Basin Research - Volume 35, Issue 4, 2023

We acquired high‐resolution (each 2 cm) magnetic susceptibility (MS) data of latest Sinemurian through the earliest Aalenian time interval from the Sancerre‐Couy core. The ~20 Myr long Early Jurassic record captures long‐period cyclicities of 1.6, 3.4 and 7–8 Myr. The shortened late Jurassic orbital periods reflect the chaotic orbital motion of the inner planets.

We present high‐resolution (every 2 cm) magnetic susceptibility (MS) data from the Sancerre‐Couy drill core (Paris Basin), spanning the latest Sinemurian to the earliest Aalenian (Early Jurassic). This record allows to build a 20‐million‐year cyclostratigraphic interval using the stable 405 kyr (g2–g5) orbital eccentricity cycle and to focus on long‐period cyclicities and their potential implications for the chaotic diffusion in the inner Solar System and sea‐level changes. Time series analysis indicates evidence of two long‐period cyclicities of 1.6 and 3.4 Myr. These Early Jurassic cyclicities likely correspond to the Cenozoic orbital cyclicities of 2.4 Myr (g4–g3) and 4.7 Myr eccentricity terms. Shortening of eccentricity terms during the Early Jurassic is potentially related to the chaotic orbital motion of the inner planets expressed in the resonant argument θ = 2(g4–g3) − (s4–s3). The 1.6 Myr (g4–g3) cycle matches the third‐order eustatic sequences, whereas the 3.4 Myr cycle has no equivalent in the reference eustatic chart. These cycles of several million years are superimposed a cyclicity of ca. 7.5 Myr, which may correspond to the eccentricity term of 9.5 Myr, previously detected in the Cenozoic. Such cyclicity matches the global, ‘shorter’ second‐order sea‐level sequences and is strongly documented in the sedimentological and mineralogical proxy data, hence supporting the potential key role of orbitally paced climate and sea‐level changes at this timescale.

Heat flow estimated during the IODP Expedition 385 into the Guaymas Basin show that values corrected for sedimentation are between 119 and 1003 mW/m². Heat is dissipated by conduction for plate ages greater than 0.2 Ma. Off‐axis sill intrusion is being cooled down by hydrothermal circulation.

Heat flow is estimated at eight sites drilled int the Guaymas Basin, Gulf of California, during the International Ocean Discovery Program Expedition 385. The expedition sought to understand the thermal regime of the basin and heat transfer between off‐axis sills intruding the organic‐rich sediments of the Guaymas Basin, and the basin floor. The distinct sedimentation rates, active tectonics, and magmatism make the basin interesting for scientific discoveries. Results show that sedimentation corrected heat flow values range 119–221 mW/m² in the basin and 257–1003 mW/m² at the site of a young sill intrusion, denominated Ringvent. Thermal analysis shows that heat in the Guaymas Basin is being dissipated by conduction for plate ages >0.2 Ma, whereas younger plate ages are in a state of transient cooling by both conduction and advection. Drilling sites show that Ringvent is an active sill being cooled down slowly by circulating fluids with discharge velocities of 10–200 mm/yr. Possible recharge sites are located ca. 1 km away from the sill's border. Modelling of the heat output at Ringvent indicates a sill thickness of ca. 240 m. A simple order‐of‐magnitude model predicts that relatively small amounts of magma are needed to account for the elevated heat flow in non‐volcanic, sediment‐filled rifts like the central and northern Gulf of California in which heating of the upper crust is achieved via advection by sill emplacement and hydrothermal circulation. Multiple timescales of cooling control the crustal, chemical and biological evolution of the Guaymas Basin. Here, we recognize at least four timescales: the time interval between intrusions (ca. 10³ yr), the thermal relaxation time of sills (ca. 10⁴ yr), the characteristic cooling time of the sediments (ca. 10⁵ yr), and the cooling of the entire crust at geologic timescales.

