Foreland Basin Development Research Articles

Sedimentology and structure of the Paleogene Nummulitic series of Corsica: Reconstruction of the southern termination of the western Alpine foreland basin

AbstractNummulitic Limestones deposits are preserved along the tectonic contact between the Variscan basement and Alpine units of Corsica. These marine carbonates, dated from the Late Palaeocene to the Middle Eocene, were deposited within a foreland flexural basin that is considered to be the southern continuation of the Alpine foreland basin of southeast (SE) France. However, in contrast with the Nummulitic Limestones of SE France, those of Corsica are far less documented. This field‐based study constrains the sedimentology, stratigraphy and structure of the Nummulitic Limestones of Corsica in three localities (Balagne, Corte and Sari‐Solenzara) to identify factors that controlled foreland basin development and to clarify its significance within the early alpine orogen. The microfacies, microfaunal assemblages and siliciclastic fractions are characterised throughout the succession at each locality. The results indicate the existence of an important Variscan basement relief to the west of the basin (West Corsican Massif) that supplied early alluvial fans found at the base of the foreland succession in the northernmost Balagne area. Continuous high clastic input strongly reduced the development and diversity of the overlying Nummulitic Limestones facies and fauna. Further south, limestones in the Corte and Sari‐Solenzara areas are thicker and contain richer fauna. Three depositional models corresponding to the carbonate ramp system are proposed for the Nummulitic Limestones and used to construct paleogeographic maps illustrating the transgressive evolution of the Corsican foreland basin from the Early to the Late Eocene. Based on our results and available regional tectonic data and LT thermochronological data, we propose that the Nummulitic marine transgression took place within a continuous foreland basin encompassing southern Corsica and SE France during the early development of the western alpine arc.

Reconstructing the South American Miocene puzzle: An integrated analysis of the Paraná Formation (Argentina)

In recent decades, Miocene marine transgressions in South America have sparked ongoing debates concerning their boundaries, number of events, regional connectivity, and the existence of an epicontinental sea formed during the Andean orogeny. The bed-by-bed analysis of the Late Miocene Paraná Formation deposits in Entre Ríos Province (Argentina) allow defining six facies associations: (i) beach ridge, (ii) tidal inlet channel and washover fan, (iii) outer estuarine bioherms, (iv) central estuary, (v) tidal bar and sandflat, and (vi) tidal mudflat and creek deposits. Architectural and stacking pattern analyses reveal transgressive-regressive sequences typical of the coastal strip, encompassing shallow marine to estuarine sub-environments. The analysis identified diverse ecological guilds, including bivalve and gastropod assemblages in bioherms, and crustacean traces in tidal flats. Regressive units are characterized by wave-dominated shoreface deposits, while transgressive units comprise fine-grained estuarine deposits interbedded with bioclastic sandstones and storm deposits. The paleoenvironmental and paleoecological data indicate normal salinity marine environments with macroinvertebrates and estuarine components and suggesting tropical brackish conditions. Plant remains point to a range of vegetative communities, including mangroves, palm forests, grasslands, humid riparian forests, and dry upland forests, reflecting tropical to subtropical and humid to semiarid environments. This study concludes that the Paraná Formation correspond to a third-order transgressive system tract influenced by the Miocene Paranaense Sea. The basin-scale implications suggest that the distal Chaco-Paraná basin was affected by eustatic changes due to its low accommodation and topographic relief. The Chaco-Paraná foreland basin's evolution was driven by interactions between tectonic, climatic, and eustatic processes. The comprehensive analysis of sedimentary and fossil records provides a detailed paleogeographic reconstruction of the Miocene marine transgression in southern South America.

Unraveling the sediment routing systems evolution of the south Pyrenean foreland basin during the lower to middle Paleogene period

Orogenesis plays a crucial role in creating, modifying, and disrupting sediment routing fairways. Reconstructing past source to sink systems is a critical step to unravel the geological history, mandatory to disentangle tectonic and climatic forcings, and to understand the perturbations that affected the overall system. The Paleocene-Eocene period is a debated early stage of the early Pyrenean orogeny, characterized by the piggy-back style tectonic partitioning of sub-basins as the deformation front propagates towards the southern foreland. An important issue is to understand how sedimentary systems reacted and reorganized in response to this dynamic scenario. and whether the stratigraphic and sediment provenance results align with the envisioned scenario.In this paper we aim at contributing to the paleogeographic, sedimentologic and tectono-sedimentary evolution of the South-Pyrenean foreland basin by reviewing the chronostratigraphic framework of the basin infill in its south-central sector (the Ager sub-basin in the Serres Marginals thrust sheet). We built five new magnetostratigraphic sections, which together encompass most of the Paleogene record, aimed to best reconcile magnetostratigraphic data with the defined biostratigraphic framework of the region including marine Shallow Benthic Zones (SBZ biozones) and continental mammalian localities (MP levels). Detailed trends in subsidence show the development and evolution of the foreland depozones, from forebulge to foredeep and wedge-top setting, relative to the successive emplacement of the Montsec and Serres Marginals thrust sheets.A correlation with the eastern portion of the foreland basin (Lower-Middle Pedraforca and Cadí Thrust Sheets) and adjacent northern Graus-Tremp basin (Montsec Thrust Sheet) was feasible and seeks to contribute to the tectono-sedimentary evolution of the South-Pyrenean foreland in the light of a source to sink approach. Our proposal includes a new paleogeographic evolution of the area in a series of paleogeographic maps from Late Thanetian to Late Cuisian times.

