coal is extra-low ash yield and extra-low sulfur coal. Compared with common Chinese coals and world hard coals, the trace element content in No. 4 coal is normal or depleted. The minerals in coal are mainly clay minerals, silica and sulfate minerals, and carbonates. The diagrams of Al2O3, TiO2, Sr/Y, L,a/Yb, and the REY geochemical features indicate that the Paleozoic intermediates and felsitic igneous rocks in Harlik Mountain and Eastern Bogda Mountain are the main provenance of No. 4 coal. The syngenetic siderite, Sr/Ba, Th/U, total sulfur content, and maceral indices indicate that No. 4 coal was formed in a salt-lake environment, and the climate changed from dry and hot to warm and humid.
... The Fengcheng Formation, deposited between the Late Carboniferous C4 (late Bashkirian-Moscovian) and Early Permian P1 (Asselian-early Sakmarian) glacial intervals during the Late Paleozoic Ice Age (LPIA), was regarded as the oldest alkaline lacustrine OM-rich shale in the region of the Junggar Basin (Fielding et al., 2008a(Fielding et al., , 2008bCao et al., 2020;Tang et al., 2021;Wang et al., 2022). Following the Fengcheng Formation, the world's richest and thickest alkaline lacustrine OM-rich interval (i.e., the Lucaogou Formation) was deposited in the Middle Permian Junggar Basin which was one of the largest known Phanerozoic lake basins and developed the most laterally extensive paleolake (i.e., a maximum area of approximately 300,000 km 2 ) (Demaison and Huizinga, 1991;Carroll, 1998;Wartes et al., 2002;Carroll and Wartes, 2003;Luo et al., 2018;Huang et al., 2020). The deposition of the Lucaogou Formation covered a period with an estimate of 3 Ma (Carroll and Wartes, 2003;Huang et al., 2020). ...
Organic matter (OM) accumulation in terrestrial sediments shows not only a significant carbon sink in the Earth's carbon cycle but an important origin of fossil fuels, which is closely associated with the complex and diverse depositional environments and climate conditions. The Junggar Basin developed the most laterally extensive and thickest alkaline lacustrine sediments as well as the richest hydrocarbon source rock/oil shale interval of all the world during the deposition of the Lucaogou Formation and equivalent units. In this study, the sedimentary succession of 564 m with a mudstone and dolomite matrix of the Lucaogou oil shale from the Qi 1 well located in the southern Junggar Basin was investigated, which can be divided into two members based on lithology. This paper is a synthetic use of major element oxides, trace, and rare-earth elements as well as organic petrography data as proxies to evaluate the provenance, paleotectonic setting, paleoclimate and weathering conditions, paleoenvironment, and paleoproductivity as well as origin and accumulation of OM of the Lucaogou sediments.
It is suggested that no recycling sediments in all the studied samples, based on parameters including the Th/Sc vs. Zr/Sc plot, the Chemical Index of Alteration (CIA) being linearly dependent on Weathering Index of Parker (WIP), and the high Index of Compositional Variability. The provenance mainly from intermediate-felsic volcanic rocks (e.g., granodiorite, andesite and dacite) is supported by the bivariate plots of Euanom vs. Th/Sc, La/Th vs. Hf, La/Sc vs. Co/Th, Cr/Th vs. Sc/Th, Y/Ni vs. Cr/V, and Nb/Y vs. Zr/TiO2 as well as triangular diagrams of mafic–felsic–weathering, M+–4Si–R2+ and Rb/V–Zr/Zn–Sc/Nb. The continental island arc is inferred from paleotectonic setting discrimination diagrams (i.e., the La–Th–Sc, Th–Co–Zr/10 and Th–Sc–Zr/10 ternary as well as Ti/Zr vs. La/Sc binary diagrams). The chemical weathering indices (i.e., the CIA, WIP, weathering index (W), Weathering Intensity Scale, Sodium Depletion Index, and Ga/Rb vs. K2O/Al2O3 binary diagram) indicate the paleoclimate conditions were cold/arid with weak chemical weathering in a warming and/or enhanced continental weathering episode. Besides, the Sr/Ba ratios and gallium concentrations indicate the paleosalinity was brackish to hypersaline and was higher in the lower member in relation to that in the upper member. However, the sub-oxic environment of the benthic water (inferred from the MoEF vs. UEF co-variations and Corg/P ratios as well as large sizes of framboidal pyrites) and the moderate paleoproductivity (evaluated via the Sibio, Babio, and Pbio values, as well as Ni/Al and Cu/Al ratios) caused by predominate algae and bacteria (inferred from the organic petrography) kept stable in the paleolake during this period. The OM accumulation was mainly controlled by the preservation conditions rather than the paleoproductivity and limited detrital inputs/low deposition rates, which was indicated by the plot of Co × Mn values vs. Cd/Mo ratios and the total organic carbon contents being negatively related to stable carbon isotope compositions of OM.