Seismic interpretation and structural measurements along fault complexes to derive the amount of Late Cretaceous–Palaeocene extension in the Lofoten‐Vesterålen margin in the context of breakup evolution in NE Atlantic.

Late Cretaceous–Palaeocene continental extension within the Lofoten‐Vesterålen margin is investigated by integrating 2D‐3D seismic and potential field datasets, together with updated crustal transects. Most of that deformation is recorded by two low‐angle detachment structures named West Røst High Fault Complex (WRHFC) and North Utrøst Ridge Fault Complex (NURFC) located at the southern and central‐northern portions of the studied area, respectively. Multiple extensional episodes of various intensities were mapped as different fault stages, including one Albian‐Cenomanian phase, four early Late Cretaceous phases, three Late Cretaceous to latest Cretaceous–Palaeocene phases, and one Palaeocene phase. The WRHFC is narrower in extent and has accommodated a relatively greater amount of localized extension (ca. 18–19 km), whereas the NURFC occupies a wider area with widespread extension intensity (ca. 6–8 km). In comparison, the total across‐margin average extension within the southern, central, and northern portions of the Lofoten‐Vesterålen and NE Greenland conjugate margins are ca. 192, 221, and 266 km, respectively. Such results indicate an apparent extension discrepancy between derived extension from measured fault‐block geometries within the fault complexes and the whole conjugate margin system, with only ca. 11% and 13% of the extension seen on the studied seismic profiles on the WRHFC and NURFC, respectively. The corrected maximum extension for purely sub‐seismic resolution faulting on both the WRHFC and NURFC fault complexes is ca. 164 and 46 km, respectively. Finally, both WRHFC and NURFC structures provide key evidence for a ductile mode of deformation towards breakup that is expressed through shear zones‐rift topography interactions with overlaying listric/detachment faults. These features reflect the resulting multiphase tectonic evolution across the asymmetric Lofoten‐Vesterålen and NE Greenland conjugate margins, and the obliquity in the breakup axis location along them. The study outcomes are pertinent and applicable to understand the breakup evolution of the northern NE Atlantic and its vicinity.

Deformation characteristics of the strain localization zone in the Hailar Basin.

The spatial strain variations while continental lithospheric extension progresses and leads to either a failed rift basin or a fully rifted passive margins have been extensively studied. However, determining the spatial kinematics of rift basins throughout their entire temporal evolution at rift scale is often difficult due to the sparsity of spatial data coverage; this limits our current understanding of rift evolution and our ability to ascertain the controlling factors on basin development. This study uses extensive high‐quality 2D and 3D seismic reflection data and borehole data from the Hailar Basin (northeast Asia) to investigate the spatial strain variations that occurred during multiphase rifting at both high resolution and rift scale. By coupling fault evolution and depocenter analysis, we demonstrate that the Hailar Basin experienced complex strain partitioning with a progressively eastward migration of strain and eventual strain localization onto rift axis. This strain localization occurred in the area that had been dominated by syn‐rift sagging. We propose that the observed eastward strain migration has been driven by the multiphase rollback of the Paleo‐Pacific slab. Contrastingly, during the final rift phase, crustal rheology played the dominant role in rift development, while the influence of geodynamic processes significantly decreased, resulting in the cessation of the eastward rift migration and the ultimate strain localization. This pattern of strain migration and localization is likely to be applicable to other multiphase rifts where the main controlling factors varied through time.

