Basin Research - Volume 34, Issue 3, 2022

Supradetachment basins at passive rifted margins are a key witness of major‐continental extension, and they may preserve a record from which the amount and rates of extension and metamorphic core complex exhumation may be reconstructed. These basins have mainly been recognised in back‐arc and orogenic collapse settings, with few examples from rifted margins. Using 2D and 3D seismic reflection, wellbore, and gravity anomaly data, we here characterise the three‐dimensional structural and tectonosedimentary evolution of a spoon‐shaped supradetachment basin that was formed in the necking domain of a rifted margin, at the southern limit of the Møre and Vøring segments of the Norwegian rifted margin. The basin, with an areal extent of ca. 2400 km², and a landward‐rotated syn‐tectonic succession up to ca. 30 km thick (true stratigraphic thickness), is separated from footwall continental margin core complex basement culminations by major large‐offset (>30 km) normal fault complexes characterised by a cross‐sectional geometry whereby an upper, steeper part of the fault gives way to a low‐angle detachment fault at depth. These fault complexes are associated with a tectonic thinning of the continental crust to ca. 11 km, compared with a crustal thickness of ca. 27 km in the proximal domain. The basin is filled by a succession of pre‐, syn‐ and post‐tectonic deposits, that accumulated over time as the basin evolved over a series of rift‐ and detachment faulting events. The 30 km thick syn‐tectonic succession reflects deposition during two separate rifting events, which are disconnected by deposits reflecting a relative short period of tectonic quiescence. The results are discussed in light of examples of supradetachment basins on other rifted margins globally, as well as in the context of the evolution of the Norwegian margin overall.

Schematic representation of the hypothetical, time‐varying paleofluid system at the southeastern termination of the Gypsum Valley salt wall. Different colored arrows indicate varying fluid sources or geochemistry. Circular arrows are intended to generally illustrate mixing over a certain scale, not precise flow paths. (a) Paleofluid system that existed at the end of the Pennsylvanian. (b) Paleofluid system that existed around the time of the middle Permian. (c) Paleofluid system that existed at some time after deposition of the Morrison Formation (ca. 147 Ma) and from which Group 2 fracture cements precipitated. (d) Possibly stratigraphically segregated and fault‐controlled paleofluid system that existed in the Latest Cretaceous. Note that unit thicknesses near the diapir are highly schematic because the cross section cuts through the plunging nose of the salt wall, geometries vary rapidly into and out of the plane of the section and there may have been substantial out of plane motion of material. See the stratigraphic column in Figure 2 for the unit names, ages and labels. The fundamentals of each cross‐section geometry are based on the present day geometry shown in Figure 4b as well as the regional cross section restoration of Rowan et al. (2016).

This study combines field structural analysis with thin‐section petrography, U‐Pb dating, and strontium, carbon and oxygen isotopic analysis of calcite fracture fills to constrain the evolution of the 2‐5 km scale paleofluid system around the faulted, plunging fold nose comprising the southern termination of the Gypsum Valley salt wall in the Paradox Basin, U.S.A. Brittle deformation in this region began with the formation of a down‐to‐the‐northeast, counter‐regional fault and then progressed into jointing and faulting in a radial pattern, followed by jointing in a concentric pattern. Coupled with increases in fracture abundance toward the faults, multiple stages of mineralization suggest that the faults served as efficient and long‐lived conduits for vertical fluid migration. Although fracture cement textures and calcite colour are variable throughout the area, the distribution of these characteristics does not correlate with fracture orientation, relative age, stratigraphic or structural position. Irrespective of the type of calcite comprising the fracture cements, δ¹³C values average near −7‰ (VPDB), whereas δ¹⁸O values cluster into groups whose averages are roughly 6‰ apart, with the more negative grouping stratigraphically restricted to fracture cements in Jurassic rocks. The stratigraphic segregation of δ¹⁸O values suggests the paleofluid system contained two distinct paleofluids, a more recent one comprised of meteoric waters and an older one comprising brine that originated in Pennsylvanian strata. ⁸⁷Sr/⁸⁶Sr ratios in fracture‐filling calcite cements indicate that the older fluid underwent fluid‐rock interaction with Permian strata and that this evolved fluid migrated upwards along the faults until the Triassic or Jurassic. Thereafter, fluid migrating along the faults was more meteoric and appears to have migrated downward along the faults, where it interacted with Permian strata. Consistent U‐Pb dates from carbonates precipitated from the older fluid suggest this stage of the paleofluid system was active around 240 Ma. Local burial history models and published temperatures for fracture cements elsewhere in the basin suggest the younger stage of the paleofluid system occurred during the Latest Cretaceous to Oligocene. This study highlights the spatial and temporal complexity of fluid systems in the vicinity of salt structures and emphasises the need to interpret them through careful integration of high resolution stratigraphic and structural data in the context of evolving salt tectonics.

