Basin Research - Volume 36, Issue 1, 2024

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This study presents an intergrated tectonostratigraphic framework of the South Korea Plateau (SKP), East Sea using 2D seismic profiles and borehole data. Based on seismic stratigraphy analysis, we proposed a four‐stage structural and sedimentary evolution model of the SKP which provides a window into understanding potential targets for sandstone reservoirs.

, Abstract

The South Korea Plateau (SKP), a typical submarine plateau, preserves an important tectono‐sedimentary evolutionary record and represents a major frontier area for petroleum exploration in the East Sea (Sea of Japan). However, its tectonic mechanisms and their controls on sedimentary fill are underexplored. Here, we present the first integrated tectonostratigraphic framework of the SKP using reprocessed, two‐dimensional, seismic‐reflection profiles and borehole data. Four regional megasequence boundaries are interpreted, delineating four tectonostratigraphic packages: the syn‐rift (MS1), post‐rift phase 1 (MS2), post‐rift phase 2 (MS3) and syn‐compression (MS4) megasequences. We propose a four‐stage structural and sedimentary evolution model for the SKP based on the megasequences and structural development. Stage‐1 (latest Late Oligocene−Early Miocene): the SKP was rifted and extended through block faulting, resulting in the formation of rift basins dominated by fan‐delta and shallow‐lacustrine depositional systems. Stage‐2 (late Early Miocene−Middle Miocene): hemipelagic sedimentation prevailed with gravity‐controlled slope failures under a tectonically stable environment associated with slow thermal subsidence. Stage‐3 (late Middle Miocene−Late Miocene): continued thermal subsidence allowed the predominance of hemipelagic biogenic deposits accompanied by intermittent mass‐wasting‐induced turbidites and resulted in the development of a polygonal fault system. Stage‐4 (Early Pliocene−present): the SKP was influenced by E−W compression caused by an eastward movement of the Eurasian plate. Turbiditic and hemipelagic sedimentation was predominant with turbidity‐flow‐leveed channels derived from direct riverine input or through slope failures. Based on this tectonostratigraphic analysis, we reveal the variation in depositional systems and sand‐dispersal patterns for the SKP, highlighting potential targets for sandstone reservoirs: MS1, fan‐deltas and lacustrine‐fan turbidites; MS3, deepwater fan turbidites; and MS4, deepwater fan turbidites, channel‐levee complexes and turbidite frontal‐splay deposits. This study proposes a structural and sedimentary evolution model for the SKP that could enhance our understanding of reservoir potential for petroleum‐exploration in the future.

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[Abstract

Developing an accurate understanding of the ways in which faults have grown within a particular region and stratigraphy can aid risk management for CO2 storage sites. Areas of fault interaction lead to differences in the stress field, resulting in an increased strain, which is often accommodated by a high intensity of deformation bands and/or fracturing, dependent on host rock properties. These structures alter the permeability surrounding faults. Hence, detecting areas of interaction of structures throughout the fault growth history allows the identification of locations where high risk may occur in terms of the hydraulic properties of a fault zone. The Vette Fault Zone (VFZ), bounding the Alpha prospect within the potential CO2 Smeaheia storage site, Northern Horda Platform, is shown to have grown from a minimum of seven fault segments. By utilising a comparison with the adjacent Tusse Fault Zone (TFZ), we can identify potential areas of high risk, where fluids may have the ability to flow across or along the VFZ. The high seal strength of the TFZ holding back a large gas column is likely to be created by shale juxtaposition and smearing with cataclastic processes. The same could be assumed for the VFZ, associated with similar tectonics and displaced stratigraphy. However, rather than membrane breaching causing fluids to flow across the fault, potential areas of high risk have been identified at locations of relict breached relay zones, where the initial displacement of the intersecting faults and area of overlap was high. These areas appear to correspond with the location of hydrocarbon contact depth (spill point) along the TFZ. Using the same assumptions for the VFZ, we can observe one potential area of high risk, which lies within the area of suggested CO2 accumulation.

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The size of relict breached relay zones has shown to have a potential influence on fault seal integrity, where breached relay zones that had large overlap areas and high throw amplitudes have a greater potential to fracture and allow fluids to flow along/across the fault zone.