The provenance of Early Cretaceous sandstones dredged on the northern margin of the Demerara Plateau, offshore French Guiana and Suriname, reveals the sediment routing system that prevailed through the Equatorial Atlantic rifting. Fission‐track analysis and U–Pb dating of 310 and 111 detrital zircons, respectively, have been performed. Microfacies analysis and inherited cooling ages suggest that the sandstones were deposited in shallow marine environments during the Early Cretaceous, before the Late Albian drowning of the marginal plateau. Most of the U–Pb zircon crystallisation ages are comprised of between 700 and 600 Ma and are attributed to the Pan‐African‐Brasiliano orogeny. Statistical and chronological evidence suggest that the zircon fission‐track cooling ages were inherited from source materials. Triassic peak ages (>50% of the population) are attributed to the early phase of Central Atlantic rifting. One sample records a cooling phase at ca. 170 Ma, presumably following volcanic hotspot activity and the opening of the Central Atlantic Ocean. Two other samples record the rapid exhumation of the French Guiana transform margin during the Equatorial Atlantic rifting (127 ± 11 and 106 ± 8 Ma). We propose a source‐to‐sink model in which the Pan‐African‐Brasiliano basement of the margin was eroded as a result of flexural uplift along the French Guiana margin, and the detrital material funnelled in the Cacipore graben sustained the Early Cretaceous syn‐rift sedimentation on the marginal plateau.

Microfacies, fission‐track analysis and U–Pb dating of detrital zircons shed light on the provenance of Early Cretaceous sandstones dredged on the northern margin of the Demerara Plateau, offshore French Guiana and Suriname, revealing the sediment routing system that prevailed through the Equatorial Atlantic rifting.

For basins that evolve adjacent to large‐magnitude normal faults, tectonic controls on sedimentation involve isostatic back rotation of an exhuming footwall and, commonly, the evolution of kilometre‐scale extension‐parallel folds. Based on observations from classic localities in western Norway, we propose a three‐stage evolution scenario for transtensional supradetachment basins where the basins become progressively re‐arranged because of core complex exhumation and subsequent orthogonal shortening. Extension‐parallel transverse synclines initially form due to a normal displacement gradient, but when displacements accumulate beyond a certain magnitude, the hanging wall increasingly responds to core complex exhumation and the original depocentre, formed close to the original area of maximum displacement, will become inverted and dismembered above the core complex. Two new synclinal depocentres will develop along the flanks. Because these synclines form by extensional fault growth rather than by shortening, they will be associated with widening of the basin and onlap onto basement at high angles to the maximum elongation trend with overall grain‐size decrease and retrogradational stacking patterns. Further, because these synclines grow away from the evolving core complex, sedimentary units will become asymmetrically distributed inside each syncline in such a way that the oldest deposits in the syncline will be preserved on the flank most proximal to the core complex. In transtensional environments, a third evolutionary stage may involve constrictional strain where extension‐parallel folds and reverse faults produced by orthogonal shortening enhance or interact with other structures. Ultimately, initial extensional sub‐basins may become warped across extension‐parallel folds. Hanging wall deformation will be manifested in shifting accommodation patterns, with depocentres that generally migrate in the direction of the detachment fault. Accommodation patterns initially related to megafault growth may conceptually evolve into depocentres controlled by orthogonal shortening.

The 3D evolution of extensional detachment faults results in core complex exhumation in the area of maximum displacement and an associated basin architecture reflecting the growth of synclinal depocentres away from the core complex.

We investigate the complex evolution of the northern intracontinental North German Basin and elucidate the impact of regional tectonics on salt movement in Permian‐to‐Jurassic times. We describe the onset and characteristics of salt movement throughout the study area and explain observed isolated diapirism at the basin margin.

