Basin Research - Volume 34, Issue 6, 2022

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Knickpoint mapping in the Middle Shire Basin shows to main sets of mobile knickpoints associated with baselevel fall events at the downstream end of the Middle Shire River.The baselevel fall events triggered knickpoint migration through the fluvial network since at least the Mid. Pleistocene. We suggest that Middle Shire basin opening, associated with rift linkage, is likely a recent event (at least Mid. Pleistocene) relative to the Late Oligocene activation of Cenozoic rifting in the East African Rift’s Western Branch

, Abstract

Tectonic and paleo‐environmental reconstructions of rift evolution typically rely on the interpretation of sedimentary sequences, but this is rarely possible in early‐stage rifts where sediment volumes are low. To overcome this challenge, we use geomorphology to investigate landscape evolution and the role of different forcing mechanisms during basin development. Here, we focus on the humid Middle Shire River basin, located within the zone of progressive interaction and linkage between the southern Malawi Rift and Shire Rift Zone, East Africa. We used a digital elevation model to map knickpoints and knickpoint morphologies in the Middle Shire River basin and examined the relationships with pre‐rift and syn‐rift structures within the rift interaction zone. The main axial stream, Shire River, descends steeply, 372 m over a 50 km distance, across exposed metamorphic basement along the rift floor, exhibiting a strongly disequilibrated longitudinal elevation profile with both ‘mobile’ and ‘fixed’ knickpoints. In particular, we identify two clusters of mobile knickpoints, which we interpret as associated with baselevel fall events at the downstream end of the exposed basement that triggered knickpoint migration through the fluvial network since at least the Mid. Pleistocene. We infer that after the integration of the axial stream across the Middle Shire Basin, the knickpoints migrate upstream in response to fault‐related subsidence in the Shire Rift Zone. Conversely, the fixed knickpoints are interpreted to reflect local differential bedrock erodibility at lithologic contacts or basement‐hosted fault scarps along the basin floor. The results suggest that Middle Shire basin opening, associated with rift linkage, is likely a recent event (at least Mid. Pleistocene) relative to the Late Oligocene activation of Cenozoic rifting in the East African Rift's Western Branch. These findings support the hypothesis that the Western Branch developed from the gradual propagation, linkage and coalescence of initially nucleated distinct rift basins.

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, Abstract

Volcanic eruptions can provide large amounts of sedimentary materials and expose fluvial valleys or lakes to catastrophic hyperpycnal flood events, but this process has not been documented in detail. The Ordos Basin Permian fluvio‐deltaic system reveals evidence for abundant volcanism linked to the tectonic evolution of orogenic belt around the basin and provides a ‘natural laboratory’ for investigating hyperpycnal flows associated with volcanic activity. This study analysed volcanic matrix‐rich sandstone (VMS) samples for petrology, mineralogy and geochemistry in order to address current and uncertain volcanogenic material provenance explanations and the lack of systematic investigations into the depositional and diagenetic processes of volcanic related sediments in the southwestern basin. By combining tectonic background surveys, detrital zircon geochronology and spatial distribution of volcanogenic materials, it was found that volcanogenic materials were not derived from the Yinshan‐Yanshan Orogenic Belt (YYOB) as previously thought, but instead evidence a southwestern origin from the North Qinling Orogenic Belt (NQOB). Volcanogenic materials retained in the provenance area during frequent volcanic eruptions were transported to the basin via fluvial systems shortly after the eruption. The associated sediments meet the criterion for hyperpycnites based on lithofacies associations, suggesting the occurrence of hyperpycnal flows during the deposition of VMS. During the subsequent burial stage, the VMS became three distinct types defined by their volcanic matrix content and have similar paragenetic sequences but different diagenetic intensities. The differing content of authigenic minerals in the three types was closely related to the sandstone pore structure characteristics and the evolution of volcanic‐matrix alteration materials. This article proposes a possible explanation for the previously unidentified tectonothermal events in the NQOB during the Permian, validating and reinforcing the theoretical work of hyperpycnal flow. This contribution provides new insights and understanding of the depositional and diagenetic processes of lacustrine basins with similar tectonic settings.