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Since the Late Cretaceous, the equatorial margin of Brazil has undergone a rift phase, with the erosion of a moderate rift escarpment, a Late Cretaceous–Paleogene post‐rift phase, with major drainage reorganization and significant vertical erosion, and a Late Oligocene‐to‐Recent post‐rift phase, with moderate vertical erosion and river headwater migration.

, Abstract

The equatorial margin of Brazil is an example of a rift margin with a complex landscape, dominated by an escarpment perpendicular to the continental margin, which testifies to an equally complex rift and post‐rift surface and tectonic evolution. This has been the focus of a long debate on the driving mechanism for post‐rift tectonics and on the amount of exhumation. This study contributes to this debate with new petrographic and thermochronologic data on 152 samples from three basins, Pará‐Maranhão, Barreirinhas and Ceará, on the offshore continental platform. Our detrital record goes back to the rift time at ca. 100 Ma ago and outlines three major evolutionary phases of a changing landscape: a rift phase, with the erosion of a moderate rift escarpment, a Late Cretaceous‐Palaeogene post‐rift phase of major drainage reorganization and significant vertical erosion and a Late Oligocene‐to‐Recent post‐rift phase of moderate vertical erosion and river headwater migration. We estimate that along the equatorial margin of Brazil, over a large onshore area, exhumation since the Late Cretaceous has totalled locally up to 2–2.5 km and since the late Oligocene did not exceed 1 km.

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[Abstract

Tectonic subsidence on rifted, passive continental margins are largely controlled by patterns of extension and the nature of strain partitioning in the lithosphere. The Sunda Shelf, adjacent to the SW South China Sea, is characterized by deep basins linked to regional Cenozoic extension associated with propagating seafloor spreading caused by slab pull from the south. Analysis of seismic reflection profiles and drilled sections crossing the Nam Con Son and Cuu Long basins highlight Oligocene extension, with most of the thinning concentrated in the ductile mid‐lower crust. Upper crustal extension was modest and ductile flow is inferred to be directed northwestwards, towards the oceanic crust. Basin inversion occurred in the Mid Miocene, associated with the collision of the Dangerous Grounds Block and Borneo. Subsequent accelerated tectonic subsidence exceeded predictions from uniform extension models assumed to relate to extensional collapse after inversion. We correlate this to a period of faster erosion onshore driven by strong monsoon rains in Indochina and Peninsular Thailand at that time. Erosion of the onshore basement, inducing rock uplift and coupled with loading of the basins offshore, drives ductile mid‐lower crustal flow, likely to the northeast under Indochina, and/or to the west where Plio‐Pleistocene subsidence of the shelf is very slow. Significant sediment delivery from the Mekong River into the Cuu Long Basin began in the Late Miocene and migrated seawards as the basin filled. Mass balancing suggests that the basins of this part of the Sunda Shelf are filled through erosion of bedrock sources around the Gulf of Thailand. There is no need for sediment delivery from a major river draining the Tibetan Plateau to account for the deposited volumes.

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Maps showing the lateral variations in extension in A) the total crust, B) lower crust and C) upper crust based on the seismic profiles shown as black lines. C) shows the upper crustal extension with the same color scale as the lower and total crustal maps while D) shows the upper crustal extension with an enhanced color scale in order to demonstrate variations in brittle faulting across the area.

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[Abstract

Stratigraphy along the Brazilian Equatorial Margin is a crucial guide to the geodynamic history of rifting of Pangea and formation of the South Atlantic Ocean. Understanding the evolution of the Brazilian Equatorial Margin, which intersects the Saint Paul and Romanche Fracture Zones on the western margin of South Atlantic Ocean, is also key for reconstructing eustatic histories and natural resource exploration. In this study, we quantify the stratigraphic and subsidence histories of three sedimentary basins—Barreirinhas, Ceará, Potiguar—that sit within the margin. Stratigraphy was mapped using ca. 900‐line‐km of two‐dimensional seismic data. Biostratigraphic and check‐shot data from 23 wells drilled on the continental shelf, slope and in the distal parts of these basins were used to date and depth‐convert stratigraphy. Check‐shot data were also used to parameterise compaction. The mapped stratigraphy was backstripped to calculate subsidence histories for the basins. Subsidence curves were decompacted, water‐loaded and corrected for palaeo‐water depths using biostratigraphic data from well reports. The mapped stratigraphy of the Barreirinhas and Ceará Basins and theoretical subsidence curves indicate that stretching factors did not exceed 1.6. These values suggest that these basins can be regarded as failed rifts. In contrast, more distal stratigraphy mapped in the Potiguar Basin to the south indicates that it stretched by a factor of 5–6. Calculated subsidence histories indicate that this basin formed primarily because of Cretaceous rifting and Cretaceous to Recent post‐rift thermal sag, with amplitudes governed by the amount of initial stretching.