The formation and structural evolution of complex intracontinental basins, like the North German Basin, mark fundamental earth processes. Understanding these is not only essential to basic research but also of socioeconomic importance because of the multitude of resources, potential hazards, and subsurface use capability in such basins. As part of the Central European Basin System, major subsidence and structural differentiation affected the Baltic sector of the North German Basin in Permian‐to‐Jurassic times. A dense network of high‐resolution 2D seismic data together with nearby wells allow the creation of regional maps with refined stratigraphic subdivision of unprecedented spatial resolution covering the bays of Kiel and Mecklenburg (Baltic Sea). Cross sections along the basin margin allow reconstruction of the structural evolution of the Zechstein salt and its overburden. At the northern basin margin, near the Kegnaes Diapir, thinning of the Buntsandstein and divergent reflectors indicate Early Triassic faulting and salt movement. In the Late Triassic, tectonic activity increased as expressed by the onset of salt movement in the north‐eastern Glückstadt Graben, major growth of the Kegnaes Diapir and faulting at the north‐eastern basin margin during deposition of the Keuper (Erfurt, Grabfeld, Stuttgart and Weser formations). At the north‐eastern basin margin, we interpret the accumulation of Keuper and Jurassic deposits as an infill of a local sub‐basin bordered by the Werre Fault Zone and Agricola Fault System. Between the Glückstadt Graben and the north‐eastern basin margin, the Eastholstein–Mecklenburg Block formed a more stable area, where salt movement first began during the latest Triassic. In the peripheral part of the basin, salt movement was triggered by thin‐skinned extension associated with thick‐skinned faulting within the axial parts of major graben systems. Indications for gravity gliding are absent. Reactive diapirism is restricted to the basin margin, where reduced overburden thickness and Late Triassic erosion allowed diapiric breakthrough.

Challenges and opportunities for subsurface storage in the different salt structures described in this study.

Halite beds in the upper Permian Zechstein Group represent an opportunity for the future development of underground storage caverns. However, geological factors such as lithological heterogeneities, cap rock characteristics and depth can affect the sealing capacity and the integrity of the cavern or contaminate the stored fluid. The main objective of this paper is to evaluate these factors focusing on the compositional variation of the Zechstein Group in different salt structures in the Norwegian North Sea, and related opportunities and challenges for salt cavern storage. Based on deformation style, geometry, height and thickness of the salt structures, we have divided the Zechstein Group into four main categories: (1) thin beds, which can be either carbonate‐anhydrite or clastic dominated. Halite is absent and therefore there is no potential for the development of salt caverns. (2 and 3) bedded to weakly deformed evaporites and intermediate size salt structures, where thick halite beds of more than 300 m are present, but they are usually deeper than 2000 m. Lithological heterogeneities in the halite consist of a mix of competent and incompetent (K‐Mg salts) lithologies. (4) Tall diapirs, characterized by shallower structures (<2000 m), with large deformation and poor seismic image. Thin layers of incompetent K‐Mg salts are observed in these diapirs. The composition, thickness and deformation of the cap rock vary greatly in the area. Thick halite beds are recognized in most salt structures, suggesting an opportunity for underground storage. The challenges are related to the depth of the halite, amount and type of heterogeneities, characteristics of the cap rock and deformation in the different salt structures. These results also have implications for the distribution of reservoir and source rocks, and the evolution of the Northern Permian Basin.

Contrary to widely used traditional sequence‐stratigraphic models, highstand sublacustrine fans developed in the deep‐water environment in the middle of the third member of the Eocene Shahejie Formation (Es3z, 42–40.5 Ma), Dongying Sag. Using four cores and 3D seismic data, we describe the sequence stratigraphy frameworks, sediment characteristics, depositional elements, and depositional model of the Liangjialou highstand sublacustrine fan. The Es3z is a complete third‐order sequence stratigraphic, bounded by T4 at the top and T6 at the base. Between T4 and T6, there is the maximum flooding surface (MFS) distributed from proximal to distal of the system in the third‐order stratigraphic sequence. Therefore, the Liangjialou Fan at the top of the Es3z, between MFS and T4, is a typical highstand sublacustrine fan as determined from seismic data. This highstand sublacustrine fan is characterised by gravel to fine‐grained sand, abundant amalgamation surfaces, and crude stratifications common in coarse‐grained structureless‐ to normal‐ graded sandstones. Climbing ripples and thin laminae rich in plant debris are common in inverse‐then normal‐graded sandstones. Channel‐levee, channel‐lobe transition zone, and lobes developed in this highstand sublacustrine fan, with clear internal progradation stacking patterns caused by hyperpycnal flow. The sudden increase in sediment supply in an accommodation decrease situation and the narrow shelf width are the primary controlling factors for forming highstand sublacustrine fans. In the late stage of the highstand systems tract, gravity‐flow deposits caused by hyperpycnal flow are developed from another new point source to form this highstand sublacustrine fan. Hyperpycnal flow in lacustrine basins can transport coarse‐grained sediments with the help of strong flood energy and steep slopes. Gravity flow deposits in lacustrine basins are not confined by sequence stratigraphy frameworks, and many additional areas of sand caused by hyperpycnal flow could be found in lacustrine basins.

