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A classification of rhizogenic (root-formed) calcretes, with examples from the Upper Jurassic-Lower Cretaceous of Spain and Upper Cretaceous of Southern France
Abstract and Figures
Studies of fluvial sequences from the Upper Jurassic and Lower Cretaceous of the Cameros Basin (Spain) and the Upper Cretaceous of the Aquitaine Basin (France) have revealed large numbers of calcrete profiles which have formed wholly or largely through the action of plant root calcification. Those from Spain have formed by the calcification of root mats, whereas those from France resulted from the accumulation of calcified root cells (Microcodium). These root-related (rhizogenic) calcretes developed in stages allowing developmental time-series (chronosequences) to be identified, contrasting with existing models for calcrete profile growth. They provide unique examples where vegetation has been the dominant control on profile development and probably indicate specific vegetation types and palaeoecological settings. The role of root activity in forming some Quaternary calcretes is also discussed.
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... The biogenic micromorphological features formed close to the roots in the active zones in the rhizosphere, where carbonate precipitation is the dominant process (Wright et al., 1988;Alonso-Zarza and Jones, 2007). Septal alveolar textures are common in modern carbonate soils (Khalaf and Al-Zamel, 2016;Zamanian et al., 2016;Alonso-Zarza, 2018) and are widely documented in paleosols (Wright et al., 1995;Alonso-Zarza et al., 1998;Nascimento et al., 2019;Valera-Fern andez et al., 2020). Microcodium is commonly associated with intracellular root calcification (Klappa, 1980;Wright et al., 1988Wright et al., , 1995Alonso-Zarza et al., 1998;Košir, 2004), and the spherulites result from the activity of cyanobacteria and bacteria activity in the root systems (Dupraz et al., 2009). ...
... Septal alveolar textures are common in modern carbonate soils (Khalaf and Al-Zamel, 2016;Zamanian et al., 2016;Alonso-Zarza, 2018) and are widely documented in paleosols (Wright et al., 1995;Alonso-Zarza et al., 1998;Nascimento et al., 2019;Valera-Fern andez et al., 2020). Microcodium is commonly associated with intracellular root calcification (Klappa, 1980;Wright et al., 1988Wright et al., , 1995Alonso-Zarza et al., 1998;Košir, 2004), and the spherulites result from the activity of cyanobacteria and bacteria activity in the root systems (Dupraz et al., 2009). ...
The present work analyzes the monotypic Taenidium barretti ichnofabric developed in abandoned channels and floodplains to understand its paleoecological significance in highly seasonal fluvial systems. The data come from the Cretaceous paleosols of the Marília Formation (Bauru Basin, SE Brazil), in which the T. barretti ichnofabric and rhizoliths represent the biotic record. Beetle larvae are considered to be the most likely T. barretti tracemakers in these paleosols. The intensity and recurrence of bioturbation suggest that the windows of opportunity for beetle population growth were very short and controlled by the progressive substrate desiccation after flood cessation. The characteristics of the trace fossil assemblage suggest soil colonization by r-strategic organisms triggered by flooding events that provided the resource inputs necessary for their living during rainy seasons. The results also show that in fluvial systems with high discharge variations, the boundaries between a "pre-desiccation suite" and a "desiccation suite" in the Scoyenia ichnofacies can be diffuse due to the short duration of colonization windows and progressive terrestrialization of floodplains and channels. Therefore, the frequency of flooding events, the lowered water table, and the rapidity of substrate desiccation constrained colonization by other burrowers and may be considered as first-order factors controlling the generation of a monotypic T. barretti ichnofabric. Thus, the monotypic Taenidium ichnofabric in the Scoyenia ichnofacies context is an ichnomarker of brief windows for colonization in highly seasonal environments.
... This imbalance has been cited as the reason for the halt and slowdown in travertine formation in the Aggay River catchment, which is a sub-catchment of the Middle Sebou (Gourari, 2001). The predominance of coarse detrital facies and the presence of various pedogenic carbonate morphotypes (nodular calcrete and root calcrete) in paleosols and weakly pedogenized fine deposition more particularly on low terraces, as well as the formation of laminar carbonates indicate arid to semi-arid climatic conditions during their formation (Wright et al., 1995;Alonso-Zarza and Wright, 2010a,b;Arenas-Abad et al., 2010;Srivastava et al., 2021;El Asmi et al., 2023a, 2023b. The presence of localized oncholitic concretions testifies to a certain climatic humidity that could be linked to springs (Arenas-Abad et al., 2010). ...