Sedimentary basins can receive first‐ or recycled detritus or a mixture of both. Inherited features of mineral grains (age, isotope, and trace elements) can help in characterizing the source area of the detritus. Acquired features during the sedimentary cycle (changes in the diversity of mineral assemblages and grain roundness) can help in differentiating between recycled versus first‐cycle sediments.

An implicit assumption of most sedimentary provenance analyses is a direct link between source and sink. However, recycling of sedimentary detritus from pre‐existing strata interrupts the direct source‐to‐sink link and can result in incorrect interpretations of paleogeography and paleodrainage. Detrital zircon is the favoured proxy of contemporary provenance studies, but its physiochemical resilience makes it particularly prone to recycling. In this study, we integrate geochemical (age, isotope, and trace elements) and grain roundness data of multiple detrital minerals with different physiochemical stabilities (zircon, tourmaline, rutile, and apatite) to evaluate the importance of recycling in an ancient sedimentary basin. We focus on the early Cambrian Lalun Formation of Iran, which forms part of a laterally extensive sandstone‐rich succession deposited along the northern margin of Gondwana. The Lalun Formation preserves a distinct change of compositional maturity between lower arkose and shale units and an upper unit of quartz‐rich sandstone. Detrital zircon, rutile, and apatite data demonstrate that all units of the Lalun Formation share a common source in the Arabian‐Nubian Shield. Whole‐rock geochemical data further indicate that all units have similar chemical alteration indices, suggesting the change in compositional maturity is not a product of differential weathering of the source region. Analysis of grain roundness reveals that detrital zircon, rutile, and tourmaline in the upper quartz‐rich unit are typically more rounded than those in the underlying arkose and shale units. In contrast, detrital apatite grains are nearly all angular in the quartz‐rich unit but mostly rounded in the lower arkose and shale. Together, the detrital mineral provenance, whole‐rock geochemistry, and morphological data are consistent with recycling of the lower arkose and shale units of the Lalun Formation into the uppermost quartz‐rich unit, with the latter also receiving a component of first‐cycle detritus represented by angular detrital apatite. Our findings demonstrate that integrating the features of detrital minerals acquired during a sedimentary cycle (grain rounding and diversity of mineral assemblages) with features inherited from their ultimate source rocks (age, isotopic, and geochemical proxies) can assist in recognising sediment recycling in ancient strata.

Although volcanism is an important process in the evolution of rift basins, current tectono‐sedimentary models largely neglect its impact on sediment supply, transport pathways, and depositional systems. In this paper, we integrate core, well logs, and 3D seismic data from the Palaeogene‐Neogene Shaleitian (SLT) uplift and surrounding sub‐basins, Bohai Bay Basin, China, to investigate the sedimentology and geomorphology of a volcanic rift basin. Results of this study show that the spatial distribution of extrusive centres was strongly controlled by basement‐involved intra‐basin normal faults. During the early part of the syn‐rift stage, the SLT uplift supplied sediments to transverse fan deltas and braided‐river deltas that fringed the adjacent syn‐rift depocentres. Volcanic deposits mainly occurred as relatively thin lava flow and pyroclastic facies that partially filled fault‐controlled topographic lows, reducing topographic rugosity, and enhanced breaching of basement highs between syn‐rift depocentres. Integration of drainage to the syn‐rift depocentres and development of through‐flowing axial depositional systems was enhanced. During the later part of syn‐rift and in early post‐rift stages, the SLT uplift was progressively inundated, reducing sediment supply to the fringing transverse depositional systems. In contrast, axial braided‐river deltas became the main depositional systems, sourced by large hinterland drainage from the Yanshan fold‐belt to the northwest. Volcanism in the late syn‐rift and early post‐rift occurs as thick lava flow and pyroclastic facies that infill rift topographic lows and locally blocked axial fluvial systems creating isolated lakes. Within hanging wall depocentres, volcanic topographic highs split and diverted axial fluvial and deltaic systems. Furthermore, volcanism supplied large volumes of volcanic sediment to the rift resulting in increased sedimentation rates, and the development of unstable subaerial and subaqueous slopes and deposits, increasing the occurrence of landslides. Based on the observations of this study we update tectono‐sedimentary models for rift basins to include volcanism.