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Evolving geography and geology of the NW Himalayas showing how the provenance of the sections at Jawalamukhi and Joginder Nagar have evolved because of their motion towards the mountains, the changing course of the river and progressive unroofing of the Lesser Himalaya in the Kullu‐Rampur Window.

, Abstract

Testing models that link climate and solid Earth tectonics in mountain belts requires independent erosional, structural and climatic histories. Two well‐preserved stratigraphic sections of the Himalayan foreland basin are exposed in NW India. The Jawalamukhi (13–5 Ma) and Joginder Nagar sections (21–13 Ma) are dated by magnetostratigraphy and span a period of significant climate change and tectonic evolution. We combine sediment geochemistry, detrital zircon U–Pb dating and apatite fission track analyses to reconstruct changes in the patterns of erosion and exhumation in this area from the Early Miocene to Pliocene. The provenance of the foreland sediments reflects a mixture of Tethyan and Greater Himalayan sources from 21 to 11 Ma, with influx from the Inner Lesser Himalaya starting after 11 Ma, and a strong increase in Crystalline Inner Lesser Himalayan erosion after 8 Ma. This distinct shift in provenance most likely reflects exhumation of the Kullu‐Rampur Window, as well as the northward motion of the Jawalamukhi section towards the Himalayas, drainage reorganization in the foreland, and/or tectonically driven drainage capture in the mountains. Prior to 10.5 Ma sediment came from a large river whose sources were Greater Himalaya and Haimanta dominated, likely a palaeo‐Sutlej, while after 8 Ma the source river was dominated by a more local drainage. Our work is consistent with Nd isotope and mica Ar‐Ar constraints from the same sections that demonstrate initial Inner Lesser Himalayan unroofing in this region from 11 Ma, earlier than the 2 Ma implied from the marine record and during a period of summer monsoon weakening when fission track data indicate very rapid cooling and erosion of the Lesser Himalaya sources from no later than 10 Ma. Tectonically driven rock uplift coupled with southerly migration of the maximum rainfall belt during a time of drying, may have focused erosion over the Lesser Himalayan Duplex and created the Kullu‐Rampur Window.

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A grid of 2D seismic data (∼75,000 km) and combined with lithological information from ODP Site 1165 is applied to investigate the Cenozoic paleoceanographic changes of the Enderby Basin, East Antarctica. The study provides comprehensive new insights into the evolution of the Antarctic Bottom Water (AABW) via the morphology, geometry and distribution of bedforms in a newly recognized mixed turbidite‐contourite system.

, Abstract

The thick sequence of mid‐late Cenozoic sediments preserved within the Enderby Basin of the East Antarctica margin contains key information regarding glacial history and palaeo‐oceanographic conditions during the last 34 My. The interplay between glacial processes and ocean circulation can be reconstructed from seismic stratigraphic studies. Here, interpretation of seismic sequences and geomorphology from an extensive 2D seismic dataset (∼75,000 km) are correlated with lithological data of the ODP site 1165 drilled on the continental rise, and used to assess the age and origin of the sediment, and the possible influence of oceanic currents on its distribution. Mapping of seismic units and facies reveals that, in addition to glacial sediments derived from the Antarctic mainland, the upper Cenozoic succession includes drift units with prograding sequences building out from the Mac. Robertson Land margin, west of the Prydz Bay. Three contourite drifts grew on the western side of submarine channels and large sediment wave fields suggest a mixed system of turbidity currents influenced by west‐flowing bottom currents. The drifts are composed of four seismic units representing stages of onset (Lower Oligocene), main growth (Early‐Middle Miocene), maintenance (Middle‐Late Miocene) and burial (Pliocene). The internal geometry and reflection patterns of the drifts imply an intensified current activity from the Early to Middle Miocene. The results plausibly reflect that the formation of proto Antarctic Bottom Water (AABW) started around the Eocene–Oligocene boundary and intensified episodically from the early to middle Miocene.