... The timing of exhumation of the Bogda Shan and the associated implications for the relationship (i.e. former connection) between the Junggar and Turpan-Hami Basins throughout the late Palaeozoic and Mesozoic remain highly debated Wartes et al. 2002;Greene et al. 2005;Tang et al. 2014;Ji et al. 2018;Wang et al. 2018a). Since late Palaeozoic-early Mesozoic thermochronological signals embedded in the basement rocks have been largely eroded by later tectonic events, current interpretations of the exhumation history of Bogda Shan rely heavily on sedimentological evidence (e.g. ...
Bogda Shan is a mountain belt located at the eastern extremity of the Chinese Tianshan and records a complex and debated exhumation history. Previous studies have reported a young Cenozoic thermal history for the exhumation of Bogda Shan, which is in conflict with the observation of preserved Mesozoic erosion surfaces in the area. This study re-evaluates the Meso-Cenozoic thermo-tectonic evolution of Bogda Shan using apatite fission track (AFT) thermochronology. Palaeozoic basement (meta-sandstone) samples collected from the northern and southwestern flanks of the mountain ranges reveal apparent Mesozoic AFT ages ranging from ~202 Ma to ~97 Ma. Inverse thermal history modelling results reveal slow to moderate basement cooling during the early Mesozoic, corresponding to relatively low levels of exhumation. This accounts for the preservation of low-relief Mesozoic peneplanation surfaces recognized at elevations of ~3500–4000 m. None of the presented AFT data and thermal history models show any evidence for significant deep Cenozoic exhumation. In the neighbouring Junggar Basin, a Middle Jurassic sandstone sample records partial resetting of the AFT system during the Cretaceous. This observation conflicts with previous data (from the same Jurassic strata) where complete resetting of the AFT clock during the Cenozoic was suggested. Furthermore, Lower Cretaceous and Palaeogene sediments from the Turpan-Hami Basin show non-reset detrital AFT age populations of ~197, ~135, and ~104 Ma, which are coincident with the main pulses of exhumation recorded in the Chinese North Tianshan. Based on a comprehensive summary of the published data, we argue for a Mesozoic building of the Bogda–Balikun–Harlik mountain chain in the eastern Chinese Tianshan. Subsequent Cenozoic exhumation must have been relatively modest at most (<2 km) as it was not recorded by AFT thermochronology.
The linkages between nitrogen cycling, nitrogen isotopes, and environmental properties are fundamental for reconstructing nitrogen biogeochemistry. While the impact of ocean redox changes on nitrogen isotopes is relatively well understood, it is poorly known how nitrogen responds to changes in pH and salinity. To fill the knowledge gap, we explore the effects of these environmental parameters using a well-controlled set of samples from Carboniferous–Paleogene lake sediments in China. Our results show that the threshold of 10–12‰ in δ15N works to distinguish alkaline (pH > 9) from circum-neutral conditions. Elevated Mo levels in the alkaline samples support the idea of NH3 volatilization from a reducing water column in an alkaline setting. For non-alkaline lakes, δ15N values tend to be higher (up to +10‰) in more saline, anoxic settings, which is attributed to either the expansion of stagnant anoxic waters spurring water-column denitrification or a shift from plant-based toward more microbially dominated ecosystems or both. Our results imply that salinity-induced redox stratification and basicity can alter nitrogen biogeochemical cycling beyond what is shown by the marine nitrogen isotope record alone. This finding will result in an improved understanding of the dynamic controls of δ15N in sediments and lead to better biogeochemical interpretations of paleo-environmental conditions from unknown environmental settings on Earth and beyond Earth.
