Transgressive Erosion Research Articles

High-resolution geophysical and geochronological analysis of a relict shoreface deposit offshore central California: Implications for slip rate along the Hosgri fault

Abstract The Cross-Hosgri slope is a bathymetric lineament that crosses the main strand of the Hosgri fault offshore Point Estero, central California. Recently collected chirp seismic reflection profiles and sediment cores provide the basis for a reassessment of Cross-Hosgri slope origin and the lateral slip rate of the Hosgri fault based on offset of the lower slope break of the Cross-Hosgri slope. The Cross-Hosgri slope is comprised of two distinct stratigraphic units. The lower unit (unit 1) overlies the post–Last Glacial Maximum transgressive erosion surface and is interpreted as a Younger Dryas (ca. 12.85–11.65 ka) shoreface deposit based on radiocarbon and optically stimulated luminescence (OSL) ages, Bayesian age modeling, seismic facies, sediment texture, sediment infauna, and heavy mineral component. The shoreface was abandoned and partly eroded during rapid sea-level rise from ca. 11.5 to 7 ka. Unit 2 consists of fine sand and silt deposited in a midshelf environment when the rate of sea-level rise slowed between ca. 7 ka and the present. Although unit 2 provides a thin, relatively uniform cover over the lower slope break of the older shoreface, this feature still represents a valuable piercing point, providing a Hosgri fault slip rate of 2.6 ± 0.8 mm/yr. Full-waveform processing of chirp data resulted in significantly higher resolution in coarser-grained strata, which are typically difficult to interpret with more traditional envelope processing. Our novel combination of offshore radiocarbon and OSL dating is the first application to offshore paleoseismic studies, and our results indicate the utility of this approach for future marine neotectonic investigations.

Changjiang coastal mud‐belt deposits in Taiwan Strait: Controls on its distribution and facies

ABSTRACTCoastal mud belts, which lie parallel to the coast just seaward of the shoreface, are one of the most important settings where shallow‐marine muddy deposits accumulate. However, sedimentary processes and facies distributions of coastal mud belts remain largely uninvestigated. This study uses process‐oriented sedimentology, coupled with provenance analysis and organic geochemistry of sedimentary records at the northern entrance to the Taiwan Strait, to investigate one of the largest such systems in the world, the Changjiang coastal mud belt. Carbon‐14 dates indicate that deposition has been intermittent throughout Marine Isotope Stages 1, 2 and 3, with several cryptic gaps in the muddy succession as a result of exposure or transgressive erosion during periods of lower or rising sea level, respectively. The palaeowater depth of each depositional unit was roughly estimated based on a global sea‐level curve and the elevation and age of the depositional units. Integrated provenance and sedimentology studies suggest that an incised valley most likely cut by the Minjiang, a nearby river draining the Chinese mainland, probably during Marine Isotope Stage 6, was filled with shelf mud during Marine Isotope Stage 3. These results demonstrate that deposition of mud on the Changjiang coastal mud belt was influenced by sea‐level changes that controlled water depth and flow patterns through Taiwan Strait. The Changjiang coastal mud belt migrates landward when sea level rises and moves seaward when sea level drops. In addition, changes in oceanic circulation associated with opening and closing of Taiwan Strait due to sea‐level changes causes significant changes in the source of the mud, with an alternation between sediment contributions from the Changjiang in the north, the nearby Minjiang, and the island of Taiwan to the south‐east. This study also reveals that the Changjiang coastal mud belt deposits are composed of both shallower‐water and deeper‐water facies. The shallower‐water facies predominantly comprises abundant fluid‐mud beds generated by waves and tidal currents. The deeper‐water facies contains shell‐rich mud and bioturbated mud, which were likely deposited by the Chinese coastal current and Taiwan Warm current. The bed types and interpretation of sedimentary processes of the Changjiang coastal mud belt are useful for recognition of ancient deposits of coastal mud belts elsewhere.

Antecedent geological control on transgressive delta and shoreline preservation: Examples from the SE African shelf