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The Kalkarindji Large Igneous Province was erupted across much of northern and western Australia at ca. 510 Ma, blanketing the region below extensive basaltic lava flows. Within the McArthur Basin in the North Australian Craton, new apatite fission track data provide evidence for heating of the shallow basin during this event. Sub‐surface heating was expressed as short‐lived, high magnitude heating by heat transfer from hot lavas, which subsequently transitioned to longer‐lived heating of the basin by insulation below these basalts.

, Abstract

The McArthur Basin of the North Australian Craton is one of the very few places on Earth where extensive hydrocarbons are preserved that were generated from Mesoproterozoic source rocks, prior to the development of extensive multicellular life. It is, however, unclear precisely when hydrocarbons from these source rocks matured, and if this occurred as a singular event or multiple phases. In this study, we present new apatite fission track data from a combination of outcrop and sub‐surface samples from the McArthur Basin to investigate the post‐depositional thermal history of the basin, and to explore the timing of hydrocarbon maturation. Apatite fission track data and thermal modelling suggest that the McArthur Basin experienced a basin‐wide reheating event contemporaneous with the eruption of the Cambrian Kalkarindji Large Igneous Province in the North and West Australian cratons, during which thick (>500 m) basaltic flows blanketed the basin surface. Reheating at ca. 510 Ma coinciding with Kalkarindji volcanism is consistent with a proposed timing of elevated hydrocarbon maturation, particularly in the Beetaloo Sub‐basin, and provides a mechanism for petroleum generation throughout the basin. Subsequent regional cooling was slow and gradual, most likely facilitated by gentle erosion (ca. 0.01–0.006 km/Ma) of overlying Georgina Basin sediments in the Devonian–Carboniferous with little structural reactivation. This model provides a framework in which hydrocarbons, sourced from Mesoproterozoic carbon‐rich rocks, may have experienced thermal maturation much later in the Cambrian. Preservation of these hydrocarbons was aided by a lack of widespread structural exhumation following this event.

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New detrital zircon provenance data from the Upper Cretaceous Straight Cliffs Formation and equivalent Iron Springs Formation of southern Utah indicates that orogen‐transverse deposition was limited to the most proximal sections within ca. 75 km of the Sevier fold‐thrust belt. The majority of foredeep deposits record long‐distance fluvial transport (>400 km) of zircons from the active Cordilleran magmatic arc and Yavapai‐Mazatzal basement rocks in central Arizona (the Mogollon Highlands). The data suggest that the apex of a large basin‐axial fluvial system may have been located at the structural syntaxis between the Sevier fold‐thrust belt and western margin of the Mogollon Highlands, and point to extrabasinal controls on sediment supply, which was largely decoupled from flexural accommodation controls within the Sevier foreland basin.

, Abstract

Depositional models for retroarc foreland basin systems commonly imply a direct link between thrust belt dynamics and sediment supply, despite documented complexity between orogen‐transverse and orogen‐parallel (axial) sediment routing systems. Previous detrital zircon provenance studies from the Straight Cliffs Formation in the Kaiparowits Plateau of southern Utah indicate primary axial source areas to the south (Mogollon Highlands), and southwest (Cordilleran magmatic arc), with relatively minor transverse input from the Sevier fold‐thrust belt to the west. Complementary data from 32 new samples are presented to investigate whether these trends persist in correlated strata from sections most proximal to the active fold‐thrust belt, as well as strata proximal to the Mogollon Highlands. This study aims to improve statistical robustness through higher‐n datasets (>300 analyses/sample) and addresses the possibility of recycling of zircon grains from strata in the exhumed Sevier fold‐thrust belt. Based on this provenance dataset, we infer that orogen‐transverse deposition was mainly limited to the most proximal sections (within ca. 75 km of the fold‐thrust belt), but included episodic distribution across the foredeep that was associated with distinct intervals of widespread amalgamated sand and gravel sheet deposits. In contrast, more distal parts of the foredeep record long‐distance fluvial transport (>400 km) of zircons from the active Cordilleran magmatic arc as well as Yavapai‐Mazatzal basement rocks in central Arizona (the Mogollon Highlands). Palaeogeographic reconstructions suggest the apex of a large basin‐axial fluvial system may have been located at the structural syntaxis between the Sevier fold‐thrust belt and western margin of the Mogollon Highlands. This interpretation is supported by detrital zircon data presented here, thus extending the possibility that the Palaeogene ‘California River’ may have existed by at least Late Cretaceous time. Ultimately, these data point to extrabasinal controls on sediment supply, which was largely decoupled from flexural accommodation controls in this archetype retroarc foreland basin.