Foreland basin development in response to Proto-Tethyan Ocean closure, western North China Block

Closure timing of the northeastern Proto-Tethyan Ocean between the Qilian-Qinling Terranes and the North China Block remains unsolved, with assumptions ranging from end-Ordovician to the Devonian. To address this issue, integrated studies of stratigraphy and geochronology were conducted on Late Ordovician strata in the southwestern Ordos, which recorded the earliest tectonic transition from passive margin to foreland basin in the westernmost North China Block. Stratigraphic and paleontologic syntheses demonstrate that pre-Katian strata are shallow-marine deposits dominated by benthonic faunas. Meanwhile, Katian successions above a paraconformity are characterized by deep-water debrites and turbidites containing abundant planktonic graptolites. Provenance analysis reveals an evolving source from the North China Block basement to the Qilian-Qinling arc terranes at the beginning of Katian (ca. 450 Ma). Detrital zircons from pre-Katian quartz arenites yield ages of ca. 1600–2800 Ma, significantly older than their depositional timing. In contrast, Katian turbidites in the southwestern Ordos and the North Qilian Orogen display similar age patterns dominated by ca. 450–900 Ma ages. These clues imply a basin-filling shift from passive margin to underfilled foreland, separated by forebulge unconformity formation at Sandbian-Katian boundary. Stratigraphic correlation further suggests that sequence of foreland basin-infill transitions was broadly simultaneous along strike more than ~1200 km across the southwestern North China Block, including forebulge flexural uplift, followed by rapid shallow shelf drowning to abyssal plain, until arrival and superimposition of arc-derived turbidites. Along-strike synchronicity of orogenic activities implies that closure of the northeastern Proto-Tethys was nearly synchronous. The L-shaped orogen-parallel foreland encompassing the southwestern North China Block unveils oroclinal bending of the Qilian-Qinling collision belt, which still dominates the geology of present northeastern Tibet. Our new insights provide a stratigraphic constraint for the timing and mode of the initial elimination of the northeastern Proto-Tethys.

Neogene to modern foreland basin development in the Sub-Andean zone of southern Bolivia and northern Argentina, 21−23°S

The Sub-Andean retroarc region is a unique example of an active continental-scale retroarc foreland basin system. Heavily targeted for hydrocarbon exploration, the region hosts a large volume of subsurface data coupled to surface studies dedicated to refining its evolution in time and space. This paper presents a regional correlation of stratigraphic markers from seismic reflection and well logs across the Sub-Andean foothills at 23−21°S in southern Bolivia and northern Argentina, which reveals the contrasting along-strike history of Mesozoic to Cenozoic tectonics that preceded the foreland basin setting. Supported by published geochronological data and new zircon U-Pb maximum depositional ages, we describe the depositional transition from pre-Andean to Andean stratigraphy and discrete episodes of foreland basin subsidence and shortening. Based on interpreted stratigraphic breaks, we define the extent and stepwise evolution of this foreland basin, which was characterized by the progressive eastward migration of foreland basin depozones. Based on restored thickness profiles, we present flexural models of basin subsidence for the Sub-Andean foothills region. The modeling of discrete episodes of foreland basin subsidence refines the widely accepted bimodal elastic strength in the foreland basin at 21−23°S, which is weaker in the western ranges (∼20 km effective elastic thickness) and stronger eastward (>40 km). Modeling results also reveal minimum values of subsidence rates (up to 1.2 mm/yr) in the sequential foredeep depozones and suggest that the modeled tectonic load migration—as constrained by the record of syntectonic strata—probably increased over time through the incorporation of Sub-Andean rocks into the orogenic wedge.