... However, they are mostly found as isolated grains in micrite matrix. Despite its controversial origin (Freytet and Plaziat, 1982;Wright et al., 1995;Freytet et al., 1997;Kabanov et al., 2008), ancient Microcodium shares many attributes with modern intracellularly calcified roots (Klappa, 1978;Jaillard et al., 1991;Košir, 2004). In-situ Microcodium structures normally exhibit corrosive contact with the substrate whereas disintegrated aggregates are often reworked as individual polyhedral elements and incorporated in calcretes and palustrine facies and also resedimented in marine sediments (Martín-Chivelet and Giménez, 1992;Košir, 2004). ...
The sedimentary and stratigraphic features of the low-gradient slope carbonate palustrine system of the Fortanete Formation (northeast Spain) during the early Campanian have been studied. The succession of the Fortanete Fm provides depositional record of a progressive transition to continental settings in an area that was predominantly marine for most of the Late Cretaceous. Palustrine systems can be developed in a wide variety of environments, however, modern analogues are scarce. The Fortanete Fm thus presents a great opportunity to study the stratigraphy, the sedimentary features and isotopic composition of a palustrine‐–marine influenced system. A total of 10 stratigraphic sections distributed along a 40 km wide and 80 km long area have been carried out in order to decipher the facies distribution in the system. This study led to the differentiation of three palaeoenvironments based on predominant lithofacies and sedimentary structures: (1) marshes and shallow low-gradient slope lakes; (2) ephemeral lakes, lake margins and prairies with variable water level and marine influence; and (3) mudflats with evaporites and fine siliciclastic input. The Fortanete Fm serves as an excellent case study for palustrine environments with the presence of typical palustrine features such as pseudomicrokarst, brecciation, black pebbles, desiccation cracks, nodules, Microcodium, and fenestral fabric. Most palustrine systems are associated with continental freshwater settings, whereas the Fortanete Fm displays evidence of intermittent marine influence in the distal studied sections. Carbon and oxygen isotope composition is within the range of typical freshwater lake carbonates and also corresponds to isotope composition of palustrine carbonates with marine influence. The Fortanete Fm shares a number of characteristics with freshwater continental and freshwater modern analogues. Comparison with these systems provides new insights on how modern analogues can be used to decipher the complexity of the processes developing these systems in the past.
... o Association 3 -Sols carbonatés : les argiles de l'association 4 sont parfois recoupées par des concrétions calcaires alignées en horizons, indicatrices de sols carbonatés ( Fig.7E; Wright et al., 1995;Rettalack, 2001). ...
Faced with actual climatic issues, Geosciences have to better understand the intense and brutal greenhouse periods, known as hyperthermals, that have unfolded in the past. Middle-Eocene Climatic Optimum (or MECO) was one of them and occurred between 40.0 and 40.5 My (Bartonian). MECO have been essentially studied in deep-marine environment, in which most of hyperthermals are defined, but few studies have been conducted on their expression on continental systems. In this paper we aim to identify and quantify effects of MECO within Escanilla Formation fluvial system in Ainsa piggy-back basin (South-Pyrenean Zone, Spain). Paleo- climate and sedimentary system evolutions were studied through (1) an alluvial facies characterization, (2) isotopic analysis of organic and inorganic δ13C, (3) minerals and clays spectra, (4) chemical index of alteration and (5) an analysis of the channel paleohydraulic parameters.
We demonstrate that during an interval contemporaneous with the end of the MECO – isotopically identified by δ13Corganic signal – the Escanilla fluvial system recorded a doubling of water and sediment flux, related to the transition from meandering to braided rivers system. During an overall semi-arid period identified on the full section, flux’s increase interval is correlated with a wetter episode, marked by a rising of detrital clays fraction. If the observed flux variations really constitute a response to the MECO hyperthermal, then a precipitation increase on the nascent Pyrenean chain during the thermal maximum of the MECO, towards ~ 40.1 My, could be the reason.