Using a suite of ultra-high resolution geophysical tools, remote operated vehicle dives, legacy cores, and isolated grab samples, we demonstrate at a regional shelf scale, the influence of antecedent geology (basement topography, shelf gradient and submerged shoreline features) on the evolving transgressive shelf stratigraphy of a subaqueous delta. The unconsolidated uppermost delta occurs as isolated remnants scattered across the low-gradient shelf. Seismic data reveal across-shelf heterogeneity in bedrock elevation, with prominent bedrock highs and depressions and several well-preserved aeolianite palaeo-shoreline complexes at water depths of −100, −60 and − 40 m. Analysis of bathymetric and seismic data demonstrates that these pre-existing shoreline complexes exert an overarching control on the distribution patterns of i) deltaic sediments, where they abut the landward flank of the shoreline form, acting as a barrier to seaward dispersal, and ii) shoreface sediments which remain sequestered on the mid-shelf on the seaward flank of the shoreline complex, hampering the landward translation of the shoreface in step with rising sea levels. Moreover, sediment distribution/accumulation is further constrained where elevated portions of the basement topography provide little to no accommodation, and act as zones of sediment bypass, and adjacent basement depressions accommodate sediment to seaward. The gentle antecedent slope, coupled with the gentle shoreline trajectory mediated transgressive erosion directly to landward. This is reinforced where local inflections in the wave ravinement profile form due to the presence of palaeo-shoreline complexes, aiding in the ultimate preservation of these submerged delta facets in the palaeo-shoreline lee. This study shows that low antecedent gradients, and palaeo-shoreline features, lead to development of transgressive coastal profiles that are predisposed to delta overstepping. The pre-existing basement topography and palaeo-shorelines constrain the positioning and morphology of the delta. We suggest that antecedent conditioning has partitioned accommodation for delta accumulation and moderated wave ravinement associated with transgression since the Last Glacial Maximum (LGM). The geological framework has acted as a recurring primary control on the geomorphic evolution of the submarine delta and shelf.

Development and preservation of transgressive sandy versus rocky shorelines: Observations from the SE African shelf

Relict rocky shorelines are seemingly rarely preserved in the geological record, often underrepresented in the literature and the reasons for their scarcity, as well as the conditions needed for preservation are not well understood. On the “Wild Coast” shelf along the subtropical east coast of South Africa a series of submerged shorelines are present. High-resolution multibeam bathymetry, side scan sonar, ultra-high-resolution seismic profiling and legacy seismic reflection data show several fluvial channels incised into the shelf, associated with the Last Glacial Maximum (LGM) lowstand. Palaeo-shorelines are preserved at −85, −75, −60 and −40 m. The −85 and −75 m shorelines reflect sediment abundance and depositional features preserved as lagoonal depressions, lithified barrier-spits and coastal barrier dunes with limited bedrock influence. The shallower shorelines at −60 and −40 m occur on a steeper shelf gradient and reflect the development of rocky headlands, palaeo-cliffs and rock shore platforms. Their occurrence implies increasing bedrock control, erosion and sediment bypass during transgression.The preservation of rocky shoreline signatures on the inner shelf is aided by competency of the bedrock lithologies, in addition to their depth relative to sea-level stillstands and rapid rises in sea level associated with meltwater pulses (MWPs) (notably the −60 m shorelines and MWP1B). MWPs may have helped facilitate the overstepping and eventual preservation of these shoreline landforms on the seabed. Preservation of the −40 m shoreline is considered to correlate with accelerated rates in sea- level rise prior to MWP1C. The preservation of the −85 and −75 m aeolianite shorelines are not linked to meltwater pulses, however, their substantial sediment volume precluded total loss through transgressive erosion and this, coupled with early cementation, promoted their partial preservation. Such deposits rarely survive subaerial exposure but could be preserved if covered by marine sediments. Conversely, the existence of well-preserved, Pleistocene-age rocky shorelines on the shelf points to their high potential for inclusion into the longer-term geological record under subsequent high or low sedimentation. The formation of distinctive rocky shorelines, however, may reflect alternating slow and fast rates of sea-level rise.

Syn-depositional oil seeps in the Late Albian Paddy Member of the Peace River Formation (Early Cretaceous), north-central Alberta

Abstract Controversy exists regarding the timing of emplacement of oil in the giant Athabasca and Peace River oil sands. Bitumen-cemented sandstones are present in the late Albian Paddy Member of the Peace River Formation; some cemented sandstones formed burrowed firmgrounds and reworked intraclasts, showing that oil was reaching surface by about 101–102 Ma. Estuarine and shallow-marine sandstones of the Paddy Member are exposed on the Peace and Heart rivers in the vicinity of the town of Peace River. Bitumen-cemented sandstone occurs in four stratigraphic settings: 1) An in-situ cement forms locally bedding-transgressive, sheet-like masses in cross-bedded estuarine sandstone; 2) A bitumen-cemented and heavily burrowed layer, 30–60 cm thick, lies immediately beneath a marine transgressive surface and shows that oil infiltrated downward into, and cemented, the upper surface of a shoreface sandbody. The firm sand was subsequently exhumed by transgressive erosion, fractured and burrowed by arthropods; 3) Rounded pebble- to cobble-sized clasts of bitumen-cemented sand lie on the floor of a tidal channel, and the channel floor surface is also stained and burrowed. This case suggests floating oil infiltrated the channel floor at low tide, subsequently hardening prior to erosion and burrowing; 4) Rounded cobble-to boulder-size clasts of bitumen-cemented sandstone, up to 1 m wide and 0.5 m thick, lie on a regional marine ravinement surface cut on the uppermost Paddy shoreface sandstone. The sand was permeated with oil, top-down, then oxidized to form a tough bitumen cement. The bituminous sand was subsequently scoured by waves during marine transgression to form a boulder lag that is enclosed in transgressive marine claystone. All four of these examples show that oil was reaching the Earth’s surface during Paddy time where it infiltrated porous sands to a depth of several decimetres, subsequently degrading and oxidizing to form a tough ‘asphalt pavement’ that attracted a burrowing firmground fauna and was eroded into cohesive intraclasts weighing many tens of kilograms. Pyrolysis proved the bitumen cement to be degraded oil, but it was not possible to identify the source-rock using gas chromatography-mass spectrometry. The Paddy ‘asphalt pavements’ are comparable to cements that form in beach sediments in the wake of catastrophic spills from tanker groundings. A direct analogue is provided by Eocene estuarine sediments in Dorset, U.K. There, bitumen-cemented estuarine sands also formed firmgrounds and intraclasts on channel-floor and marine transgressive surfaces.