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[Abstract

Reconstructing 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 <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.

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Multi‐sample thermal models and 1D thermal modeling for the Ischigualasto‐Villa Union Basin. Our study exemplifies that peak heating (Triassic) and the onset of the cooling (earliest Late Miocene) cannot be directly associated with peak burial (Cenozoic) and the onset of thrust induced exhumation (younger than 5 Ma), as it is commonly done in thermochronologic studies in sub‐andean basins. Thermal signatures in this basin are decoupled from subsidence and exhumation signals and rather respond to changes in basal heat flow.

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Schematic diagram illustrating the range of gravity‐driven processes and the factors controlling their distribution along a margin,

, Abstract

This paper presents a semi‐quantitative analysis of gravity‐driven deformation along the Namibian margin using extensive 2D depth converted seismic data. The geometries, internal characters and distribution of gravity‐driven systems were investigated through regional and detailed seismic studies. The research shows that surficial slumps are typically ca. 50 m thick and are characterised by contorted seismic facies commonly occurring along the slopes of the margin. They commonly funnel and cluster within high relief areas such as canyons and pre‐existing landslide scars. These contrast with coherent slides that are up to 2 km thick which extend laterally along the margin for tens to hundreds of kilometres. Slides preferentially occur in the proximal part of the margin and are constrained within the main margin depocenters. Here, high sedimentation rates and loading promote the generation of distinct, weak, overpressured layers that favour initiation of sliding of relatively coherent sediment masses. This research also shows that one‐third of volume of the post‐rift sediments on the Namibian margin were affected by slides and slumps. This demonstrates that gravity‐driven deformation is a key geological process that can strongly modify the evolution of rifted passive margins.

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[Abstract

The Santos Basin, offshore Brazil contains a complex set of salt‐tectonic structures, the origins of which are debated, that is, the Albian Gap and the São Paulo Plateau (SPP). The Albian Gap is a ca. 450 km long, 60 km wide feature characterized by a post‐Albian, counter‐regional rollover overlying depleted Aptian salt, and in which the Albian is largely absent. The SPP, located immediately downdip, is defined by a pre‐salt structural high overlain by ca. 2.5 km thick salt. Another prominent feature is the Merluza Graben, a rift‐related depocentre that underlies the southern portion of the Albian Gap and displays significant (3–4 km) base‐salt relief along its main faults. Two competing hypotheses have been proposed to explain the kinematics of these provinces. One invokes post‐Albian extension in the Albian Gap and kinematically‐linked contraction in the SPP. The other invokes post‐Albian salt expulsion in the Albian Gap and salt inflation in the SPP. Recent studies, however, suggest these processes likely alternate in time and space, contributing nearly equally to the evolution of these domains. We apply 3D physical modelling to (i) test this hypothesis; and (ii) to more generally understand how gravity gliding and spreading over three‐dimensionally variable base‐salt relief control regional salt tectonics. The results show a similar salt‐related evolution and structural styles to those proposed in the most recent studies. They also (i) explain the origin of the ca. 25 km wide diapir precursor of the Albian Gap by early salt inflation against base‐salt steps; (ii) show that normal faults with different polarities and rollover types form due to the interplay between gliding and spreading over different base‐salt domains and (iii) provide a mechanism for the origin of strata encased within salt structures. This improves our understanding of the distribution and origin of salt‐related structural styles in worldwide salt basins.

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Regional model sections from (a) North Domain, and (b) South Domain illustrating the main salt‐related structural styles of both domains and their relationship with the updip Merluza Graben, the Albian Gap, where the first post‐salt interval is absent, and the inflated salt and associated fold‐belt formed in the downdip end of the model, over the Sao Paulo Plateau.