... La présence de Microcodium en place et non remanié sous forme de prismes isolés est un indice de diagenèse en milieu subaérien (vadose) ainsi que la preuve de l'existence d'une surface d'exposition subaérienne continentale au dessus des faciès affectés (Klappa, 1978;Plaziat, 1984). -En plus des paléosols, les Microcodium ont été décrit dans d'autres milieux continentaux comme les calcrètes, les milieux fluviatiles et palustres (Esteban, 1982), les plaines d'inondations et rarement dans les paléokarsts (Esteban, 1982 ;Freytet et Plaziat, 1982 ;Wright et Tucker, 1991;Wright et al., 1995 ;Alonso-Zarza, 1999, 2003Alonso-Zarza et al., 1998…). Alonso-Zarza et al., 1998). ...
Les Microcodium sont des structures énigmatiques de nature encore incertaine désignant des cristallisations de calcite limpide, en prismes ou pyramides allongés, groupés, soit en « épis de maïs », soit en lames encroûtantes plus étendues. Les prismes de calcite montrent des microstructures internes qui consistent en de très fins filaments (diamètre de l'ordre du micron), plus fins que ceux des Cyanophycées et qui pourraient évoquer des Bactéries filamenteuses. Ils caractérisent les formations carbonatées continentales et même marines pour certains auteurs. On ne connaît pas d'exemples certains de Microcodium antérieurs à la fin du Crétacé ; ils caractérisent notamment le Paléogène du pourtour méditerranéen. Dans le bassin d'Anzal, une nouvelle forme énigmatique de Microcodium, appelée «calcite fibreuse pseudosphérulitique» (PFC), a été découverte dans des paléosols hypercalcimorphes. Cependant, alors que les Microcodium affectent essentiellement des substrats calcaires, les PFC d'Anzal n'affectent que des substrats dolomitiques. Ces microstructures se présentent sous forme de niveaux massifs décimétriques associés à des structures organo-sédimentaires racinaires. Il s'agit d'une mosaïque particulière en forme de « bouquets » de cristaux subsphériques de calcite (entre 30 μ et 4 mm) montrant chacun une microfabrique fibreuse radiale intracristalline. Cette microfabrique est formée par des inclusions fibreuses sombres et anastomosées qui émergent à partir du centre de chaque cristal. L'étude des gisements de PFC a permis de mettre en évidence trois conditions nécessaires pour leur formation et leur développement : la présence d'un substrat dolomicritique ; l'existence d'un paléosol (rhizosphère) hypercalcimorphe et l'influence d'une nappe phréatique. La répartition géographique et stratigraphique de ces structures doit refléter un genre spécifique de plantes vasculaires méconnues et appartenant à des communautés végétales pionnières, capables de coloniser des substrats carbonatés durant les premières périodes d'exposition subaérienne. Toutefois, une origine néomorphique des PFC à partir d'un précurseur sulfaté ne peut pas être écartée. Mots Clés : Microcodium; Anti-Atlas ; calcite fibreuse pseudosphérulitique; dolomie lacustre ; paléosol hypercalcimorphe ; cathodoluminescence. Abstract-Microcodium is a problematic calcitic microfeature of many calcretes and calcareous paleosols, especially in early Paleogene continental successions of the peri-Tethyan realm. Massive, decimetric-thick intervals formed by mosaics of pseudospherulitic fibrous calcite (PFC) crystals displaying an intracrystalline fibrous microfabric have been found associated with plant roots and paleosols (rhizoliths) in the Paleogene lacustrine deposits of Anzal basin. The fibrous microfabric is marked by inclusion-defined fibers that radiate from the central parts of the crystals, wich can be termed pseudospherulites, because they are single crystals and not polycristalline aggregates. These "spherulites" present a strong resemblance with Microcodium colonies in morphology, internal structure, and environment of formation. Microcodium is almost invariably associated with calcic paleosols. In contrast, the PFC described in this paper are developed within dolomicritic host sediments, in part by pedogenic alteration and in part by palaeo-groundwater precipitation. Cathodoluminescence microscopy, morphology and structure of the PFC indicate that, first, these aggregates grew with plant roots in paleosols, and second, at or near the water table, under periodically fluctuating geochemical conditions (calcite saturation and redox) caused by fluctuations of the water-table during periods of subaerial exposure. The analogy between PFC and Microcodium could be explained interpreting PFC as calcified root cells of other vascular plant group, which were able to colonize dolomitic substrates during early phases of subaerial exposure. However, the presence of gypsum inclusions within the PFC crystals, makes the hypothesis of a gypsum precursor as a possible solution for the PFC origin.