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Sketch showing stratigraphy, shape and structure of Barreirinhas, Ceará and Potiguar Basins, highlighting their different tectono‐sedimentary evolution. Barreirinhas and Ceará Basins are considered to be failed rifts, Potiguar Basin formed as a result of Cretaceous rifting and subsequent post‐rift thermal sag.

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[Abstract

Geological process models typically simulate a range of dynamic processes to evolve a base topography into a final two‐dimensional cross section or three‐dimensional geological scenario. In principle, process parameters may be updated to better align with observed geophysical or geological data. However, it is hard to find any process model realisations that fit all observations if data sets are complex and sparse in space or time because the simulations typically depend highly non‐linearly on base topography and dynamic parameters. As an alternative, geophysical probabilistic tomographic methods may be used to estimate the family of models of a target subsurface structure that are consistent both with information obtained from previous experiments and with new data (the Bayesian posterior probability distribution). However, this family seldom embodies geologically reasonable images. Here we show that the posterior distribution of tomographic images obtained from travel time data can be fully geological by injecting geological prior information into Bayesian inference and that we can do this near‐instantaneously by using trained mixture density networks (MDNs). We invoke two geological concepts as prior information about the possible depositional environment of an imaged target structure: a braided river system and a set of marine parasequences. Each concept is parameterised by the latent parameters of a generative adversarial network. Data from a target structure can then be used to infer the family of compatible latent parameter values using either geological concept using MDNs. Our near‐instantaneous MDN solutions closely resemble those found using relatively expensive Monte Carlo methods. We show that while the use of incorrect geological conceptual models provides significantly less accurate results, a classifier neural network can infer which geological conceptual model is most consistent with the data. It is thus demonstrated that even apparently barely related geophysical data may contain information about abstract geological concepts, and that geological conceptual models are key to creating reasonable images from geophysical data.

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The true marine parasequences parameter matrix (left) and the summary statistics for the posterior estimates from the MDN (top) and McMC (bottom). The statistics are the posterior mean, posterior standard deviation, and the travel time misfits.

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This study utilizes regional scale 2D seismic reflection data and presents the crustal and tectonic map of the Florida margin in the Eastern Gulf of Mexico. We propose an updated tectonic model, where the formation of the Seaward Dipping Reflections (SDR) preceded the mantle exhumation and serprentinization. The latter could have been partly covered by an ustable state of magma supply prior to the seafloor spreading.

, Abstract

The Gulf of Mexico is an intraplate oceanic basin where rifting commenced in the Late Triassic, leading to drifting and ensuing oceanic accretion by Middle‐Late Jurassic, which ceased by the Early Cretaceous. Its tectonic evolution encompasses multiple rifting phases dominated by orthogonal extension, variable magmatism and salt deposition. This complex tectonic history is recorded within the rifted margins of the Gulf of Mexico, including along the eastern part of the basin, where considerable uncertainty remains regarding the tectonic evolution and resulting crustal configuration. This study presents new insights into the crustal types and an updated tectonic framework for the Florida margin. An integrated analysis of seismic and potential field data allows us to characterize the nature of the crust, which shows wide zones of hyperextended continental crust, seaward dipping reflection (SDR) packages, exhumed mantle and magmatic crust. Our results propose elements that could improve the plate model of the Gulf of Mexico, by accounting for the polyphase nature of rifting, the counter‐clockwise rotation of the Yucatan block and the observed increase in magmatic supply.