The volcanism in the Shaleitian Sub‐basin enhanced sediment dispersal by smoothing out fault induced topographic rugosity in the syn‐rift stage and created topographic barriers, resulting in damming depocentres and blocking or diverting routing systems in the post‐rift stage.

Evolution of rift basin fill and geometry depend on the complex interactions between fault growth, sediment supply, base level changes and pre‐existing basement fabric. This study integrates multiple datasets in the Great South Basin (GSB), southeast New Zealand, and provides key insights into the evolution of depositional environments in rift basins, including the interplay between normal faulting, sediment supply and sediment dispersal patterns. It also examines the control of pre‐existing basement fabric on rift geometry and sediment distribution in the syn‐ and post rift successions. The syn‐rift is up to ~5.5 km thick in the GSB, and is underlain by several different basement terranes. Three syn‐rift stages are recognised; c.105–101, 101–90 and 90–83 Ma. During the initial syn‐rift, isolated northeast‐trending graben developed, with resultant alluvial fan/fan delta, fluvial, coastal and lacustrine sediment fill. The balance between sediment supply and accommodation space exerted considerable control on facies, especially the presence of lacustrine facies. During the later stages of syn‐rift, marine transgression occurred and connectivity between the graben developed, with shelfal, shoreface and marginal‐marine facies deposited. With marine transgression across the hinterland, sediment supply was significantly reduced in the northeast of the basin, leading to underfilling of graben, and preservation of rift topography for up to ~20 Myr after the cessation of faulting. In the west, where sediment supply was higher, rift topography was quickly filled. The NW‐trending basement terrane boundaries controlled accommodation space development during the initial stages of graben formation. Later in the syn‐ and post rift stages, these terrane boundaries formed long‐lived sediment input points into the basin, and controlled the position of repeated large deltaic depositional units.

Evolution of syn‐ to early post‐rift facies in rift basins.

Early Miocene (a) and middle Miocene (b) schematic paleogeography and inferred oceanic current flow. Eastward (clockwise) marine transport of western‐sourced sediment along the shelf or slope deflected the paleo‐Tennessee signal >150 km eastward to feed the deep‐sea fan further east in the Miocene, which perhaps reflected intensification of a precursor to the Gulf of Mexico Loop Current.

Sediment routing from hinterland to the deep sea is complicated because it involves evolution of river drainage from source areas to coastal plains and sediment mixing on the shelf and slope by marine currents. Previous regional paleogeographic mapping in the Gulf of Mexico (GOM) has observed a >150 km offset between the middle Miocene paleo‐Tennessee fluvial axis and the associated deep‐sea fan depositional axis, indicating a complicated sediment pathway. We integrate new and published detrital zircon (DZ) U‐Pb age data from fluvial, shelf and deep‐sea deposits to examine the complex Miocene sediment routing system in the northern GOM. These data suggest an increase in sediment load derived from western North America (increased Western Cordillera terranes; <300 Ma zircon age component) from the early to middle Miocene in the deep‐water Green Canyon protraction area. The early Miocene Green Canyon area received sediments mainly from fluvial axes located directly updip: the paleo‐Mississippi River (44%–56%; characterized by Yavapai‐Mazatzal, Mid‐Continent and Western Cordillera sourced 1800–1600 Ma, 1500–1300 Ma and <300 Ma, respectively, and Grenville‐Appalachian sourced 1300–950 Ma and 500–300 Ma age components) and smaller rivers and tributaries draining the Appalachian Mountains (e.g. paleo‐Tennessee River, 18%–43%; mainly Grenville‐Appalachian sourced 1300–9500 Ma and 500–300 Ma age components). In contrast, the middle Miocene Green Canyon deep‐sea fan shows a strong DZ signal from the paleo‐Red River (38%; increased <300 Ma zircon age component), which requires input of additional sediment sources from west of the paleo‐Mississippi system. In addition, the paleo‐Tennessee River, which was a major middle‐Miocene sediment source for the central‐eastern GOM due to uplift and increased erosion of the Appalachian Mountains, is underrepresented (34%; decreased 1300–950 Ma zircon age component) in the middle Miocene Green Canyon fan. We suggest that two mechanisms combined to produce the increased middle Miocene input from western sediment sources and restriction of locally up‐dip Tennessee River sources: (1) regional drainage changes involving middle Miocene capture of the paleo‐Red River and its tributaries by the paleo‐Mississippi River, which at the same time lost some of its eastern tributaries owing to expansion of the paleo‐Tennessee and (2) eastward (clockwise) marine transport of western‐sourced sediment along the shelf or slope, which deflected the paleo‐Tennessee signal >150 km eastward to feed the deep‐sea fan further east, perhaps reflecting intensification of a precursor to the GOM Loop Current.