... La présence de Microcodium en place et non remanié sous forme de prismes isolés est un indice de diagenèse en milieu subaérien (vadose) ainsi que la preuve de l'existence d'une surface d'exposition subaérienne continentale au dessus des faciès affectés (Klappa, 1978;Plaziat, 1984). -En plus des paléosols, les Microcodium ont été décrit dans d'autres milieux continentaux comme les calcrètes, les milieux fluviatiles et palustres (Esteban, 1982), les plaines d'inondations et rarement dans les paléokarsts (Esteban, 1982 ;Freytet et Plaziat, 1982 ;Wright et Tucker, 1991;Wright et al., 1995 ;Alonso-Zarza, 1999, 2003Alonso-Zarza et al., 1998…). Alonso-Zarza et al., 1998). ...
Les Microcodium sont des structures énigmatiques de nature encore incertaine désignant des cristallisations de calcite limpide, en prismes ou pyramides allongés, groupés, soit en « épis de maïs », soit en lames encroûtantes plus étendues. Les prismes de calcite montrent des microstructures internes qui consistent en de très fins filaments (diamètre de l'ordre du micron), plus fins que ceux des Cyanophycées et qui pourraient évoquer des Bactéries filamenteuses. Ils caractérisent les formations carbonatées continentales et même marines pour certains auteurs. On ne connaît pas d'exemples certains de Microcodium antérieurs à la fin du Crétacé ; ils caractérisent notamment le Paléogène du pourtour méditerranéen. Dans le bassin d'Anzal, une nouvelle forme énigmatique de Microcodium, appelée «calcite fibreuse pseudosphérulitique» (PFC), a été découverte dans des paléosols hypercalcimorphes. Cependant, alors que les Microcodium affectent essentiellement des substrats calcaires, les PFC d'Anzal n'affectent que des substrats dolomitiques. Ces microstructures se présentent sous forme de niveaux massifs décimétriques associés à des structures organo-sédimentaires racinaires. Il s'agit d'une mosaïque particulière en forme de « bouquets » de cristaux subsphériques de calcite (entre 30 μ et 4 mm) montrant chacun une microfabrique fibreuse radiale intracristalline. Cette microfabrique est formée par des inclusions fibreuses sombres et anastomosées qui émergent à partir du centre de chaque cristal. L'étude des gisements de PFC a permis de mettre en évidence trois conditions nécessaires pour leur formation et leur développement : la présence d'un substrat dolomicritique ; l'existence d'un paléosol (rhizosphère) hypercalcimorphe et l'influence d'une nappe phréatique. La répartition géographique et stratigraphique de ces structures doit refléter un genre spécifique de plantes vasculaires méconnues et appartenant à des communautés végétales pionnières, capables de coloniser des substrats carbonatés durant les premières périodes d'exposition subaérienne. Toutefois, une origine néomorphique des PFC à partir d'un précurseur sulfaté ne peut pas être écartée. Mots Clés : Microcodium; Anti-Atlas ; calcite fibreuse pseudosphérulitique; dolomie lacustre ; paléosol hypercalcimorphe ; cathodoluminescence. Abstract-Microcodium is a problematic calcitic microfeature of many calcretes and calcareous paleosols, especially in early Paleogene continental successions of the peri-Tethyan realm. Massive, decimetric-thick intervals formed by mosaics of pseudospherulitic fibrous calcite (PFC) crystals displaying an intracrystalline fibrous microfabric have been found associated with plant roots and paleosols (rhizoliths) in the Paleogene lacustrine deposits of Anzal basin. The fibrous microfabric is marked by inclusion-defined fibers that radiate from the central parts of the crystals, wich can be termed pseudospherulites, because they are single crystals and not polycristalline aggregates. These "spherulites" present a strong resemblance with Microcodium colonies in morphology, internal structure, and environment of formation. Microcodium is almost invariably associated with calcic paleosols. In contrast, the PFC described in this paper are developed within dolomicritic host sediments, in part by pedogenic alteration and in part by palaeo-groundwater precipitation. Cathodoluminescence microscopy, morphology and structure of the PFC indicate that, first, these aggregates grew with plant roots in paleosols, and second, at or near the water table, under periodically fluctuating geochemical conditions (calcite saturation and redox) caused by fluctuations of the water-table during periods of subaerial exposure. The analogy between PFC and Microcodium could be explained interpreting PFC as calcified root cells of other vascular plant group, which were able to colonize dolomitic substrates during early phases of subaerial exposure. However, the presence of gypsum inclusions within the PFC crystals, makes the hypothesis of a gypsum precursor as a possible solution for the PFC origin.