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The diffusion coefficient values vary according to the depositional domain. The diffusion assumption is optimal for the deltaic slope domain.

, Abstract

Diffusion‐based stratigraphic models are widely used to simulate sedimentary systems and margin deltas. Diffusion‐based models assume that the topographic evolution primarily depends from its slope. Limited attention has however been given to the calibration of the transport coefficients. Here, we evaluate transport coefficient values from natural examples, the Ogooué and Zambezi rifted margin deltas over the last 5 to 12 Ma respectively. We developed a method to estimate transport coefficients based on high resolution seismic stratigraphy analysis of the stratigraphic architecture of these deltas. For each stratigraphic sequence, we calibrated the sand/shale ratios of the deposits, we restored their depositional slopes, we estimated their uncompacted accumulated volumes and we calculated the transport coefficient (Kd) from the sediment flux/slope ratio. Estimated values of Kd fall within one order of magnitude (×0.1 km²/ka), a much narrower range than previously published values (×0.0001 to ×100 km²/ka). We show that the diffusion approximation is optimal at 10–100 km scale and 0.5–1 Ma time resolution, independently of the stratigraphic context. We show that the diffusion assumption is appropriate for the formation of the clinoforms (mainly gravity driven). It is however not optimal for the shelf and distal domains where additional processes (e.g., wave, flood, hemipelagic, turbidites, oceanic current), not accounted for it the diffusion assumption, significantly impact sediment transport. We documented a significant increase of Kd values after 0.9 Ma, coeval of an increase in the amplitude of eustatic variations at this time indicating that the calibration of Kd from present day sedimentary systems might not be optimal for simulations of sedimentary systems before the last million years.

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[Abstract

Over the last decade, there has been a resurgence of interest in the climatic and tectonic mechanisms that drove the Messinian salinity crisis (MSC) and the associated deposition of thick evaporites. The MSC represents an unprecedented palaeoceanographic change that led to a very short (ca. 640 kyr) ecological and environmental crisis. However, across the Levantine offshore basin, the sedimentological nature of the top evaporitic units and the mechanisms that controlled the transition from a hypersaline evaporitic unit to brackish deposits (final MSC stage 3) are still disputed. Here, we re‐evaluate the deposits associated with the terminal phase of the MSC, named in offshore Lebanon as the Nahr Menashe Unit (NMU). We describe the NMU seismic facies, characterize and map its internal seismic stratigraphy and provide a new interpretation of its depositional environment, which persisted during the late Messinian and then evolved through a regional reflooding event. The base of the NMU overlies semicircular depressions, randomly distributed linear marks and surface collapse features, which are indicative of a period of intense evaporite dissolution. The NMU seismic facies observed from the slope to the deep part of the basin support the interpretation of a layered salt‐evaporite‐sand depositional system subject to complex reworking, dissolution, deposition and final erosion. A drainage network of valleys and complex tributary channels incising into the top NMU shows marked erosional characteristics, which indicate a dominant southwards sediment transfer following deposition of the NMU. The drainage network was subsequently infilled by layered sediments interpreted here to represent the post‐MSC marine sediments. Our analysis adds important details regarding previous interpretations of the NMU as fluvial in origin. Specifically, the presence of subcircular, linear dissolution features coupled with mound‐like features indicates that the NMU is composed dominantly of evaporites that were subject to dissolution prior to erosion associated with the drainage network. The NMU is interpreted to represent the deposition/redeposition of a mixed evaporite‐siliciclastic succession in a shallow marine or lacustrine environment during the tilting of the offshore Lebanese basin.

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Cicular and linear depressions on the bottom of Nahr Menashe, indicating extensive dissolution and erosion processes.