The ‘Base Cretaceous Unconformity’ in a basin-centre setting, Danish Central Graben, North Sea: A cored record of resedimentation and condensation accompanying transgression and basinal overturn

A cored section (North Jens-1) in the Danish Central Graben spans the ‘Base Cretaceous Unconformity’ (BCU) in a seismically conformable situation in a basin-centre setting. Multidisciplinary analysis of the core revealed that the apparent simplicity of the boundary in seismic profiles at this location disguises a complex, dynamic geological history. Dark, organic-rich, laminated hemipelagic/turbiditic Farsund Formation mudstones of late Ryazanian (Surites stenomphalus Zone) age are overlain by a thick slump sheet of upper Ryazanian mudstones, probably derived from relict rift-fault scarps on the basin floor. An erosionally-based gravity-flow bed (a hybrid density-flow deposit) of latest Ryazanian (Peregrinoceras albidum Zone) age forms the lowermost unit of the Valhall Formation. The lower boundary of this bed may record a minor hiatus across the stenomphalus–albidum zonal boundary and represents the BCU, compatible with seismic and log data. The hybrid bed is capped by an intensely burrowed omission surface that represents the first clear indication in this basin-centre location of watermass overturn and the demise of the stratified, poorly oxygenated basin. The biostratigraphy, ichnofabric and fauna/flora of the succeeding bioturbated marlstones and marly chalks of the lower Valhall Formation (Platylenticeras/Paratollia Zone) testify to increasingly oxygenated bottom waters and low sedimentation rates in a well-mixed, open marine, oligotrophic system; palynological data indicate that the onset of these conditions was linked to transgression. The North Jens-1 core illustrates the risk of assuming stratigraphic simplicity at the BCU surface in basin-centre, seismically conformable settings. Sedimentary reworking, already characteristic of the Farsund Formation setting, was probably exacerbated by transgressive erosion and accompanying sediment de-stabilization along the graben flanks.

Depositional and sequence stratigraphic model of transgressive shelf sandstone: The Late Cretaceous Tocito Sandstone, San Juan Bain, New Mexico, U.S.A.

ABSTRACT There has long been debate about the origin of long, linear, isolated shallow marine sandstones. In the 1980s these were routinely interpreted as wave- or tide-modulated shelf sandstones. With the advent of sequence stratigraphy, many of these sandstones were reinterpreted as top-truncated forced-regressive to lowstand delta or shoreface, inviting the question regarding whether the concept of a shelf sandstones remain a viable depositional model. More recent syntheses demonstrate that shelf sands tend to be derived by reworking of underlying sediment through transgressive erosion and show varying levels of detachment and reworking into discrete ridges and bars by wave and tidal processes as they become more evolved. The Late Cretaceous Tocito Sandstone in the Western Interior Seaway, which overlies the regressive Gallup Formation, is a candidate for a transgressive shelf sandstone. In this study, we present detailed facies analysis, based on sedimentological sections measured from well-exposed outcrops of the Tocito Sandstone and its related stratigraphic units in northwest New Mexico, USA. With high-resolution sequence stratigraphic control, facies analysis, detrital-zircon anatomy, and petrographic analysis are integrated to investigate the sequence stratigraphic evolution and depositional processes of the Tocito Sandstone. Detrital zircons indicate that the Tocito, Torrivio, and Gallup deposits are derived from different source areas, and therefore imply separate sequence stratigraphic origins. The Tocito Sandstone is interpreted to have been deposited as shelf sand ridges in retrogradational estuarine to drowned-barrier-system environments with tidal and wave influence, characterized by intense bioturbation. The Tocito was deposited in an overall transgressive systems tract and sourced from reworking of the underlying Torrivio and Gallup units through processes of transgressive erosion.