... Consequently, in the present study, it is possible to differentiate calcretes and calcarenites according to the following attributes: a. Macro and micromorphological features. Some of the features that identify the presence of calcretes include the presence of root systems (Jones, 1992;Wright et al., 1995;Alonso-Zarza and Jones, 2007;Alonso-Zarza, 2018) as well as laminations with typical micromorphology related to the microbial activity, such as cyanobacteria, bacteria, fungi and lichens (Verrecchia et al., 1995, Freytet et al., 1997Alonso-Zarza and Wright, 2010). In the sections that were studied, the three calcretes show laminations that are observable at the macro-scale in the C3.1 and C1.A, and C1.C regions of Calcretes 3 and 1, respectively ( Fig. 3d and 4b, c). ...
Striking examples of calcretes have been formed in alluvial and aeolian sediments of the Qazvin Plain in Iran. In the present research, eight profiles were studied, varying from recent proximal alluvial fans to adjacent alluvial plains and sand dunes. Macroscopic, microscopic, isotopic, and elemental composition studies show that both pedogenic and non-pedogenic processes have been responsible for the formation of these horizons. The remarkable features of the calcretes in this study are the prevalence of spores, nanofibers, and pisoids. This research provides clues for organisms involved, and the interface of soil with the atmosphere or the rhizosphere in the calcretization process. Nanofibers only occur around root traces in the pisoides and gypcretes. The shape of nanofibers reflects the contribution of roots and their associated microorganisms in the formation of calcrete and gypcrete. According to the results of the present study, the main factors controlling the calcretization are climate, calcrete position in the landscape, a host material, and topography. The stable isotope analysis of two alluvial fan profiles show that δ13C and δ18O values range for the groundwater calcretes in the phreatic zone from −4.34 to −1.98‰ and −9.26 to −6.97‰, respectively. These values suggest a high contribution of soil-derived light isotopes in meteoric water. These ranges in the vadose zone vary from −1.98 to −7.32 ‰ and −9.26 to −7.32‰, respectively. This result can be attributed to the density and types of vegetation, host detrital sediments, meteoric water, and evaporation that have controlled the isotopic composition of the calcretes.
Entre 1873 et 1884, Florentino Ameghino consacre tous ses efforts à documenter et prouver la coexistence des hommes etde la mégafaune pampéenne dans la valle de Luján. Il identifie ainsi une dizaine des sites en contexte de plaine alluvial desvallées incisés dans le lœss pampéen. Ce sites, caractérises par l’abondance des os modifies de mégafaune et quelquelithiques, ne seront jamais réétudies. L’objectif principal de cette étude est leur réévaluation et validation geoarchéologique.L’analyse séquentielle, et l’étude de l'architecture des séquences fluviolacustres et des facies loessoides associées, et lesdatations par AMS et OSL entre 13 000 AP et 50 000 AP, permettent d’attribuer un âge pléistocène supérieur aux niveauxdécrits au XIX siècle comme porteurs des évidences culturelles. Ces dépôts sont scellés par une couche noire, riche enmatière organique (Terres Noires), qui se corrèlent avec des niveaux similaires des sites Clovis et Monte Verde, signalant unchangement climatique abrupt comme déclencheur du processus d'extinction. Une attention particulière a été donne àl’étude des évidences taphonomiques et à l’expérimentation sur les traces de découpe et percussion. Ainsi sont présentées lesrésultats de ces expériences et des nombreux exemples d’os de mégafaune pampéenne striés et incisés. Ces premièresdécouvertes emmènent à considérer l’'utilisation de technologie osseuse expéditive et à reconsidérer la l’hypothèse d’unpeuplement pampéen antérieur au dernier maximum glaciaire (OIS 2 - OIS 3).