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[Abstract

Passive margin stratigraphy contains time‐integrated records of landscapes that have long since vanished. Quantitatively reading the stratigraphic record using coupled landscape evolution and stratigraphic forward models (SFMs) is a promising approach to extracting information about landscape history. However, there is no consensus about the optimal form of simple SFMs because there has been a lack of direct tests against observed stratigraphy in well‐constrained test cases. Specifically, the extent to which SFM behaviour over geologic space and timescales should be governed by local (downslope sediment flux depends only on local slope) versus nonlocal (sediment flux depends on factors other than local slope, such as the history of slopes experienced along a transport pathway) processes is currently unclear. Here, we develop a nonlocal, nonlinear SFM that incorporates slope bypass and long‐distance sediment transport, both of which have been previously identified as important model components but not thoroughly tested. Our model collapses to the local, linear model under certain parameterizations such that best‐fit parameter values can indicate optimal model structure. Comparing 2‐D implementations of both models against seven detailed seismic sections from the Southeast Atlantic Margin, we invert the stratigraphic data for best‐fit model parameter values and demonstrate that best‐fit parameterizations are not compatible with the local, linear diffusion model. Fitting observed stratigraphy requires parameter values consistent with important contributions from slope bypass and long‐distance transport processes. The nonlocal, nonlinear model yields improved fits to the data regardless of whether the model is compared against only the modern bathymetric surface or the full set of seismic reflectors identified in the data. Results suggest that processes of sediment bypass and long‐distance transport are required to model realistic passive margin stratigraphy and are therefore important to consider when inverting the stratigraphic record to infer past perturbations to source regions.

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We evaluate a simple stratigraphic forward model that incorporates nonlocal marine sediment transport processes and find that it improves on purely local models in its ability to produce observed stratigraphy on the Southeast Atlantic Margin.

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[Abstract

Early synrift half‐grabens, herein named the Chilpo and Sinheung subbasins, have been newly recognised in the north‐central area of the Miocene Pohang Basin, Korea. The subbasins are closely aligned along an array of NNW‐trending strike‐slip faults and are bounded by ENE‐striking normal faults. The sediment infill of the subbasins consists of a deepening‐ and fining‐upward sequence of alluvial cobble to boulder conglomerate (FA1), alluvial to nearshore granule to pebble conglomerate and sandstone (FA2) and hemipelagic mudstone (FA3). The younger strata sequentially onlap the older strata in the hangingwall basement or transfer zones, whereas the footwall basement is directly onlapped by the younger hemipelagic mudstone. These patterns may have resulted from a series of domino‐style block rotations, in which subsidence along normal faults caused the differential creation of accommodation space, whilst simultaneous uplift in the upslope of rotated hangingwall blocks controlled denudation, the shedding of sediments to downslope areas and asymmetric stacking of sediments in both subbasins. In contrast to the western margin of the Pohang Basin, where large footwall‐derived fan‐delta systems developed along the bounding faults, in the Chilpo and Sinheung subbasins, small hangingwall‐derived alluvial‐fan deltas formed during rifting. The development of these domino‐style half‐grabens would have resulted from the detachment and simultaneous transrotation of the crustal block away from the northwest during accelerated extension in the Pohang Basin. Recent geophysical studies have detected very thin basin‐fill sediments in the north‐central Pohang Basin and faults that are similar in orientation to those identified in this study, thereby supporting our observations. These findings confirm the early opening of the Pohang Basin, supporting the hypothesis that all of the Miocene basins in southeastern Korea formed in a setting of NW–SE extension, and further suggest the development of the wedge‐shaped transtensional basin, in response to the opening of the East Sea (Sea of Japan) after 17 Ma.

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Opening mode and morphology of the synrift half‐graben (Chilpo and Sinheung) subbasins and the possible movement of bounding faults during formation of the Miocene Pohang Basin. The opening of the subbasins was initiated along the NNW‐trending strike‐slip faults and extended by subvertical rotation of hangingwall blocks under the NW–SE extensional regime, suggesting a wedge‐shaped transtensional model for the Pohang Basin.

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Amplitude map from the Oligocene–Miocene Marlim unit, which records the stratigraphic evolution of a turbidite system on a dynamic stepped submarine slope with multiple entry and exit points and fill‐and‐spill cycles.