Late Pleistocene-Holocene sedimentary evolution in the coastal zone of the Red River Delta

The Red River Delta is considered one of the largest megadelta systems in Asia. The formation of this delta has been controlled by the continent-ocean interaction and sea-level fluctuation during the Cenozoic. In this study, we present a new sequence stratigraphic framework of the Red River Delta based on borehole lithofacies analysis and high resolution seismic data. The Late Pleistocene–Holocene sediments in the coastal zone of the Red River Delta were subdivided into three systems tracts: (1) the lowstand systems tract (LST) is characterized by a Late Pleistocene alluvial silty sand facies complex (arLSTQ13b); (2) the transgressive systems tract (TST) is illustrated by the coastal marsh facies complex and the lagoonal greenish-gray clay facies of Early-Middle Holocene (amt, mtTSTQ21−2); and (3) the highstand systems tract (HST) is composed of the Middle-Late Holocene deltaic clayish silt facies complex (amhHSTQ22−3). The boundaries between these three systems tracts are not isochronous, namely: (1) The LST-HST boundary has been associated with the Würm 2 Glaciation, which occurred at ~40-18 Ka.; (2) The TST-LST boundary is identified by a transgressive erosion surface, whose age ranges from ~12-5 Ka.; and (3) the HST-TST boundary is an unconformity between the submarine deltaic facies complex and the Middle Holocene marine flooding plain.

Sedimentology and basin-fill history of the Cenozoic succession of the Sylhet Trough, Bengal Basin, Bangladesh

The Sylhet Trough, a petroleum province of the Bengal Basin, accommodates a huge thickness of Eocene to Recent sedimentary successions. However, the basin-fill history of the trough is poorly understood; specifically, constrains on timing of deposition of the individual units are yet to be established. Therefore, we aimed at establishing sedimentation and basin-fill history of the Sylhet Trough based on detailed lithofacies analysis of the outcropping Cenozoic succession. We have divided the entire Cenozoic succession into three megasequences which can be further sub-divided into nine lithostratigraphic units based on bounding discontinuities, such as transgressive erosion surface, regressive erosion surface, transgressive surface, marine flooding surface, and incised valley floor. The oldest is the Megasequence 1, comprised of shallow marine shelfal deposits overlain by shallow marine to nearshore deposits. In the middle, the Megasequence 2 is representing tide-dominated marine to coastal (deltaic) depositional systems with evidence of cyclic marine regression and transgression. Repetitive occurrence of incised channel, tidal inlet, tidal ridge/shoal, tidal flat and other tidal deposits are separated by shelfal deposits. The top of the Megasequence 2 is marked by a pronounced erosion surface interpreted as an incised valley floor indicating the final phase of marine regression followed by the gradual establishment of the overlying continental-fluvial depositional systems (i.e., the Megasequence 3). This youngest megasequence is characterized by stacked braided river sand bars that pass up-sequence into meandering river deposits. Based on the Cenozoic eustatic sea level curve, we suggest that the upper boundaries of the Megasequence 1 and Megasequence 2 are approximately at 39.5 Ma and 5.0 Ma, respectively.

The stratigraphy of the Cretaceous (Albian stage) Gault and Upper Greensand formations: Dorset to Buckinghamshire and the western Weald, UK

The present account is a review of the successions in the Albian Gault and Upper Greensand formations, their lithology and their ammonite biostratigraphy, proved at outcrop between Dorset and Buckinghamshire, in the western Weald and that recorded in boreholes in the intervening area. It is concluded that the Wessex and Weald regions formed a single sedimentary basin during the Albian Stage. The lithostratigraphy and biostratigraphy are described area by area and are combined with recent accounts of the Albian sediments of the adjacent regions to describe lateral and stratigraphical variations in the depositional environments and sources of sediment. We conclude that although the steady overall rise in sea level in the Albian had an important effect on the local depositional environments and in the distribution of Albian sediments in England and adjacent areas, the influence of intermittent tectonic phases on the development of transgressive erosion surfaces was such that individual pulses in global eustatic sea-level rise cannot be correlated with confidence with most of the transgressions in the Albian.

Late Pleistocene Post-Glacial Sea Level Rise and Differential Preservation of Transgressive “Sand Ridge” Deposits in the Adriatic Sea

Linear sand bodies with ridged or mounded morphology are commonly referred to as “sand ridges”. Their origin may reflect a great variety of depositional processes and environments, although many examples from modern shelves include near-shore transgressive deposits formed during the last post-glacial sea level rise. These transgressive sand ridges, however, are not present on all the margins drowned during this interval, indicating that local environmental factors contribute to their formation and preservation. Transgressive sand ridges are recognized on the South Adriatic shelf in water depths between 90 and 120 m, where they overlay a major unconformity originated during the last glacial sea level fall and lowstand. In contrast, they are absent on the Central Adriatic shelf, where transgressive deposits above the glacial unconformity fill erosional scours without forming relief on the seafloor. The transgressive sand ridges on the South Adriatic shelf appear roughly elongated parallel to the bathymetric contour and are locally as thick as 15–20 m. Core data indicate an overall coarse-grained composition, with a relevant bioclastic component. Their limited distribution reflects the importance of local variation in shelf morphology and sediment sourcing at the onset of sea level rise. In particular, their deposition and preservation are favored in less subsiding shelf sectors characterized by higher gradients and tectonic relief, where transgressive erosion and reworking of pre-existing lowstand deposits is also more efficient.