The dynamic inter‐relationships between marine and freshwater carbonate depositional environments are illustrated in the Sian Ka’an Wetlands, a 5,280 km2 complex of groundwater‐fed freshwater marshes, lakes and brackish coastal lagoons in the South East Yucatán Peninsula (Mexico). The Yucatán Platform was subaerially emergent and extensively karstified during the last glacial maximum at 18,000 yr BP. The Late Holocene transgression has caused progressive reflooding of the continental margin, backstepping of the MesoAmerican Reef and encroachment of coastal environments into the platform interior as rising groundwaters flood an interconnected cave and sinkhole system and feed seasonal marshes above. The Sian Ka’an Wetlands form a vast palustrine carbonate factory which is directly juxtaposed and dynamically linked with the marine carbonate factory to seaward. Continuing sea‐level rise has caused synchronous landward migration of marginal marine and freshwater environments as beach barriers were breached and palustrine sloughs flooded to form marginal marine seagrass lagoons. The Rio Hondo Fault conditions fluid inflow while the sub‐environments of the Sian Ka’an Wetlands reflect tectonic controls on microtopography and hydroperiod. Modern analogues for the Sian Ka’an Wetlands include the Florida Everglades, formed during transgression of the Florida Platform, and relict marsh environments preserved on leeward shores of Andros, Abaco and other Bahama islands. A wide range of ancient examples deposited in coastal and continental interior settings similarly reflect seasonal aquifer rise in response to marine transgression and/or onlap of late‐stage basin fill onto a karstified pediment. Freshwater palustrine carbonate factories on carbonate platforms are transient deposystems, controlled by subtle water depth, climate, vegetation and hydrological factors while being critically sensitive to sea‐level changes. The preservation potential of palustrine carbonates may be relatively low in coastal settings due to erosion or shallow marine overprinting, while greater further inland where marine flooding is rarer and in tectonically subsident continental interior basins where accommodation space is continuously created.
The Cameros Basin forms the northwestern extension of the Iberian Chains and exhibits many geological similarities with southwestern Cantabria and with mainland and offshore areas of southern and southwest Britain.
Jurassic facies and thickness changes reflected differential subsidence of blocks bounded by NE_SW and NW-SE faults. Superposed Late Cimmerian unconformities are attributed to successive phases of extension. Footwall uplift resulted in locally severe truncation.
The Late Jurassic - Early Cretaceous continental sediments mark a period of rapid sedimentation. The "Purbeck" sequence (Tierra de Lara Group) comprises two formations. The Señora de Brezales Formation, consisting of red-bed sandstones and conglomerates, was deposited in small-scale alluvial fans and sandflats in a semi-arid climate. The overlying Rupelo Formation comprises fresh-water marginal and open lacustrine carbonates. Open lacustrine facies of the Rupelo Formation exhibit a fauna and flora of charophytes, ostracods, gastropods and bivalves. Marginal lacustrine facies display pedogenic features including mottling, desiccation brecciation, microkarst solution cavities, rhizoliths and laminar crusts. Rare cherts are evaporite replacements. Carbon and oxygen isotope data indicate progressive vadose diagenesis.