Lithospheric- and crustal-scale controls on variations in foreland basin development in the Northern Alpine Foreland Basin

The Northern Alpine Foreland Basin (i.e., Molasse Basin) developed due to flexural subsidence from slab- and topographic loading during continental collision between the Adriatic and European plates. Previous studies highlight a diachronous transition from underfilled- to overfilled conditions in the Molasse Basin in response to orogen-parallel variations in flexural subsidence and sediment supply. In this contribution, we investigate the lithospheric- and crustal-scale orogenic process(es) that generated this diachronous transition. For this, we conducted a tectonostratigraphic analysis of the Molasse Basin. Firstly, we constructed a 3D geological model to derive the architecture of the European margin during the Eocene onset of flexure. Second, 2D/3D reflection seismic- and borehole data were used to characterise the spatiotemporal development of depositional environments and syn-flexural normal fault kinematics in the German Molasse. Our data show a stepwise rather than continuous eastward migration of the underfilled- to overfilled transition. Furthermore, syn-flexural fault kinematics document a Rupelian to early Burdigalian northward younging trend and higher cumulative Cenozoic offsets in the Eastern German Molasse (< 230 m) compared to the Western German Molasse (< 150 m). This implies a west-to-east increase in the curvature of bending of the European plate, induced by along-strike variations in the magnitude of applied loads and/or European lithospheric strength variations. These variations drove lateral variations in accommodation space and sediment supply. Subsequently, this led to the diachronous underfill-to-overfilled transition in the Molasse Basin. Taken together, we suggest that the diachronous transition was driven by spatiotemporal variations in the thickening of the orogenic wedge supported by slab detachment, promoted by subduction- and collision of the irregular European margin.

Tectono-stratigraphic evolution of a deep-water foreland basin: a case study from the Marnoso-arenacea basin, central Italy

Tectono-stratigraphic evolution of a deep-water foreland basin: a case study from the Marnoso-arenacea basin, central Italy

Geochemistry and depositional environment of black shale beds of the Belata Formation, Peninsular Malaysia

Shale deposits from Belata Formation outcrops were examined geochemically and mineralogically to establish their paragenesis and, subsequently, to identify factors that govern their depositional environment. Fifty representative samples were examined through SEM/EDX, XRD and XRF. The findings show the mineralogical composition was abundant in quartz, with pyrite and calcite in trace amount. Zr–TiO2 bivariate diagram showed predominantly felsic and intermediate igneous rocks, suggesting that the shales may be coming from the hidden basement rocks of Peninsular Malaysia. The diagenetic controls on the geochemistry are suggested by the changes in compositional trends. The samples exhibit CIA values ranging from 62.2 to 80.2, and PIA values ranging from 65 to 100. According to the V/Ni, Ni/Co, and V/Cr ratios, these black shales were formed in a suboxic environment. Additionally, the high V/Ni ratios (1.47) point to a marine source for the organic content that gives the shales their dark color. The paleoclimate during deposition was interpreted to be semiarid to warm. The black shale samples are inferred to have originated from a passive margin tectonic setting that followed continental collision and rifting stages of the foreland basin development phase of Peninsular Malaysia based on geochemical proxies and associated diagrams.

Paleoenviromental evolution of the Cenozoic foreland basin to intermontane basins in the Eastern Cordillera, North-Western Argentina

The stratigraphic and sedimentological characteristics of the Cenozoic deposits are very important aspects to investigate the tecto-sedimentary evolution of the Andean foreland basin to intermontane hinterland basins in north-western Argentina. These deposits lie in an unconformity or paraconformity relationship over the Salta Group (Cretaceous-Paleocene) or older deposits. During the middle Eocene-Pleistocene, the changes in the sedimentary paleoenvironment and its evolution are the direct result of tectonic effects, a change in base level and accommodation space. This study provides the results of the integrated analysis of the middle-upper Eocene to Plio-Pleistocene basins in the Eastern Cordillera, in north-western Argentina. These basins that have been generated in the Central Andes region in north-western Argentina during the Cenozoic tectonic convergence, we divided in four sequences in the evolution from middle to upper Eocene to Plio-Pleistocene. They present fluvial deposits of different topographic slopes, with development of ephemeral fluvial systems associated with dune fields, meandering fluvial systems in arid climates, interlaced sandy-gravelly fluvial systems, sinuous sandy-gravelly fluvial systems of intermediate to high sinuosity, with the development of lagoons and swamps of tropical climate during ∼10 to ∼5 Ma, and the change to arid - semiarid claimed condition from ∼5 to ∼2.5 Ma. The sedimentary record of the Plio-Pleistocene hinterland basin in the Eastern Cordillera and the adjacent Andean Puna Plateau in the Central Andes of NW Argentina constitutes geological archive that provide spatio-temporal insights into geological evolution and associated surface uplift.These basins, probably connected at their onset during the middle Eocene, are characterized by different subsidence histories, differences in sedimentary paleoenvironment evolution, variations in topographic slopes, changes in provenance and paleocurrents, resulting in differences in tecto-sedimentary history.