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[Abstract

We investigated calcites and dolomites precipitated during burial diagenesis of the Upper Triassic (Norian) continental siliciclastics from sub‐surface reservoirs of the northern Paris Basin (Chaunoy Formation) that experienced a thermal maximum >100°C during Late Cretaceous times. Relative carbonate precipitation timing was established via petrographic analyses. The diagenetic carbonates were further investigated by fluid inclusion and clumped isotope (Δ47) thermometry. The two thermometric datasets were interpreted by evaluating the possible occurrence of inclusion thermal reequilibration and Δ47 solid‐state reordering, based on the known basin thermal history and the three existing Δ47 reordering models. By considering the fluid inclusion and Δ47 datasets obtained and the various Δ47 reordering models, different carbonate precipitation scenarios, in terms of timing and parent fluid composition (δ¹⁸Ofluid), were inferred. These results underline that in samples having experienced thermal maximum >100°C, accuracy and interpretation of fluid inclusion and Δ47 thermometry data (especially on calcite) may be biased by thermal reequilibration and solid‐state reordering. The results converge towards the need of jointly applying fluid inclusion and Δ47 thermometry on the same carbonate phases to evaluate all the possible precipitation scenarios. The most likely carbonate precipitation scenarios, based on Δ47 thermometry data, point at the precipitation of two calcite phases during Early to Late Jurassic times and of one dolomite phase during the Late Cretaceous. The parent fluids possibly were original formation waters of the Chaunoy Fm. that mixed with brines migrating from the East, where time equivalent evaporitic deposits occur. The proposed precipitation model for calcites and dolomites, involving different pulses of brine migration, and the dominance of calcite phases were not recorded by previous studies on the Upper Triassic units. These latter results may be of interest to evaluate the reservoir potential of the Chaunoy Fm. in this underexplored portion of the Paris Basin.

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Precipitation timing and δ¹⁸Ofluid for the studied diagenetic carbonates obtained from different temperature datasets (fluid inclusions, ∆47).

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(a) Simulated log permeability versus depth (m TVD) for Skarv Field pressure cell with 1000 realizations, compared to published shale permeability data. For own shale samples are smectite and mixed layer conted plotted in percent. (b) Corresponding simulated overpressures versus depth for the same 1000 realizations. Magnitude of measured overpressures observed for the pressure cell is shown in blue. No data for the Lysing Fm was available.

, Abstract

The smectite‐illite transition in shales due to subsidence, temperature changes and diagenesis influences many processes in a sedimentary basin that can contribute to overpressure build up like reducing the shale permeability. The smectite‐rich layers can form sealing barriers to fluid flows that will influence pore pressure prognosis for drilling campaigns, contribute to sealing caprocks for possible CO2 storage and to sealing of plugging and abandonment wells. In this work, we have included the diagenetic smectite‐illite transition into a three‐dimensional pressure simulation model to simulate its effect on pressure build‐up due to reduced shale permeabilities over geological time scale. We have also tested effect of thermal history and potassium concentration on the process of smectite‐illite transition and the associated smectite‐illite correction on permeability. A new smectite‐illite correction has been introduced, to mimic how shale permeability will vary dependent on the smectite‐illite transition. Stochastic Monte Carlo simulations have been carried out to test the sensitivity of the new correction parameters. Finally, a 3D Monte Carlo pore pressure simulation with 1000 drawings has been carried out on a case study covering Skarv Field, and Dønna Terrace offshore Mid‐Norway. The simulated mean overpressures are in range with observed overpressures from exploration wells in the area for the Cretaceous sandy Lysing Formation and for the two Cretaceous Intra Lange Formation sandstones. The simulated smectite content versus depth is in line with published XRD dataset from wells. The corresponding modelled present‐day permeabilities for the shales including the smectite‐illite transition are two magnitudes higher than measured permeabilities on small samples in the laboratory using transient decay method. The measured permeabilities are in the range of 2.66·10⁻¹⁸ to 3.94·10⁻²² m² (2695 to 0.39 nD) for the North Sea database and represent the end members for shales‐permeabilities with the lowest values, since the small samples are selected with no or minor natural fractures. This work shows that by upscaling shale permeabilities from mm‐scale to km scale, natural fractures and sedimentary heterogeneities will increase the shale permeabilities with a factor of two and that by including permeability correction controlled by the smectite fraction, pressure ramp can be simulated due to diagenesis effect in shales.

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Normal faults in Messina Strait define an active conjugate relay zone where strain is transferred along strike between facing normal faults in southern Calabria and northeast Sicily. Stratigraphic and geomorphic data record tectonic narrowing of the strait in the past ca. 2.5 Myr by inward migration of facing normal faults and rapid mantle‐driven uplift.