Interpretations of possible courses of the latest Triassic El Alamar River based on possible sediment sources to the El Alamar Formation. No data require headwaters of the El Alamar River east of the Coahuila Block.

Upper Triassic to lowest Upper Jurassic strata in north‐eastern Mexico record surface processes during the early rifting that led to the opening of the Gulf of Mexico. Exposed near Ciudad Victoria, Tamaulipas and northwest to Galeana, Nuevo Leon, these continental deposits are called the Huizachal Group. Several key questions about these strata hamper their integration into a regional understanding of Late Triassic and Jurassic sediment routing and deposition. First, the depositional age of the stratigraphically lowest unit, the El Alamar Formation, is less well established than the depositional ages of the stratigraphically higher parts of the succession, leading to questions about the timinig of the onset of tectonic context, and regional correlation of these strata. Second, better knowledge of provenance and sediment routing, including fluvial transport distance, can help determine whether and when the depositional basin was split into subbasins versus connected by a throughgoing river system. This study uses sandstone detrital zircon U‐Pb isotopic ages and clast compositions to constrain depositional ages and reconstruct tectonic setting and provenance. Detrital zircon U‐Pb ages yielded a maximum depositional age of 210 Ma for the part of the El Alamar Formation exposed near Galeana, younger than previously determined. Although long‐distance (>100 km) sediment transport cannot be ruled out, the sources for Huizachal Group sandstone framework grains and detrital zircon could have been entirely local, within about 100 km. Alluvial deposits and pyroclastic and lava flows likewise indicate transport of no more than 100 km. We therefore infer deposition in partitioned rift basins receiving sediment from local sources. These data and interpretations contribute to understanding surface processes during the initial rifting that eventually led to opening of the Gulf of Mexico.

Early post‐rift deposition associated with the breached relay structure can be divided into two domains: (1) systems which entered the basin at the breached relay structure via the lower abandoned ramp; (2) systems which entered directly across the hangingwall of structure bounding faults. Into the early post‐rift, systems continue to enter at the same locations, however, fan geometries and sources change in response to overall basin subsidence becoming the main accommodation driver.

Fault relay ramps are important sediment delivery points along rift margins and often provide persistent flow pathways in deepwater sedimentary basins. They form as tilted rock volumes between en‐echelon fault segments, which become modified through progressive deformation, and may develop through‐going faults that ‘breach’ the relay ramp. It is well established that hinterland drainage (fluvial/alluvial systems) is greatly affected by the presence of relay ramps at basin margins. However, the impact on deepwater (deep‐marine/lacustrine) subaqueous sediment gravity flow processes, particularly by breached relay ramps, is less well documented. To better evaluate the complex geology of breached relay settings, this study examines a suite of high‐quality subsurface data from the Early Cretaceous deep‐lacustrine North Falkland Basin (NFB). The Isobel Embayment breached relay‐ramp, an ideal example, formed during the syn‐rift and was later covered by a thick transitional and early post‐rift succession. Major transitional and early post‐rift fan systems are observed to have consistently entered the basin at the breached relay location, directed through a significant palaeo‐bathymetric low associated with the lower, abandoned ramp of the structure. More minor systems also entered the basin across the structure‐bounding fault to the north. Reactivation of basin‐bounding faults is shown by the introduction of new point sources along its extent. This study shows the prolonged influence of margin‐located relay ramps on sedimentary systems from syn‐rift, transitional and into the early post‐rift phase. It suggests that these structures can become reactivated during post‐rift times, providing continued control on deposition and sourcing of overlying sedimentary systems. Importantly, breached relays exert control on fan distribution, characterised by laterally extensive lobes sourced by widespread feeder systems, and hanging walls settings by small‐scale lobes, with small, often line‐sourced feeders. Further characterising the likely sandstone distribution in these structurally complex settings is important as these systems often form attractive hydrocarbon reservoirs.