The sedimentary succession of the last ~ 3.50 Myr in the western South Yellow Sea: Paleoenvironmental and tectonic implications

The South Yellow Sea (SYS) Basin, which is part of the West Pacific Continental Margin, began to form in the late Mesozoic to early Cenozoic as a result of regional tectonic activity in Asia. The pre-Middle Pleistocene sedimentary history of the area remains poorly understood or controversial, mainly due to a lack of long borehole cores penetrating pre-Quaternary strata. Borehole core CSDP-1 (300.10m long), the longest sediment core recovered thus far from the SYS, was drilled in 2013 in the western SYS. Recently, a reliable timescale for the core for the last ~3.50Myr was obtained by magnetostratigraphy (Liu et al., 2016a). Here we report the results of integrated analyses of the core, including sedimentary characteristics, downcore changes in environmental proxies (benthic foraminifera and ostracods), and accelerator mass spectrometry 14C and optically stimulated luminescence dates. In addition, we analyze 546km of single-channel seismic profiles and ~50km of a boomer seismic profile around the core site, and correlate the regional stratigraphy. The core sedimentary succession contains 17 depositional units (lithofacies associations) corresponding to three major seismic units, reflecting a variety of depositional environments during the last ~3.50Myr. Our results show that fluvial environments prevailed in the western SYS from ~3.50 to 1.66Ma, followed by alternations of tidal-flat to shallow-subtidal environments with fluvial environments from ~1.66 to 0.83Ma, and alternations of modern-like marine environments and fluvial environments from ~0.83Ma to the present-day in response to glacio-eustatic sea-level cycles. The depositional succession during the last ~3.50Myr was dominantly controlled by tectonism and glacio-eustatic sea-level cycles. The presumed first marine transgression since the Neogene in the western SYS at ~1.66Ma is interpreted to suggest that sea water could partially flood the Zhe-Min Uplift (ZMU) into the SYS due to tectonic subsidence, and further subsidence of the ZMU and the SYS shelf is inferred to have resulted in establishment of normal marine environments in the SYS during the highstands of glacio-eustatic sea-level cycles since ~0.83Ma. The average subsidence rates in the western SYS were ~120–165m/Myr during the last ~1.66Myr. The sedimentary succession comprises fifth- and fourth-order sequences, which are interpreted to have been driven by the 20-kyr or 41-kyr and by the 100-kyr glacio-eustatic sea-level cycles, respectively, since the late Pliocene. The sequences deposited since the Late Pleistocene are relatively complete, recording deposits from MIS 5 to MIS 1. However, the pre-Upper Pleistocene sequences show significant stratigraphic hiatuses, presumably caused by fluvial erosion during the glacial sea-level lowstands and subsequent transgressive erosion. This type of significant discontinuity in the sedimentary record can be considered to be diagnostic of shelf sequences controlled by glacio-eustatic sea-level cycles.

Preserved Stratigraphic Architecture and Evolution of A Net-Transgressive Mixed Wave- and Tide-Influenced Coastal System: The Cliff House Sandstone, Northwestern New Mexico, U.S.A.

Abstract: The Cretaceous Cliff House Sandstone comprises a thick (400 m) net-transgressive succession representing a mixed wave- and tide-influenced shallow-marine system that migrated episodically landwards. This study examines the youngest part (middle Campanian) of the Cliff House Sandstone, exposed in Chaco Cultural Natural Historical Park, northwest New Mexico, U.S.A. Detailed mapping of facies architecture between a three-dimensional network of measured sections has allowed the character, geometry, and distribution of key stratigraphic surfaces and stratal units to be reconstructed. Upward-shallowing facies successions (parasequences) are separated by laterally extensive transgressive erosion (ravinement) surfaces cut by both wave and tide processes. Preservation of facies tracts in each parasequence is controlled by the depth of erosion and migration trajectory of the overlying ravinement surfaces. In most parasequences, there is no preservation of the proximal wave-dominated facies tracts (foreshore, upper-shoreface), resulting in thin (4–7 m) top-truncated packages. Four distinct shallow marine tongues (parasequence sets) have been identified, consisting of ten parasequences with a total stratigraphic thickness of ∼ 100 m. Each tongue records an episode of complex shoreline migration history (multiple regressive–transgressive phases) in an overall net-transgressive system. The ravinement surfaces provide a stratigraphic framework in which to understand partitioning of tide- and wave-dominated deposits in a net-transgressive system, and a model is presented to account for the sediment distribution and stratigraphic architecture observed in each parasequence. Despite a complex internal architecture, parasequences exhibit a predictable pattern which can be related to the regressive and transgressive phases of deposition. Preservation of wave-dominated facies tracts is associated with shoreline regression, while tide-dominated facies tracts are interpreted to record sediment accumulation during shoreline transgression that also resulted in significant erosion of the underlying regressive deposits. The interplay between erosion, sediment bypass, and deposition during regression and transgression is shown to ultimately control the preservation and stratigraphic architecture of the larger-scale net-transgressive coastal system. While the Cliff House Sandstone exhibits a facies composition and quantitative stacking patterns (shoreline trajectory) similar to other studied examples, differences in the dip-extent of the wave-dominated sandstone tongue has resulted in a more disconnected architecture between the high-frequency cycles. Understanding the variety of stratal geometries that ravinement surfaces can generate is therefore crucial to predicting the spatial distribution and facies architecture in transgressive systems.