Formation and evolution of an Early Cambrian foreland basin in the NW Yangtze Block, South China

The Ediacaran to Cambrian succession in the NW Yangtze Block has long been considered to have formed in a passive margin. The Ediacaran is dominated by thick carbonate rocks, whereas the Lower Cambrian comprises a thick clastic succession. The transition from carbonate to clastic rocks and the provenance and tectonic setting of the clastic succession are poorly understood. Stratigraphic correlation shows a distinct stratigraphic absence from the Lower Cambrian to Devonian in the Bikou terrane. A regional seismic profile shows a wedge geometry of the Lower Cambrian from NW to SE. Outcrops reveal an overall coarsening-upward Lower Cambrian succession. The petrographic analysis of clastic rocks indicates immaturity, implying a proximal source. Palaeocurrent measurements of clastic rocks point to dominant SE-vergent orientations. The age spectra of detrital zircons imply that they were derived from Early Cambrian continental arcs and older continental crust. Geological, geophysical and geochemical evidence indicates that an Early Cambrian foreland basin was formed in the NW Yangtze Block. This foreland basin appears to have been strongly influenced by orogenic loading northwestward. We propose that this orogenic event should be named the Motianling orogeny, the origin of which may be related to subduction of the Proto-Tethys ocean beneath the NW Yangtze Block. Supplementary material: Supplementary tables A–D and a petrological description of detrital zircons are available at https://doi.org/10.6084/m9.figshare.c.6332980

A corrected finite-difference scheme for the flexure equation with abrupt changes in coefficient

Abstract. The fourth-order differential equation describing elastic flexure of the lithosphere is one of the cornerstones of geodynamics that is key to understanding topography, gravity, glacial isostatic rebound, foreland basin evolution, and a host of other phenomena. Despite being fully formulated in the 1940s, a number of significant issues concerning the basic equation have remained overlooked to this day. We first explain the different fundamental forms the equation can take and their difference in meaning and solution procedures. We then show how numerical solutions to flexure problems as they are currently formulated are in general potentially unreliable in an unpredictable manner for cases in which the coefficient of rigidity varies in space due to variations of the elastic thickness parameter. This is due to fundamental issues related to the numerical discretisation scheme employed. We demonstrate an alternative discretisation that is stable and accurate across the broadest conceivable range of conditions and variations of elastic thickness, and we show how such a scheme can simulate conditions up to and including a completely broken lithosphere more usually modelled as an end-loaded, single, continuous plate. Importantly, our scheme will allow breaks in plate interiors, allowing, for instance, the creation of separate blocks of lithosphere which can also share the support of loads. The scheme we use has been known for many years but remains rarely applied or discussed. We show that it is generally the most suitable finite-difference discretisation of fourth-order, elliptic equations of the kind describing many phenomena in elasticity, including the problem of bending of elastic beams. We compare the earlier discretisation scheme to the new one in one-dimensional form and also give the two-dimensional discretisation based on the new scheme. We also describe a general issue concerning the numerical stability of any second-order finite-difference discretisation of a fourth-order differential equation like that describing flexure wherein contrasting magnitudes of coefficients of different summed terms lead to round-off problems, which in turn destroy matrix positivity. We explain the use of 128 bit floating-point storage for variables to mitigate this issue.

Organic matter accumulation of the Upper Triassic Ma'antang shales in the Longmenshan Foreland Basin (western Sichuan, China)

The Upper Triassic Ma'antang Formation is not only an important set of source rocks in the Sichuan Basin but may also be a successor for shale gas in China. In this study, the provenance, tectonic setting, chemical weathering, environment and productivity of the middle–late Carnian (Tuvalian) are reconstructed from petrological and geochemical data to determine the origin of the Ma'antang shales. The brachiopod-dominated fossil composition, type II-dominated organic matter, and fluctuating water salinity reflect that the shale formed in a marine environment influenced by terrestrial influx. TiO2 vs. Zr and La/Th vs. Hf diagrams, as well as the combination of major elements indicate that the Sichuan Basin was a collisional setting in the Tuvalian, and the provenance was felsic igneous source rocks. Redox-sensitive elements indicate dysoxic–anoxic conditions. Samples with Cd/Mo < 0.1 and Co*Mn > 0.4 show normal levels of Cd and relative enrichment of Mn and Co, which supports that the shale is more likely to be a product of anoxic preservation mode associated with the restricted basin. In contrast, the moderate productivity conditions reflected by Babio and P/Ti are also positive factors for organic matter accumulation. Based on the reconstruction of these environmental parameters, several driving factors that promote the formation of high TOC shales are explored. It is believed that the rapid tectonic subsidence in the early stage of foreland basin evolution (undercompensation period) created accommodation space for the shale, and the deep water also promoted anoxic bottom water, which facilitated the preservation of organic matter. The Carnian Pluvial Episode (CPE) increased global weathering and rainfall, allowing the oceans to receive more terrestrial nutrients, and freshwater influx caused stratification due to differences in salinity between the surface water and bottom water. Volcanic ash from the Wrangellia Large Igneous Province (W-LIP) and volcanic ash from local volcanoes were transported over long distances in the Late Triassic megamonsoons. Both were able to stimulate marine productivity, maintain and expand oceanic anoxia, and were beneficial for organic matter accumulation.