ALLUVIAL FANS AND THEIR NATURAL DISTINCTION FROM RIVERS
BASED ON MORPHOLOGY, HYDRAULIC PROCESSES, SEDIMENTARY PROCESSES,
AND FACIES ASSEMBLAGES
Terence C. Blair and John G. McPherson
Contrary to common contemporary usage, alluvial fans are a naturally unique phenomenon readily distinguishable from other sedimentary environments, including gravel-bed rivers, on the basis of morphology, hydraulic processes, sedimentologic processes, and facies assemblages. The piedmont setting of alluvial fans, where the feeder channel of an upland drainage basin intersects the mountain front assures that catastrophic fluid gravity flows and sediment gravity flows, including sheetfloods, rock falls, rock slides, rock avalanches, and debris flows, are major constructional processes, regardless of climate. The unconfinement of these flows at the mountain front gives rise to the high-sloping, semiconical form that typifies fans. The plano-convex cross-profile geometry inherent in this form is the inverse of the troughlike cross-sectional form of river systems, and precludes the development of floodplains that characterize rivers. The relatively high slope of alluvial fans creates unique hydraulic conditions where passing fluid gravity flows attain high capacity, high competency, and upper flow regime, resulting in sheetfloods that deposit low-angle antidune or surface-parallel planar-stratified sequences. These waterlaid facies contrast with the typically lower-flow-regime thick-bedded, cross-bedded, and lenticular channel fades, and associated floodplain sequences, of rivers. The unconfinement of flows on fans causes a swift decrease in velocity, competency, and capacity as they attenuate, inducing rapid deposition that leads to the angular, poorly sorted textures and short radii typical of fans. This condition is markedly different than for rivers, where sediment gravity flows are rare and water flows remain confined by channel walls or spill into floodplains, and increase in depth downstream. The distinctive processes that construct alluvial fans, coupled with the secondary surficial reworking of their deposits, yield unique facies assemblages that permit the easy differentiation of fan sequences even where the geomorphic context has been lost, including in the rock record. The fault-proximal piedmont setting critical for their preservation makes properly identified alluvial-fan deposits in the rock record an invaluable tool for reconstructing and interpreting the tectonic and stratigraphic evolution of ancient sedimentary basins and their contained register of Earth history.
Journal of Sedimentary Geology, v. A64, No. 3, July 1994, p. 450–489.
This book provides comprehensive introductory text for the wide-ranging field of petrology. The section on igneous rocks is divided into 10 chapters covering: introduction to igneous environments; igneous minerals and textures; chemistry and classification of igneous rocks; crystallization of magmas; origin of magmas by melting on the mantle and crust; evolution of magmas - fractional crystallization and contamination; petrology of the mantle; igneous rocks of the oceanic lithosphere; igneous rocks of convergent margins; igneous rocks of continental lithosphere. The section on sedimentary rocks is comprised of 7 chapters covering: the occurrence of sedimentary rocks; weathering and soils; conglomerates and sandstones; diagenesis of sandstones; mudrocks; limestones and dolostones; other types of sedimentary rocks. The final section, on metamorphic rocks, contain 7 chapters covering; metamorphism and metamorphic rocks; isograds, metamorphic facies and P/T evolution; assemblages, reactions, and equilibrium; controls on metamorphic reactions; metamorphism of mafic and ultramafic rocks; metamorphism of aluminous clastic rocks; and metamorphism of calcareous rocks. Appendices cover CIPW norm calculations and methods of P/T determination.
The Junggar, Tarim, and Qaidam Basins are commonly considered to be cratonic blocks surrounded by orogenic belts and have thus been called intermontane basins. I propose that those basins be compared to the Black Sea and Caspian Basins, and suggest that Junggar was formed during the Carboniferous Period, and Tarim and Qaidam during the Permian Period, as back-arc basins behind volcanic arcs on the southern active margin of Paleozoic Asia. The relatively thin crust and the presence of very large positive magnetic anomalies under the Mesozoic and Cenozoic sediments of those basins indicate that their deepest depressions are floored, at least in part, by oceanic rocks. The oldest sediments in those basins are very likely marine shales. After arc-continent collisions during late Paleozoic and Triassic time, the basins became partially enclosed, and the euxinic sediments in those partially restricted basins could well be the source beds of the crude oils found recently in major oil fields of those basins. Junggar, Tarim, and Qaidam became inland basins with continental sedimentation after their communications to open sea were severed by the rising mountain chains. Isostatic basin subsidence permitted the accumulation of thick Mesozoic and Paleogene sediments before tectonic rejuvenation along Neogene faults.
The Junggar Basin is one of the largest and most important oil-producing basins in China, in which Upper Permian lacustrine oil shales are among the thickest and richest source rocks in the world. The Upper Permian Pingdiquan Formation was deposited predominantly in fan-delta sequences within a lacustrine setting. In the northeastern Junggar Basin, the Pingdiquan Formation sandstones (volcanic and feldspathic litharenites) constitute the principal oil reservoirs, whereas the interbedded black shales are the predominant oil source rocks. The early diagenetic minerals in the sandstones include siderite, pyrite, analcime, albite, calcite, and trace amounts of halite. Late diagenetic minerals include K-feldspar, ankerite, and minor mixed-layer clay minerals. A similar diagenetic sequence was recognized in the intercalated mudrocks, which contain abundant analcime, albite, and microcrystalline dolomite. Early authigenic mineral formation (e.g., calcite versus analcime/albite) was controlled by alternating periods of lower and higher salinity in the lacustrine environment. Cementation by siderite, analcime, calcite, and albite substantially occluded sandstone porosity early in diagenesis. However, extensive dissolution of analcime cement and detrital feldspars during burial produced significant secondary porosity in the sandstones, improving their reservoir potential. Organic acids generated during oil-shale maturation may be responsible for secondary porosity in the interbedded sandstones.