Structural controls on the stratigraphic architecture of net‐transgressive shallow‐marine strata in a salt‐influenced rift basin: Middle‐to‐Upper Jurassic Egersund Basin, Norwegian North Sea

AbstractIn this study, we integrate 3D seismic reflection, wireline log, biostratigraphic and core data from the Egersund Basin, Norwegian North Sea to determine the impact of syn‐depositional salt movement and associated growth faulting on the sedimentology and stratigraphic architecture of the Middle‐to‐Upper Jurassic, net‐transgressive, syn‐rift succession. Borehole data indicate that Middle‐to‐Upper Jurassic strata consist of low‐energy, wave‐dominated offshore and shoreface deposits and coal‐bearing coastal‐plain deposits. These deposits are arranged in four parasequences that are aggradationally to retrogradationally stacked to form a net‐transgressive succession that is up to 150‐m thick, at least 20 km in depositional strike (SW‐NE) extent, and >70 km in depositional dip (NW‐SE) extent. In this rift‐margin location, changes in thickness but not facies are noted across active salt structures. Abrupt facies changes, from shoreface sandstones to offshore mudstones, only occur across large displacement, basement‐involved normal faults. Comparisons to other tectonically active salt‐influenced basins suggest that facies changes across syn‐depositional salt structures are observed only where expansion indices are >2. Subsidence between salt walls resulted in local preservation of coastal‐plain deposits that cap shoreface parasequences, which were locally removed by transgressive erosion in adjacent areas of lower subsidence. The depositional dip that characterizes the Egersund Basin is unusual and likely resulted from its marginal location within the evolving North Sea rift and an extra‐basinal sediment supply from the Norwegian mainland.

Rapid Mud Accumulation On the Central Texas Shelf Linked To Climate Change and Sea-Level Rise

Abstract The Holocene Texas Mud Blanket (TMB) is a large (~ 300 km3 containing ~ 5 × 1011 t of sediment) depocenter on the central Texas shelf and represents a stratigraphic anomaly in that it is the only significant transgressive-highstand depocenter in the northern Gulf of Mexico other than the Mississippi River Delta. Given its size and remoteness from large rivers, the origin of the TMB has remained problematic. Our results demonstrate that the TMB accumulated mainly in the late Holocene as a result of increased sediment supply from rivers and an increase in delivery of Mississippi River sediments to the region by marine currents. Approximately 3000 km of high-resolution 2D seismic data was used to construct detailed isopach maps of the TMB. A robust radiocarbon age model derived from long sediment cores is used to measure its accumulation through time. Mineralogical (XRD) results provide information about temporal variability in the source of sediments to the TMB and indicate that sediments came mainly from the Colorado, Brazos, and Mississippi rivers, with the Rio Grande and smaller rivers of Texas having been minor contributors. The second largest source is from transgressive erosion of the shelf. The TMB accumulated in a shelf depression that exists between the falling-stage Colorado and Brazos deltas to the north and the Rio Grande Delta to the south. Initial inundation of the outer shelf occurred ~ 17 ka when terrestrial and lagoonal sediments filled the deepest parts of the depocenter. Between ~ 17 and ~ 9 ka was a time of rapid eustatic rise and low sedimentation (VAR, volumetric accumulation rate, = 0.4 km3/ka; MAR, mass accumulation rate, = 0.7 × 106 t/y). At ~ 9 ka, sediment accumulation dramatically increased to 41 km3/ka (69 × 106 t/y). This sediment was mainly generated by ravinement of shelf strata, including the ancestral Brazos and Colorado deltas. Between ~ 7.5 and ~ 4 ka, Texas was experiencing maximum temperature and minimum precipitation for the Holocene (the Climatic Optimum). This was a time of minimum sedimentation in the TMB. This was followed by a period of remarkable growth during the late Holocene (~ 3.5 ka to Present) when the TMB accumulated 172 km3, total volume, or 2.86 × 1011 t of sediment, accounting for 57% of its volume. Mineralogical results, plus the sheer volume of sediment deposited during this time, calls for a corresponding increase in sediment delivery to shelf from the Brazos and Colorado rivers and an increase in the delivery of sediment from the Mississippi River. By the late Holocene the Brazos and Colorado rivers had filled their lower valleys with sediment, thus eliminating onshore accommodation and increasing sediment delivery to the Gulf. The dramatic increase in sediment delivery from the Mississippi River to the TMB during the late Holocene is best explained by an increase in southeasterly winds, which drive westward-flowing marine currents in the northwestern Gulf.