The Liangjialou Fan at the top of the Es3z is a typical highstand sublacustrine fan. Channel‐levee, channel‐lobe transition zone, and lobes developed in this highstand sublacustrine fan, with internal progradation caused by hyperpycnal flow. The sudden increase in sediment supply are the primary controlling factors for forming highstand sublacustrine fans.

Outcrops of coarse‐grained, high‐angle clinoform sets are mainly thought to represent Gilbert‐type deltas. Superficially similar clinoform sets may, however, form in marine, sub‐wave base settings. They are interpreted to have formed as a result of storms where downwelling, seaward‐directed currents transported sand from the coastal area and shoreface across the shelf in suspension or as bedload to be deposited as clinothems. An additional transport of sand took place by strong coast‐parallel currents. The clinoform sets appear to be associated with rift events, the creation of accommodation space and an increasing supply of coarse‐grained sediment. A major protracted rift phase was initiated in East Greenland in Middle Jurassic times and intensified through the Late Jurassic to reach a climax close to the Jurassic–Cretaceous boundary. Rifting caused uplift of borderlands, creation of accommodation space and development of shallow marine shelves passing offshore into submarine slopes and deeper basinal areas. Each rift event was accompanied by the formation of clinoform sets prograding seawards towards the east and southeast away from the cratonic coastline in water depths below the wave base. The clinoform sets are interpreted as typical motifs for rift events in the relatively shallow epeiric Jurassic seaway between East Greenland and Norway. Outcrops of such sets represent a little‐known, commonly misinterpreted sedimentary system and may serve as motifs for rifting in shallow marine areas elsewhere in the geological record. Similar sets have been recorded in outcrop from the Mediterranean, and elsewhere and are probably more common than hitherto realized.

Coarse‐grained, high‐angle clinoform sets, up to 50 m thick, formed in marine, sub‐wave base settings described from outcrops in East Greenland. This, somewhat neglected sedimentary system may represent a typical motif for rift events in the Jurassic seaway of the North Atlantic Realm.

Fluvial architecture in Palaeozoic foreland basins in the Appalachians is used to reconstruct drainage evolution and, thereby, provides insights on progressive terrane accretion in the hinterland. Drainage basins increased in size from the Late Ordovician (Taconic) to Late Devonian (Acadian) then decreased in size in the Pennsylvanian (Alleghanian).

Source‐to‐sink studies typically utilize the sedimentary record preserved in basins to infer source area parameters such as estimates of drainage area and discharges of fluvial systems in the hinterland, the characterization of which remains elusive as few geomorphic elements of the hinterland are preserved. Clastic wedges within foreland basin settings often contain fluvial deposits, and the scale of fluvial architectural elements can be used to estimate drainage area and discharge within the accreted hinterland. This study uses thicknesses of fluvial architectural elements within the Late Ordovician Taconic, the Late Devonian‐Early Mississippian Acadian, and the Late Mississippian to Early and Middle Pennsylvanian Alleghanian clastic wedges, together with climatic conditions interpreted from palaeosol data, to estimate the drainage areas and discharges of the river systems which drained the hinterlands of the accreted Taconic and Acadian terrains and the Alleghanian suture of Laurentia and Gondwana. Published detrital zircon age spectra provide insight into ages of source areas of sediments as well as the timing of terrane accretion. The increase in hinterland drainage areas calculated for Late Devonian Acadian Orogeny river systems compared to Late Ordovician Taconic Orogeny river systems can be attributed to the increase in size of accreted terranes to the east of the foreland basins during that period of time. The decrease in hinterland drainage area from the Late Devonian Acadian Orogeny to the Middle to Late Pennsylvanian Alleghanian Orogeny is attributed to the westward migration of the drainage divide as the Alleghanian orogeny proceeded. It is inferred that the drainage divide migrated towards the drier leeward side of the mountain range and caused drainage basins to shrink and split into smaller drainage basins.