The "Wealden" sequence (Pedroso Group) also comprises two formations. The Hortigüela Formation consists of distal alluvial clastics and sheet-like oncoidal carbonates deposited on a high-energy lake flat. The Piedrahita de Muñó Formation above contain stacked sheetflood and rare channel sands, crevasse splays, distal alluvial mudstones with pedogenic carbonates, and rare lacustrine silts and limestones. These are interpreted as terminal river deposits of a semi-arid inland drainage basin. Channelised conglomerates and sandstones of the Salas Group (Aprian) were deposited in fluvial channels on a braidplain. The Utrillas Formation (Upper Albian - Lower Cenomanian) consists of quartz-pebbly sandstones and rare mottled paleosols, recording mainly high-energy "in-channel") deposition in a low-sinuosity alluvial system characterised by multiple internal reworking.
The sedimentology and preservation of Oxfordian, Purbeck, Wealden and Aptian strata were controlled by NW-SE (half-graben bounding extensional normal) faults and NE_SW (wrench or "transfer") faults. The sense of displacement on transfer faults periodically reversed, resulting in termporally-variable subsidence patterns and marked regional stratigraphic heterogeneity. Lacustrine carbonate facies can be used to detect synsedimentary tectonic movements in central basin areas. The Aptian unconformity is related to Austrian phase rifting in the Bay of Biscay. The Middle Albian unconformity is a "steer's head" onlap produced by eustatic changes and compactional / thermal subsidence.
Geometry of Oligocene conglomerates is correlated with southwesterly-directed Pyrenean thrusting. NW_SE faults controlled the location of blind thrust and thrust-ramp culminations, while NW-SE transfer faults were reactivated as thrust-compartmental faults influencing the derivation of molasse conglomerates and conditioning the location of lateral ramps and backthrusts. E-W Variscan thrusts and backthrusts were locally reactivated as Tertiary thrusts. The San Leonardo Fault Zone is a complex stack of imbricate thrusts with decollement on Carniolas and upper Purbeck evaporites. Pliocene, Quaternary and modern drainage patters continued to reflect the Variscan structural framework. The Arlanzón River through Burgos lies on a major transfer structure offsetting the W Cameros and SW Cantabria half-graben systems.
The longevity and repeated (often inverse) reactivation of transfer faults demonstrated here has important implications for the development of foreland basins and collisional orogenic belts.
Produces spherulites identical to those found in Pleistocene calcretes, demonstrating conclusively that Cyanobacteria can be at the origin of the calcification of desert laminar crusts. The most immediate consequence is that the overlying soil is a later sedimentary deposit, related to geomorphic processes and climate change. There is an abridged English version. -English summary
Plio-Quaternary calcrete profiles from the Mesa de Ocaña, South of Madrid Basin, have developed by vertical accretionary processes and differ from the classical model of calcrete development based on studies in the United States and Australia. The profiles have laminar or chalky horizons, with the former reaching thicknesses of 2 m. Unlike previously described types, these laminar forms did not develop on hardpan horizons but appear to have formed as thin indurated layers within the soil. Internally the laminar horizons exhibit complex textures wich may reflect polyphase histories of multiple climatic changes and landscape instability. This style of calcrete formation, while differing from that seen in the c1assical model, may be more widespread than has been appreciated and is similar to that seen in the tepetate soils of Mexico.
Cemented soil horizon differentation is based on the material that is dominantly responsible for restricting root growth and retarding porosity. Chemical bonding properties of the cementing material determine whether it coheres to itself or adheres to the s-matrix. the relative amount of adhesion or cohesion of soil cement can be inferred most clearly from fabric arrangements of strongly cemented horizons. Ionicly bonded calcite and gypsum produce open-porphyric related-distribution-patterns in which the primary grains float in the expanding crystal matrix. In contrast, covalently bonded silica, iron, aluminum, and organic matter produce close-porphyric related-distribution-patterns in which isotopic cutans surround grains and fill voids and channels. Whole-horizon chemical data and quantification of cement on thin sections indicate that adhesive cementation requires less cement than cohesive cementation.
Valley calcretes form in low areas as a result of vegetative and soil-forming processes which penetrate downward and reconstitute host limestone. Enrichment in Fe and Al in valley calcretes is caused by gradual concentration of aerosol-derived material in topographically low and therefore stable settings. Ridge calcretes form on ridge crests and slopes and are associated with downward penetrating roots, but they undergo a cycle of formation and destruction (ridge calcrete cycle) which prevents them from achieving the progressive maturity seen in valley calcrete profiles. -from Authors