, Abstract

Messina Strait is a narrow fault‐bounded marine basin that separates the Calabrian peninsula from Sicily in southern Italy. It sits in a seismically active region where normal fault scarps and raised Quaternary marine terraces record ongoing extension driven by southeastward rollback of the Calabrian subduction zone. A review of published studies and new data shows that normal faults in the Messina Strait region define a conjugate relay zone where displacement is transferred along strike from NW‐dipping normal faults in the northeast (southern Calabria) to the SE‐dipping Messina‐Taormina normal fault in the southwest (offshore eastern Sicily). The narrow marine strait is a graben undergoing active subsidence within the relay zone, where pronounced curvature of normal faults results from large strain gradients and clockwise rotations related to fault interactions. Based on regional fault geometries and published age constraints, we infer that normal faults in southern Calabria migrated northwest while normal faults in NE Sicily migrated southeast during the past ca. 2–2.5 Myr. This pattern has resulted in tectonic narrowing of the strait through time by inward migration of facing normal faults and rapid mantle‐driven uplift.

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[Abstract

The seafloor morphology reflects both past and on‐going sedimentary, oceanographic and tectonic processes. Vertical movement is one of the drivers responsible for reshaping the seafloor through forming steep flanks that decrease slope stability, favour landslides, change current paths, form minibasins and control the sediment deposition, distribution and geometry. Here, we make use of these interactions to derive vertical movements and constrain the active tectonic processes at the western termination of the upper Calabrian accretionary wedge from the integrated analysis of bathymetric, backscatter, surface attributes and high‐resolution reflection seismic data. Within this area, we identify two types of deformational features and mechanisms that affect the depositional, erosional and tectonic processes at different scales. These include the deviation of channels, landslide scars, mass transport deposits (MTDs), separated drifts, sediment waves, lineaments and offset seafloor structures. The first type (long‐wavelength uplift) is an uplifted 22‐km‐wide region, in which seismic onlap relationships and the dip of deep reflectors suggest long‐lasting but slow tectonic uplift affecting sedimentation, and the second type (short‐wavelength uplift) includes three narrow elongated structures and one circular dome encircling the first region of uplift. We interpret that the first type of uplift feature was caused by tectonic deformation, while the second type is interpreted as formed by the fast uplift, tilting and faulting of modern sediments caused by diapirism due to rapid sedimentation in response to the first tectonically driven uplift. The study provides insight into the complex interaction of tectonic and sedimentary processes in the upper Calabrian accretionary wedge.

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Reconstruction of thesedimentary processes active on the basin before, during, and after thevertical movement of the long‐wavelength uplift (Central Area) and the short‐wavelengthuplift (diapirs D1 to D4). This model is based on the interpretation ofgeomorphological proxies.

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Uninterpreted and seismic lines, with the mapped stratigraphic surfaces and tectono‐stratigraphic intervals. Some reflectors are traced in black to evidence syn‐, post‐ and syn‐ to post‐tectonic configurations (sensu Péron‐Pinvidic et al., 2007).