Sediment mass‐balance analysis provides key constraints on stratigraphic architecture and its controls. We use the data‐rich Middle Jurassic Brent Delta sediment routing system in the proto‐Viking Graben, Northern North Sea, to estimate sediment budgets and mass‐balance between source areas and depositional sinks. Published studies are synthesised to provide an age‐constrained sequence stratigraphic framework, consisting of four previously defined genetic sequences (J22, J24, J26, J32). Genetic sequence J32 (3.9 Myr) records transverse progradation of basin‐margin deltas, sourced from the Shetland Platform to the west and Norwegian Landmass to the east. Genetic sequences J24 (1.1 Myr) and J26 (0.9 Myr) record the rapid progradation and subsequent aggradation of the Brent Delta along the basin axis, sourced from the uplifted Mid‐North Sea High to the south, and the western and eastern source regions. Genetic sequence J32 (2.2 Myr) records the retreat of the Brent Delta. Sediment budgets for the four genetic sequences are estimated using palaeogeographical reconstructions, isopach maps, and sedimentological analysis of core and well‐log data. The estimated net‐depositional sediment budget for the mapped Brent Delta system is 2.0–2.8 Mt/year. Temporal variations in net‐depositional sediment budget were driven by changes in tectonic boundary conditions, such as the onset of uplift before the deposition of genetic sequence J24. Over the same time period, the Shetland Platform, Norwegian Landmass and Mid‐North Sea High source regions are estimated to have supplied 2.3–5.6, 5.0–14.1, and 2.8–9.4 Mt/year of sediment, respectively, using the BQART sediment load model and independent geometrical reconstruction of eroded volumes, which are constrained by isostatic uplift estimates based on the geochemistry of syn‐depositional volcanic rocks. The net‐depositional sediment budget in the sink is an order‐of‐magnitude smaller than the total sediment budget supplied by the source regions (13.9–23 Mt/year). This discrepancy suggests that along‐shore transport by wave‐generated currents into the coeval Faroe‐Shetland Basin and/or down‐dip transport by gravity flows into the coeval western Møre Basin played a key role in redistributing sediments away from the Brent Delta system.

Middle Jurassic palaeogeographical reconstruction of the North Sea (left), illustrating interpreted dispersal of excess sediment supplied by the Brent Delta sediment routing system; this excess sediment is inferred from sediment budgets predicted from source regions by the empirical BQART model, which are oneorder of magnitude greater than mapped sediment budget in the Brent Delta depositional sink (right).

Plant fragments and distinct facies tracts are important identification criteria for hyperpycnites in the Bonan Sag. Palaeogullies determine the provenance direction of hyperpycnal flows, while the formation of synsedimentary faults and troughs controls the transport routing patterns. Local micro‐palaeomorphology and depression areas further restrict the distribution of sand bodies.