Continental breakup involves a transition from rapid, fault‐controlled syn‐rift subsidence to relatively slow, post‐breakup subsidence induced by lithospheric cooling. Yet the stratigraphic record of many rifted margins contain syn‐breakup unconformities, indicating that episodes of uplift and erosion interrupt this transition. This uplift has been linked to mantle upwelling, depth‐dependent extension and/or isostatic rebound. Deciphering the breakup processes recorded by these unconformities and their related rock record is challenging because uplift‐associated erosion commonly removes the strata that help constrain the onset and duration of uplift. We examine three major breakup‐related unconformities and the intervening rock record in the Lower Cretaceous succession of the Gascoyne and Cuvier margins, offshore NW Australia, using seismic reflection and borehole data. These data show the breakup unconformities are disconformable (non‐erosive) in places and angular (erosive) in others. Our recalibration of palynomorph ages from rocks underlying and overlying the unconformities shows: (i) the lowermost unconformity developed between 134.98–133.74 Ma (Intra‐Valanginian), probably during the localisation of magma intrusion within continental crust and consequent formation of continent–ocean transition zones (COTZ); (ii) the middle unconformity formed between ca. 134 and 133 Ma (Top Valanginian), possibly coincident with breakup of continental crust and generation of new magmatic (but not oceanic) crust within the COTZs; and (iii) the uppermost unconformity likely developed between ca. 132.5 and 131 Ma (i.e. Intra‐Hauterivian), coincident with full continental lithospheric breakup and the onset of seafloor spreading. During unconformity development, uplift was focussed along the continental rift flanks, likely reflecting flexural bending of the crust and landward flow of lower crust and/or lithospheric mantle from beneath areas of localised extension towards the continent (i.e. depth‐dependent extension). Our work supports the growing consensus that the ‘breakup unconformity’ is not always a single stratigraphic surface marking the onset of seafloor spreading; multiple unconformities may form and reflect a complex history of uplift and subsidence during continent–ocean transition.

Seismic reflection data allows us to image three unconformities (IVU, TVU, and IHU) and intervening stratigraphic packages that record the Early Cretaceous breakup of NW Australia from Greater India.

Photograph of the Dolores River Canyon showing a burial wedge formed of onlapping and syndepositionally deformed Triassic Chinle Formation. Burial wedges preserve sediment and deformed during deposition on salt.

Many Paradox Basin passive salt diapirs underwent a prolonged history (>65 my) of progressive burial during the Early Triassic to Jurassic. Burial is recorded by a series of geographically limited wedge‐shaped stratal panels of diapiric roof, termed ‘burial wedges’, which partly covered the diapirs, thereby gradually narrowing them. Tectonostratigraphic analysis of the Triassic Chinle Fm. burial wedge developed on the Gypsum Valley diapir illustrates the typical features and processes associated with burial wedges. Burial wedges represent the first description of the geometry and deposition of strata deposited on a partially buried diapir. Recognition of burial wedges allows reinterpretation of Paradox Basin diapirs where large areas of the roof strata were syndepositionally folded rather than deformed in the Neogene. The observations and concepts can also be applied to the terminal histories of passive salt diapirs in other salt basins. Preserved burial wedges cover 97 km², or 64% of the original Gypsum Valley diapir and overlie regional unconformities. Onlapping strata stack into wedge halokinetic sequences (HS). Caprock‐clast conglomerates indicate deposition on exposed diapirs. Syndepositional deformation of burial wedge strata formed small synclinal ‘microbasins’ and contractional chevron folds trending subparallel to the diapir margin. A four‐stage burial wedge model includes the following: (1) erosion of the diapir roof during formation of regional unconformities; (2) onlap and partial burial of the diapir; (3) local dissolution and possible concomitant gravity‐driven folding and (4) ongoing rotation during deposition, forming wedge HS signifying continued inflation of the top of the diapir. Post formation, the burial wedges were deformed in ways that obscure their original geometry. The most common deformation is downwarping of the burial wedge into the diapir due to dissolution or lateral movement of salt, creating an anticlinal fold that overlies the underlying diapir margin. These anticlines may be offset on small normal faults subparallel to the diapir margin. Burial wedges were faulted during later diapir breaching and solution collapse.