, Abstract

Rift basins typically comprise three main tectono‐stratigraphic stages; pre‐, syn‐ and post‐rift. The syn‐rift stage is often characterised by the deposition of asymmetric wedges of growth strata that record differential subsidence caused by active normal faulting. The subsequent post‐rift stage is defined by long‐wavelength subsidence driven by lithospheric cooling and is typified by the deposition of broadly tabular stratal packages that drape any rift‐related relief. The stratigraphic contact between syn‐ and post‐rift rocks is often thought to be represented by an erosional unconformity. However, the late syn‐rift to early post‐rift stratigraphic record is commonly far more complex since (i) the associated tectonic transition is not instantaneous; (ii) net subsidence may be punctuated by transient periods of uplift; and (iii) strain often migrates oceanward during rifting until continental breakup is achieved with crustal rupture. Previous publications on the Eastern Brazilian marginal basins have not historically used the tripartite scheme outlined above, with the post–pre‐rift interval instead being subdivided into rift, sag and passive margin tectono‐stratigraphic stages. In addition, the sag stage has been previously described as late syn‐rift, early post‐rift or as a transition between the two, with the passive margin stage being equivalent to the classically defined post‐rift, drift stage. Two (rather than one) erosional unconformities are also identified within the rift‐to‐sag succession. In this work, we use 2D and 3D seismic reflection and borehole data to discuss the expression of and controls on the syn‐ to post‐rift transition in the shallow and deep water domains of the south‐central Campos Basin, south‐east Brazil. We identified three seismic–stratigraphic sequences bounded by unconformities, named lower and upper pre‐salt and salt. The lower pre‐salt interval is characterised by wedge‐shaped packages of reflections that thicken towards graben and half‐graben‐bounding normal faults. This stage ends with the development of an angular unconformity, inferred to form as a result of the onset of the oceanward migration of deformation. The upper pre‐salt is typically defined by packages of subparallel and relatively continuous reflections that are broadly lenticular and thin towards fault‐bound basement highs, but that locally contain packages that thicken against faults. The pre‐salt to salt contact is defined by an erosional unconformity that is largely restricted to basement highs, and which is inferred to have formed due to base‐level fall and uplift associated with local fault reactivation, resulting in the formation of channels of possible fluvial origin. Based on its geometries and seismic facies, we conclude that the lower pre‐salt interval is syn‐rifting and syn‐tectonic, deposited during active continental extension and upper crustal faulting affecting the entire evolving margin, whereas the overlying upper pre‐salt is syn‐rifting and post‐tectonic in the Campos Basin, deposited when extension and faulting had migrated seaward to the future location of the spreading centre. The results of our study support the arising notion that the syn‐rift sequence does not only display syn‐tectonic sedimentary packages, and thus the tripartite tectono‐stratigraphic model for rift development is too simplistic and cannot be applied when assessing rifts in the context of the regional development of continental margins.

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Abstract

The Patagonian Andean foreland system includes several intermountain basins filled with a Miocene non‐marine record deposited under syn‐tectonic conditions related to the Andean uplift and a regional climate change triggered by a rain shadow effect. Many of those basins, such as the Collón Cura basin in Neuquén Province, Argentina, present a well‐preserved fluvial record (i.e. the Limay Chico Member of the Caleufú Formation). Sedimentological and palaeomagnetic studies have allowed the interpretation of coeval transverse distributary fan and axial mixed‐load fluvial systems deposited between 10.6 ± 0.2 and 12.8 Ma. The basin infill arrangement shows that, while the axial mixed‐load fluvial system exhibits an aggradational stacking pattern, the transverse distributary fluvial fan system denotes three different orders of stratigraphic patterns: (i) large‐scale progradation of the transverse fluvial fan system over a time scale of 10⁶ year; (ii) intermediate‐scale progradational–retrogradational transverse intra‐basinal fluvial fan episodes over a time scale of 10⁵ year; and (iii) small‐scale transverse lobe progradation over a time scale of 10⁵–10⁴ year. These patterns were interpreted as transverse sediment flux variations triggered by variable external forcings. To decouple those forcings, we estimated the Collón Cura basin equilibrium time at 3–5 × 10⁵ year and compared it with the time scale over which different external forcings varied in the Patagonian Andean and foreland regions during Miocene times. Large‐scale progradation is linked to an increase in sediment flux triggered by a long‐term tectonically driven exhumation forcing associated with the Miocene Patagonian Andean contractional phase. Intermediate‐scale progradational–retrogradational episodes are linked to variations in sediment flux due to a mid‐term tectonic forcing associated with the western fault system activity. The small‐scale fan lobe progradation is related to increases in sediment flux triggered by indistinguishable short‐term autogenic processes and/or high‐frequency tectonic and climatic forcings. This contribution shows the applicability and limitations of the basin equilibrium time concept to decouple external forcings from the geological record, considering their magnitude, nature and time scale, as well as the basin characteristics.