Hyperpycnal flow deposits, one of the most important deep‐water gravity‐flow deposits in lacustrine basins, have become the research focus in recent years. However, the sedimentary characteristics and depositional model of hyperpycnal flow deposits in lacustrine basins remain unclear due to the differences of depositional settings between lacustrine and marine environments. Hyperpycnal flow deposits observed in the middle of the third member of the Shahejie Formation (Es3z) in the Bonan Sag, Bohai Bay Basin, Eastern China, provide a rare case study to reveal the characteristics and depositional model in lacustrine basins. For the first time, detailed core analysis, high‐resolution 3D seismic data, petrology and grain size analysis were used to unravel the characteristics and depositional model of hyperpycnal flow deposits in this study. Twelve lithofacies, six bed types and four bedsets (corresponding to feeder channel, distributary channel, levee and lobe) were recognized from detailed facies analysis. Plant fragment, an important identification mark for hyperpycnal flow deposits, can be classified into three types: completely broken plant fragments, partially broken plant fragments and complete leaves. The proximal part of the deposit develops a small amount of scattered and completely broken plant fragments in massive or spaced stratified pebbly sandstone and massive sandstone due to strong erosion of sustained high‐density turbidity current. The medial part of the deposit is dominated by laminated partially broken plant fragments in planar laminated or rippled sandstone due to suspended settling of sustained high‐density turbidity current and quasi‐steady low‐density turbidity current. Layered partially broken plant fragments with some complete leaves are common in the upper part mud rich division of hybrid event bed and laminated siltstone in the distal part of the deposit. The distribution pattern of hyperpycnites is controlled comprehensively by palaeogeomorphy and sediment supply. Palaeogullies determine the provenance direction of hyperpycnal flow. The formation of synsedimentary faults and troughs control the transport routing patterns. Local micro‐palaeogeomorphy and depression areas further restrict the distribution of sand bodies. During the early stage of deposition with insufficient sediment supply, sediments are transported to the deep basin along confined faulted troughs forming elongated sandy bodies. During the late stage of deposition with sufficient sediment supply, sediments are transported to the deep basin without confinement accumulating fan‐shaped sandy bodies. This study offers insight for enhancing the recognition criteria of hyperpycnites, as well as their depositional model in lacustrine basins.

We use facies analysis of subaerial alluvial fan and subaqueous fan‐delta sequences to delineate the position of the Miocene shoreline and estimate basinal water depth after the marine flooding into the Salton Trough region in southern California

The timing and character of the initial marine flooding of extensional basins has implications for their tectonic history. Yet, the recognition of such flooding is difficult along rift basin margins due to the dominance of coarse‐grained systems and the lack of marine fauna. This study conducts detailed facies and stratigraphic analysis of a Mio‐Pliocene alluvial fan and fan‐delta succession in the Fish Creek Vallecito Basin in southern California. Our goal is to characterize the marine flooding surface, determine the paleogeographic position of the shoreline and estimate the magnitude of relative base‐level rise that occurred during the marine incursion associated with the opening of the Gulf of California. Our results show that the flooding of the Elephant Trees alluvial fans is often marked by an abrupt lithologic and facies change from meter‐scale boulder‐rich subaerial debrites (proximal alluvial fan facies association) to centimetre‐scale granule‐rich subaqueous debrites, ripple‐laminated sandstones and mudstones (prodelta facies association). By delineating the zone of transition between the subaerial and subaqueous facies, we place the initial flooding paleo‐shoreline 4 km up the alluvial fan's paleo‐depositional slope. Considering alluvial fan slope gradients between 1° and 5°, this 4 km transgression would require an estimated 70–350 m of water depth during the initial marine incursion. Interfingering of fan‐delta deposits with subaqueous marine and planktonic‐rich evaporites suggests that the basin was below sea‐level after, and perhaps even before, the marine flooding. Subaerial subsea‐level basins exist in Death Valley and the Salton Trough today within similar extensional and transtentional tectonic regimes. This subaerial subsea‐level interpretation might explain the high magnitude and abrupt relative base‐level rise recorded by the facies transitions in the Fish Creek Vallecito Basin. These results suggest that the Fish Creek Vallecito Basin underwent significant extension during its early and nonmarine depositional phase, allowing it to reach subsea‐level elevations. The tectonic history of the Fish Creek Vallecito Basin maybe similar to other extensional basins where rapid subsidence allows the accumulation of nonmarine strata below sea‐level prior to the marine flooding, then restricted and deep marine strata immediately after flooding.