, Abstract

The classic fill‐and‐spill model is widely applied to interpret topographic controls on depositional architecture and facies distributions in slope successions with complicated topography. However, this model implies a constant topographic configuration over the lifespan of a turbidite system. In contrast, the impact on patterns of erosion and deposition above dynamic slopes whose topographic configuration varies spatially over time remains poorly investigated. Here, using high‐resolution 3D seismic reflection data and more than 100 wells from a 40 km long stepped slope system (Campos Basin, offshore Brazil), we document the evolution of a sand‐prone turbidite system active during the Oligocene–Miocene transition. This turbidite system was influenced by vertical and lateral deformation, and we propose a new stratigraphic model to explain the resultant depositional architecture. Two depocentres were identified as steps, with channels on the proximal step, and channel–lobe complexes on the distal step, bounded by sediment bypass‐dominated ramps. Lateral stepping of channels on the proximal step, and oblique stacking of the down‐dip lobe complexes, each cut by through‐going channels, indicate multiple fill‐and‐spill cycles. A persistent north‐east‐ward stepping and thickening on the steps are interpreted to reflect lateral tilting of the seafloor driven by salt tectonics. The dynamic substrate prevented the establishment of a single long‐lived conduit across the proximal step, as recorded in systems with fixed topographic configurations. The filling of through‐going channels with mud at the end of each cycle suggests waxing‐to‐waning sediment supply cycles and periods of sand starvation when the lateral tilting dominated and drove avulsion of the feeder channels towards topographic lows. This study demonstrates that subtle dynamic slope deformation punctuated by discrete sediment supply cycles results in complex stratigraphic patterns with multiple phases, and multiple entry and exit points. Repeated cycles of fill‐and‐spill, tilt‐and‐repeat are likely to be present in other stepped slope systems.

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Block diagram showing fault zone fluid patwhays along conjugate shear fractures, joints, opening mode‐I fractures, stylolite, and veins. Both principal stresses (σ1 > σ2 > σ3) and strains (X > Y > Z) are shown.

, Abstract

We applied field structural data and isotope geochemical (δ¹³C, δ¹⁸O and ⁸⁷Sr/⁸⁶Sr) analyses to understand the relationship among calcite veins, fault damage zones and carbonate host rocks in a thrust fault damage zone in the Achado quarry, Irecê Basin in the São Francisco Craton, NE Brazil. Our results reveal three hydrological packages with different rheological behaviours in a stratified carbonate succession. The upper package includes the Achado fault damage zone that is characterised by interlayered dolomitised grainstones and mudstones. These rocks display high positive δ¹³C values (10‰–13‰), negative δ¹⁸O values (mean −6.34‰) and radiogenic (⁸⁷Sr/⁸⁶Sr) isotope values (0.70885–0.71519). A second package is marked by a cataclastic brittle shear zone lateral parallel to dolograinstones bedding. These rocks show low to positive δ¹³C values (−3.41‰ to +8.85‰), more positive δ¹⁸O values (mean −3.73‰) and radiogenic (⁸⁷Sr/⁸⁶Sr) isotope values (0.71039–0.71373). The lower package is characterised by well‐preserved pristine limestone succession that shows δ¹³C values ranging between −0.46‰ and +3.17‰, mean δ¹⁸O = −5.41‰ and less radiogenic ⁸⁷Sr/⁸⁶Sr values (0.70762–0.70818). In contrast to the upper and intermediate packages, rocks from the lower one exhibit very low permeability and behaved as a seal for fluid migration. Fluid flow occurred several times during basin evolution, for example along syn‐rift fault damage zones, bedding‐parallel carbonate breccia, thrust faults, cataclastic shear zones, synorogenic conjugate shear fractures or joints and opening mode I fracture‐fill calcite veins. These fractures allowed pervasive fluid flow in the porous intermediate cataclastic shear zone where fluids flowed and formed veins, as diffuse fluid flow in randomly oriented fracture swarms, or channelised fluid flow in aligned fracture corridors. They record significant centimetre‐scale to km‐scale hydrological behaviour within carbonate layers. Most carbonates that are associated with veins, fault damage zones and hydraulic breccia were formed by fluids of the same origin with low δ¹³C (−6.0 to −2.0‰) and δ¹⁸O (−6.0 to −8.5‰) values, and more radiogenic ⁸⁷Sr/⁸⁶Sr values compared to the carbonate host rocks.

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