Location, extent, and magnitude of dynamic topography in the Late Cretaceous Cordilleran Foreland Basin, USA: New insights from 3D flexural backstripping

AbstractMantle‐induced dynamic topography (i.e., subsidence and uplift) has been increasingly recognized as an important process in foreland basin development. However, characterizing and distinguishing the effects (i.e., location, extent and magnitude) of dynamic topography in ancient foreland basins remains challenging because the spatio‐temporal footprint of dynamic topography and flexural topography (i.e., generated by topographic loading) can overlap. This study employs 3D flexural backstripping of Upper Cretaceous strata in the central part of the North American Cordilleran foreland basin (CFB) to better quantify the effects of dynamic topography. The extensive stratigraphic database and good age control of the CFB permit the regional application of 3D flexural backstripping in this basin for the first time. Dynamic topography started to influence the development of the CFB during the late Turonian to middle Campanian (90.2–80.2 Ma) and became the dominant subsidence mechanism during the middle to late Campanian (80.2–74.6 Ma). The area influenced by &gt;100 m dynamic subsidence is approximately 400 by 500 km, within which significant (&gt;200 m) dynamic subsidence occurs in an irregular‐shaped (i.e., lunate) subregion. The maximum magnitude of dynamic subsidence is 300 ± 100 m based on the 80.2–74.6 Ma tectonic subsidence maps. With the maximum magnitude of dynamic uplift being constrained to be 200–300 m, the gross amount of dynamic topography in the Late Cretaceous CFB is 500–600 m. Although the location of dynamic subsidence revealed by tectonic subsidence maps is generally consistent with isopach map trends, tectonic subsidence maps developed through 3D flexural backstripping provide more accurate constraints of the areal extent, magnitude and rate of dynamic topography (as well as flexural topography) in the CFB through the Late Cretaceous. This improved understanding of dynamic topography in the CFB is critical for refining current geodynamic models of foreland basins and understanding the surface expression of mantle processes.

Causes of Variable Shortening and Tectonic Subsidence During Changes in Subduction: Insights From Flexural Thermokinematic Modeling of the Neogene Southern Central Andes (28–30°S)

AbstractThe Andes of western Argentina record spatiotemporal variations in morphology, basin geometry, and structural style that correspond with changes in crustal inheritance and convergent margin dynamics. Above the modern Pampean flat‐slab subduction segment (27–33°S), retroarc shortening generated a fold‐thrust belt and intraforeland basement uplifts that converge north of ∼29°S, providing opportunities to explore the effects of varied deformation and subduction regimes on synorogenic sedimentation. We integrate new detrital zircon U‐Pb and apatite (U‐Th)/He analyses with sequentially restored, flexurally balanced cross sections and thermokinematic models at ∼28.5–30°S to link deformation with resulting uplift, erosion, and basin accumulation histories. Tectonic subsidence, topographic evolution, and thermochronometric cooling records point to (a) shortening and distal foreland basin accumulation at ∼18–16 Ma, (b) thrust belt migration, changes in sediment provenance, and enhanced flexural subsidence from ∼16 to 9 Ma, (c) intraforeland basement deformation, local flexure, and drainage reorganization at ∼12–7 Ma, and (d) out‐of‐sequence shortening and exhumation of foreland basin fill by ∼8–2 Ma. Thrust belt kinematics and the reactivation of basement heterogeneities strongly controlled tectonic load configurations and subsidence patterns. Geo/thermochronological data and model results resolve increased shortening and combined thrust belt and intraforeland basement loading in response to ridge collision and Neogene shallowing of the subducted oceanic slab. Finally, this study demonstrates the utility of integrated flexural thermokinematic and erosion modeling for evaluating the geometries, rates, and potential drivers of retroarc deformation and foreland basin evolution during changes in subduction.