[Abstract

The northern South China Sea (SCS) margin evolved from the Mesozoic convergent to Cenozoic divergent continental margin, and thus, it developed on a heterogeneous crystalline basement with inherited Mesozoic structures. Pre‐existing structures and their interactions with rift faults have historically not been described or interpreted in the intensely stretched Baiyun sub‐basin. Large‐scale 3D seismic reflection data allow us to identify four types of Mesozoic tectonic fabrics within the basement and explain their genesis: (1) Thin, isolated and north‐dipping seismic reflections 1, interpreted as thrust faults representing orogenic processes. Tilted thick seismic reflections 2 are formed by reactivation of seismic reflections 1 during post‐orogenic extension, which are all related to the NW‐ward subduction of the palaeo‐Pacific plate. (2) Thin, isolated and shallowly dipping seismic reflections 3 and low‐amplitude, semi‐transparent and chaotic seismic reflections 4 represent the low‐angle thrust system and the associated nappe units, which are related to the shift from NW‐ to NNW‐ward subduction of the paleo‐Pacific plate. Subsequently, we investigate the structural interaction between Mesozoic intra‐basement and Cenozoic rift structures. Syn‐rift, post‐rift and long‐term faults are developed in Cenozoic strata, and quantitative statistical and qualitative analyses revealed two main types of structural interactions between them and underlying intra‐basement structures: (1) Rift faults develop with inheritance of intra‐basement structures, including fully and partially inherited faults. (2) Rift faults modify intra‐basement structures, although they are controlled by intra‐basement structures at an earlier stage. Finally, our results reveal the control of pre‐existing structures on the geometry of the Baiyun sub‐basin, especially the selective reactivation of NE‐trending shear zones (SR2), which are influenced by the regional stress field and the width and dip of the shear zones.

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Pre‐existing structures and their types of interaction with rift faults during the evolution of the northern South China Sea from a convergent to a divergent continental margin.

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[Abstract

Basin‐scale outcrop analyses of fluvial architecture in the Palaeogene San Juan Basin, New Mexico, document lateral and vertical trends in channel, floodplain and palaeosol characteristics. Herein, the uppermost part of the Palaeocene Nacimiento Formation and lower Eocene Cuba Mesa and Regina Members of the San Jose Formation are identified as deposits of large fluvial fans based on trends observed across the basin. Stratigraphic trends suggest two packages originated by fluvial fan progradation. Progradation of the lower fan system provides a new explanation for the transitional nature of a disconformity at the Nacimiento–San Jose Formation contact, previously thought to be a low‐angle unconformity. The two fan systems are separated by a retrogradational interval that culminates in a depositional hiatus at the contact between the Cuba Mesa and Regina Members. This, combined with poor age constraints, indicates that the duration of the disconformity at the base of the Cuba Mesa Member may have been overestimated. Furthermore, the succession is interpreted as deposits of variable‐discharge rivers, based on the combined abundance of upper flow regime and high deposition rate sedimentary structures indicative of intense flooding events, preservation of in‐channel bioturbation and paedogenic modification indicating periods of prolonged dryness, lack of identifiable bar strata and alternations of poorly drained and well‐drained floodplain deposits with pedofacies indicating alternating wet–dry cycles. This dataset adds to a growing body of evidence linking the formation of large fluvial fans to discharge variability and thus to hydroclimates with significant inter‐ and intra‐annual precipitation variability and intense rainfall. A long‐term stratigraphic shift from poorly drained to well‐drained floodplain deposits across two progradational fan successions indicates that a predictive model suggesting downstream decreases in soil drainage conditions is not encompassing of all large fan systems, and instead suggests a transition to a more arid climate across the Palaeocene–Eocene boundary.

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Examples of architectural styles observed in the study area and a comparison with proximal, medial, and distal areas of a modern fluvial fan. (a) Amalgamated channel fills and minor floodplain, interpreted as proximal fan deposits. The degree of channel amalgamation is highest in the most proximal sector where the fan area is smallest, and the aggradation rates and channel‐return frequency are highest. (b) Heterolithic channel fills and floodplain, interpreted as medial fan deposits. Increased spacing between channel deposits and decreased channel belt size are attributed to losses from infiltration and to the medial fan's larger area, allowing for greater preservation potential for mudrock‐prone floodplain deposits. (c) Isolated channel fills and dominant floodplain, interpreted as distal fan deposits. The more isolated channels are thought to result from a continued widening of the area covered by the fan and additional decreases in discharge due to losses from infiltration (d) Satellite image obtained from Google Earth of a 60‐km long fluvial fan in the Taklamakan Desert, Xinjiang, China with approximate locations of modern proximal, medial and distal fan annotated.

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