Tidally influenced clastic epeiric sea at a Mesoproterozoic continental margin, Rjukan Rift Basin, southern Norway

Precambrian epeiric sea paleoenvironments record information about ancient tidal conditions and nearshore sedimentation patterns. The Mesoproterozoic Brattefjell Formation, part of the Rjukan Rift Basin in southern Norway, presents diagnostic criteria of an ancient tidal influence in a shallow clastic epeiric sea environment during the post-rift stage of the sedimentary basin. Eight different sedimentary lithofacies/subfacies and two genetically related lithofacies associations were recognized from the formation. The conglomerate lithofacies records a transgressive ravinement surface due to wave scouring. The sandstone lithofacies consists of six subfacies: ripple-marked, planar cross-bedded, trough cross-bedded, low-angle cross-bedded, and horizontal laminated sandstones. The sandstone lithofacies records sedimentation by shallow current flows or oscillatory waves of lateral migrating straight- or slightly sinuous-crested sandbars and channel-fill deposits. The associated bi-polar cross-bedding, slump-folds, ladder-back type ripples, shrinkage cracks, double-mud drapes, iron ooids, load casts, and flame structures indicate variation of the flood-ebb cycles in tidal sandbars, rapid sedimentation, and sediment compaction. Tidal bundle measurements suggest that 16–18 bundles were formed during the neap-spring cycle, indicating mixed semidiurnal tides. A mudstone lithofacies (two subfacies) records waning current strength deposits, channel abandonment, and either vertical aggradation of the channels or coastal plain deposits. The lithofacies associations reflect fluctuating energy conditions in a shallow clastic shoreline influenced by tides and waves. At least 23 shallowing upward cycles caused by autocyclic tidal water level changes were recognized from the lower part of the formation.Based on the lithofacies assemblages, the Brattefjell Formation can be subdivided into sandy subtidal, lagoon-estuary, and shallow marine shoreface-offshore environments deposited above the fairwater wave base. The Brattefjell Formation records millions of years of fluctuating water levels with an overall transgressive trend, indicating allocyclic patterns caused by regional subsidence. The formation is interpreted to represent a tidally influenced epeiric sea in which shallow water levels extended over part of a continent and it was associated with marine transgression. The shoreface was fed sediment by alongshore and coastal current drifts during shoreface transgressive erosion, in a low shoreline gradient with reduced sediment input and lack of major braided-fluvial channel(s).

Evaluating delta asymmetry using three‐dimensional facies architecture and ichnological analysis, Ferron ‘Notom Delta’, Capital Reef, Utah, USA

AbstractDelta asymmetry occurs where there is strong wave influence and net longshore transport. Differences in the morphology and facies architecture between updrift and downdrift sides of asymmetric deltas are potentially significant for exploration and exploitation of resources in this class of reservoirs. Although delta asymmetry has been recognized widely from modern wave‐influenced deltaic shorelines, there are few documented examples in the ancient record. Based on an integrated sedimentological and ichnological study, the along‐strike variability and delta asymmetry within a single parasequence (Ps 6) is documented in continuously exposed outcrops of the Cretaceous Ferron Sandstone Member of the Mancos Shale Formation near Hanksville in southern Utah. Two intra‐parasequence discontinuity surfaces are recognized which allow subdivision of the parasequence into three bedsets, marked as Ps 6‐1 to Ps 6‐3. Four facies successions are recognized: (i) wave/storm‐dominated shoreface; (ii) river‐dominated delta front; (iii) wave/storm‐reworked delta front; and (iv) distributary channel and mouth bar. Dips of cross‐strata within distributary‐mouth bars and shorefaces show a strong downdrift (southward) component. Ps 6‐3 predominantly consists of river‐dominated delta‐front deposits, whereas Ps 6‐1 and Ps 6‐2 show an along‐strike facies change with shoreface deposits in the north, passing into heterolithic, river‐dominated delta‐front successions south to south‐eastward, and wave/storm‐reworked delta‐front deposits further to the south‐east. Trace fossil suites correspondingly show distinct along‐strike changes from robust and diverse expressions of the archetypal Cruziana Ichnofacies and Skolithos Ichnofacies, into suites characterized by horizontal, morphologically simple, facies‐crossing ichnogenera, reflecting a more stressed, river‐dominated environment. Further south‐eastward, trace fossil abundance and diversity increase, reflecting a return to archetypal ichnofacies. The overall facies integrated with palaeocurrent data indicate delta asymmetry. The asymmetric delta consists of sandier shoreface deposits on the updrift side and mixed riverine and wave/storm‐reworked deposits on the downdrift side, similar to that observed in the modern examples. However, in contrast to the recent delta asymmetry models, significant paralic, lagoonal and bay‐fill facies are not documented in the downdrift regions of the asymmetric delta. This observation is attributed to a negative palaeoshoreline trajectory during delta progradation and subsequent transgressive erosion. The asymmetric delta was induced by net longshore transport from north to south. The forced regressive nature of the delta precludes significant preservation of topset mud.