Thermochronometry unveils ancient thermal regimes in the NW Pampean Ranges, Argentina: From Mesozoic rifting to Miocene flat‐slab subduction

AbstractReconstructing thermal histories in thrust belts is commonly used to infer the age and rates of thrusting and hence the driving mechanisms of orogenesis. In areas where ancient basins have been incorporated into the orogenic wedge, a quantitative reconstruction of the thermal history helps distinguish among potential mechanisms responsible for heating events. We present such a reconstruction for the Ischigualasto‐Villa Unión basin in the western Pampean Ranges of Argentina, where Triassic rifting and late Cretaceous‐Cenozoic retroarc foreland basin development has been widely documented, including Miocene flat‐slab subduction. We report results of organic and inorganic thermal indicators acquired along three stratigraphic sections, including vitrinite reflectance and X‐ray diffractometry in claystones and new thermochronological [(apatite fission‐track and apatite and zircon [U‐Th]/He)] analyses. Despite up to 5 km‐thick Cenozoic overburden and unlike previously thought, the thermal peak in the basin is not due to Cenozoic burial but occurred in the Triassic, associated with a high heat flow of up to 90 mWm−2 and &lt;2 km of burial, which heated the base of the Triassic strata to ~160°C. Following exhumation, attested by the development of an unconformity between the Triassic and Late‐Cretaceous–Cenozoic sequences, Cenozoic re‐burial increased the temperature to ~110°C at the base of the Triassic section and only ~50°C 7 km upsection, suggesting a dramatic decrease in the thermal gradient. The onset of Cenozoic cooling occurred at ~10−8 Ma, concomitant with sediment accumulation and thus preceding the latest Miocene onset of thrusting that has been independently documented by stratigraphic–cross‐cutting relationships. We argue that the onset of cooling is associated with lithospheric refrigeration following establishment of flat‐slab subduction, leading to the eastward displacement of the asthenospheric wedge beneath the South American plate. Our study places time and temperature constraints on flat‐slab cooling that calls for a careful interpretation of exhumation signals in thrustbelts inferred from thermochronology only.

Sequence stratigraphy and paleoenvironmental significance of the Neoproterozoic Bambui Group, Central Brazil

The Bambui Group is a ca. 2,000 m-thick mixed biochemical-siliciclastic sedimentary succession that accumulated in the Sao Francisco Basin, central Brazil. Its age is contentious, but deposition during the Cryogenian-Ediacaran (ca. 635–550 Ma) is consistent with a new sequence stratigraphic model presented herein and previously published geochemical and structural data. Six stratigraphic sequences are recognized that record depositional evolution from an epeiric sea to foreland basin during the Marinoan snowball glaciation (ca. 635 Ma) to shortly after the Gaskiers (ca. 580 Ma) glaciation. Although this stratigraphic architecture differs from the classic lithostratigraphic and more recent chemostratigraphic and sequence stratigraphic interpretations of the Bambui Group, it is supported by correlation to other Cryogenian-Ediacaran successions globally. Collectively, these six sequences preserve a robust record of seawater redox structure leading up to the oxygenation threshold that triggered the evolution of multicellular animals in the Ediacaran.Sequence 1 is composed of Marinoan glacial deposits and an overlying cap carbonate with aragonite fan pseudomorphs and barite. Sequence 2 is composed of lime-mudstone that changes laterally into loess-rich peritidal phosphorites, suggesting oxygen stratification in shallow paleoenvironments. Sequence 3 is a carbonate succession with abundant stromatolites implying widespread oxygen production. The top of Sequence 3 is marked by a regional subaerial unconformity developed during uplift associated with the onset of the Brasiliano-Pan African Orogeny. Sequence 4 signals a return to loess derived sedimentation. Distal deposits contain abundant organic matter with framboidal pyrite reflecting bacterial sulfate reduction in low sulfate Ediacaran seawater. Sequence 5 is composed of well-developed aggradational parasequences composed of alternating deep subtidal organic-rich siltstones and shallow-water carbonate grainstones. Their well-defined cyclicity is interpreted to record extreme sea level fluctuations during the onset of the Gaskiers glaciation. Stromatolites suggest an oxygen-stratified water column. Sequence 6 marks the change from an epeiric sea to a foreland basin. Interbedded basinal glauconitic siltstone and shale loess-rich are correlative with shoreface sandstones. Authigenic glauconite requires suboxic bottom waters to precipitate and thus reflects oxygenation of distal settings. Most of the deposition ended by ca. 560–550 Ma when the entire Bambui Group was deformed during the assembly of West Gondwana.In sequences 1, 2, 3, 4, 5, and the lower portion of 6, the lateral continuity of lithofacies, predictable layer-cake stratigraphy, and lack of fluvial coarse siliciclastic deposits are characteristics of epeiric sea successions. The appearance of immature sandstone, poorly sorted conglomerate, braided fluvial deposits, and synsedimentary deformation in the upper part of Sequence 6 are interpreted as the onset of foreland basin development. Stacking relationships of redox sensitive, authigenic lithofacies in both the epeiric sea and foreland phases support recent research suggesting seawater redox conditions were in a state of flux during the apex of the purported Neoproterozoic Oxygenation Event. Organic matter and authigenic pyrite in Sequences 4 and 5 reflect increased primary productivity in surface waters fertilized by nutrients such as P and Fe adsorbed on loess particles. This mechanism could be expanded to a global scale and have contributed to the increase in atmospheric and oceanic oxygen levels. These findings redefine and highlight the significance of the Bambui Group for Neoproterozoic Earth history.

Petrography and tectonic provenance of the Permian Tunas Formation: Implications on the paleotectonic setting during the Claromecó Foreland Basin evolution, southwestern Gondwana margin, Argentina

The Claromecó Basin is located at the south-western sector of the Buenos Aires province, Argentina. This basin is considered a foreland basin closely related to the evolution of the southwestern Gondwana margin. This contribution focuses on the provenance analysis of the Tunas Formation (Permian, Pillahuincó Group), which represents the last filling stage for the Claromecó Foreland Basin. Petrographic and tectonic provenance analyses were performed in sandstones recovered from subsurface (PANG 0001 and PANG 0003 exploration wells) and outcrops located close to the basin center (Gonzales Chaves locality). In the subsurface, the analyzed succession is composed of medium- to fine-grained sandstones interbedded with tuffs, mudrocks, carbonaceous mudrocks and coal beds. In outcrops, the succession is dominated by medium-to fine-grained sandstones interbedded with siltstones. Modal composition patterns are distributed into the recycled orogen and transitionally recycled to mixed fields. Petrographic analyses, in addition to provenance and sedimentological studies, confirm that sedimentary material was derived from a mixed source, which largely comes from the Sierras Australes fold and thrust belt, located towards the W–SW, where the sedimentary succession is interbedded with volcanic material. The Tunas Formation shows clear differences in its modal composition, paleocurrent direction and paleoenvironmental conditions with respect to the underlying units of the Pillahuincó Group (Sauce Grande, Piedra Azul and Bonete formations). Source areas changed from cratonic to mixed fold belt/arc-derived material, suggesting variations in the Claromecó Basin configuration during the Late Paleozoic. Changes in the paleotectonic scenario during the deposition of the Tunas Formation have been interpreted as a consequence of a compressive post-collisional deformation event, the product of adjustment, accommodation and translation of terrains towards the equator during the Permian–Triassic to form Pangea.

Organic biogeochemical study of deeper southeastern Bengal Basin sediments in West Bengal, India

The Bengal Basin is a fluvio-deltaic basin spanning Bangladesh and part of east and northeast India. The evolution of the peripheral foreland basin has been studied, but published literature on depositional conditions, source and maturity of organic matter in the deeper sediments of the Indian section of the basin is rare, despite the fact that natural gas is often encountered during hydrocarbon exploration. Our research assesses the depositional environment and source of the organic matter (OM) in the Pleistocene-Miocene sediments from five wells drilled by Oil and Natural Gas Corporation Limited in the southeastern Bengal Basin, West Bengal, India and aims to understand its maturity and potential to yield natural gas. The total organic carbon/nitrogen ratio and stable isotope (δ13C and δ15N) signature indicate primarily aquatic and C3 terrestrial plant sources of the OM and deposition under tidal flat and marshy environments. The n-alkane and isoprenoid alkane distribution are consistent with an autochthonous source of OM and terrestrial oxic-suboxic shallow-water depositional setting. The Rock-Eval parameters, such as maximal pyrolysis temperature, hydrogen and oxygen indices, indicate the immature nature of Type III and Type IV kerogen. The presence of methanogenic archaea, as indicated by phylogenetic analysis, in two Miocene sediment samples from one well indicates an active microbial activity in Type III immature OM, derived from C3 marsh vegetation and deposited under oxic shallow-water conditions. Our research describes the presence of methanogenic archaea for the first time in Miocene Bengal Basin sediments and is one of the few reports of their presence in deep (> 4000 m) horizons.

Slab Load Controls Beneath the Alps on the Source-to-Sink Sedimentary Pathways in the Molasse Basin

The stratigraphic development of foreland basins has mainly been related to surface loading in the adjacent orogens, whereas the control of slab loads on these basins has received much less attention. This has also been the case for interpreting the relationships between the Oligocene to Micoene evolution of the European Alps and the North Alpine foreland basin or Molasse basin. In this trough, periods of rapid subsidence have generally been considered as a response to the growth of the Alpine topography, and thus to the construction of larger surface loads. However, such views conflict with observations where the surface growth in the Alps has been partly decoupled from the subsidence history in the basin. In addition, surface loads alone are not capable of explaining the contrasts in the stratigraphic development particularly between its central and eastern portions. Here, we present an alternative view on the evolution of the Molasse basin. We focus on the time interval between c. 30 and 15 Ma and relate the basin-scale development of this trough to the subduction processes, and thus to the development of slab loads beneath the European Alps. At 30 Ma, the western and central portions of this basin experienced a change from deep marine underfilled (Flysch stage) to overfilled terrestrial conditions (Molasse stage). During this time, however, a deep marine Flysch-type environment prevailed in the eastern part of the basin. This was also the final sedimentary sink as sediment was routed along the topographic axis from the western/central to the eastern part of this trough. We interpret the change from basin underfill to overfill in the western and central basin as a response to oceanic lithosphere slab-breakoff beneath the Central and Western Alps. This is considered to have resulted in a growth of the Alpine topography in these portions of the Alps, an increase in surface erosion and an augmentation in sediment supply to the basin, and thus in the observed change from basin underfill to overfill. In the eastern part of the basin, however, underfilled Flysch-type conditions prevailed until 20 Ma, and subsidence rates were higher than in the western and central parts. We interpret that high subsidence rates in the eastern Molasse occurred in response to slab loads beneath the Eastern Alps, where the subducted oceanic slab remained attached to the European plate and downwarped the plate in the East. Accordingly, in the central and western parts, the growth of the Alpine topography, the increase in sediment flux and the change from basin underfill to overfill most likely reflect the response to slab delamination beneath the Central Alps. In contrast, in the eastern part, the possibly subdued topography in the Eastern Alps, the low sediment flux and the maintenance of a deep marine Flysch-type basin records a situation where the oceanic slab was still attached to the European plate. The situation changed at 20 Ma, when the eastern part of the basin chronicled a change from deep marine (underfilled) to shallow marine and then terrestrial (overfilled conditions). During the same time, subsidence rates in the eastern basin decreased, deformation at the Alpine front came to a halt and sediment supply to the basin increased possibly in response to a growth of the topography in the Eastern Alps. This was also the time when the sediment routing in the basin axis changed from an east-directed sediment dispersal prior to 20 Ma, to a west-oriented sediment transport thereafter and thus to the opposite direction. We relate these changes to the occurrence of oceanic slab breakoff beneath the Eastern Alps, which most likely resulted in a rebound of the plate, a growth of the topography in the Eastern Alps and a larger sediment flux to the eastern portion of the basin. Beneath the Central and Western Alps, however, the continental lithosphere slab remained attached to the European plate, thereby resulting in a continued downwarping of the plate in its central and western portions. This plate downwarping beneath the central and western Molasse together with the rebound of the foreland plate in the East possibly explains the inversion of the drainage direction. We thus propose that slab loads beneath the Alps were presumably the most important drivers for the development of the Molasse basin at the basin scale.

Zircon (U-Th)/(He-Pb) double-dating constraints on the interplay between thrust deformation and foreland basin architecture, Sevier foreland basin, Utah

AbstractThe Cretaceous Cordilleran foreland basin strata exposed in the Book Cliffs of eastern Utah and western Colorado have motivated important concepts linking thrust belt deformation and foreland basin evolution largely on the basis of sequence stratigraphy, stratal architecture, and sediment provenance evolution. However, these methods and approaches generally cannot provide critical insights into the temporal or causal linkages between foreland basin architecture and thrust belt deformation. This is in part due to discrepancies in age resolution and lack of evidence with which to directly couple sediment supply and basin-fill evolution to thrust belt unroofing. New detrital zircon (DZ) geothermochronometric data from Upper Cretaceous proximal to distal foreland basin strata in the Book Cliffs provide new quantitative insights into sediment origin and dispersal in relation to thrust belt deformation and exhumation. Detailed DZ U-Pb and (U-Th)/He double dating reveals that the Book Cliffs foredeep detritus was mainly delivered by transverse routing systems from two major sources: (1) Neoproterozoic and Lower Paleozoic strata from the central Utah Sevier thrust belt, and (2) Permian–Jurassic and synorogenic Cretaceous strata recycled from the frontal part of the thrust belt. A dramatic increase in Sierran magmatic arc and Yavapai-Mazatzal DZ U-Pb ages, as well as Paleozoic DZ He ages, in the deeper marine portions of the foreland basin points to axial fluvial and littoral sediment input from the Sierran magmatic arc and Mogollon highland sources. Both transverse and axial transport systems acted contemporaneously during eastward propagation of the Late Cretaceous thrust belt. DZ He depositional lag time estimates reveal three distinct exhumation pulses in the Sevier thrust belt in the Cenomanian and Campanian. The exhumation pulses correlate with shifts in sediment provenance, dispersal style, and progradation rates in the foreland basin. These new data support conceptual models that temporally and causally link accelerated exhumation and unroofing in the thrust belt to increases in sediment supply and rapid clastic progradation in the foreland basin.