Anatomy of the seaward steps and seaward termination of the Brent clastic wedge

ABSTRACTThe northward and seaward outbuilding of the Middle Jurassic Brent delta in the Northern North Sea, comprising from the base and upwards Rannoch, Etive and Lower–Middle Ness Formations, displays a stacking pattern of coastal facies tracts described as ‘steps and rises’. Each ‘step and rise’ is formed in response to stratigraphic base‐level (A/S ratio) changes and defines high‐frequency cycles within the Brent Group. During a decreasing A/S ratio regime shoreface progradation dominated and formed the wave–storm‐dominated Rannoch facies tract with a rather flat shoreface trajectory curve resembling a step. A succeeding change to an increasing A/S ratio regime led to a transgression and stratigraphic rise (step‐up) due to aggradation of tidal bay and lagoonal–barrier complexes (Etive facies tract) on top of the previous deposited shoreface. This pattern was formed both due to sediment volume partitioning comprising spatial displacement of depocentres through time and facies differentiation involving changes in coastal geomorphology and dominant deposited facies tract. In the northern areas, where the Brent seaward stepping and the entire Brent clastic wedge terminates seaward, a vertical stack of Etive and Rannoch facies tracts occurs. The Rannoch facies tract has the northernmost extension and the termination of the Brent clastic wedge comprises a vertical stack of shoreface and shelf deposits. The aggradational nature of the tidal bay–barrier sandstone complex of the Etive facies tract explains the seaward thinning from 30 to 40 m to zero sandstone thickness within a short distance (a few kilometres). Facies tract mapping, architectural relationships and mass balance considerations do not indicate that the Etive facies tract was formed due to ‘forced regression’ or ‘lowstand’ developments. Instead, the basal surface of the Etive facies tract was probably formed by transgressive erosion. The embayments created by the transgression were filled by aggradational tidal bay and barrier complex deposition of the Etive facies tract.The seaward stepping, lower Brent Group, shows similarities to outcrop analogues of the Cretaceous, Mesa Verde Group in the Western Interior Seaway and the Palaeogene, Battfjellet Formation on Spitsbergen, supporting a model with facies differentiation within high‐frequency cycles, and that the large‐scale stratigraphic development is controlled by the stacking of these cycles.

Facies architecture of a net transgressive sandstone reservoir analog: The Cretaceous Hosta Tongue, New Mexico

Net transgressive sandstones form a significant component of many shallow-marine reservoirs, but their shale-poor character commonly masks complex facies architecture and stratigraphy associated with significant permeability variations that impact reservoir drainage patterns and ultimate recovery. In this article, the controls on net transgressive sandstone reservoir architecture are investigated through a detailed analysis of the Cretaceous Hosta Tongue of the Point Lookout Sandstone (informally termed Hosta sandstone in this article) outcrop in New Mexico. Mapping of facies architecture within a series of adjacent canyons has enabled a quantitative three-dimensional reconstruction of key stratigraphic surfaces and sand body distributions from an updip pinch-out to a downdip pinch-out of the net transgressive sandstone complex.The Hosta sandstone contains a complex arrangement of wave- and tide-dominated facies associations arranged in an overall transgressive pattern. Tidal channel-fill sandstones, tidal sheet-form sandstones, and heterolithic tidal-flat and lagoonal deposits comprise the stratigraphy in the updip part of the system. These deposits pass abruptly downdip into wave-dominated shoreface sandstones. The facies composition indicates that the Hosta sandstone represents a wave-dominated barrier shoreline and a tide-dominated back-barrier lagoon. Facies associations are partitioned both vertically and laterally by a hierarchy of transgressive erosion (ravinement) surfaces cut by wave and tidal processes. Reconstructing the geomorphology and spatial organization of these surfaces is critical to understanding sand body distribution and facies architecture at high-resolution (intrareservoir) scale. The exceptional quality of the Hosta Sandstone outcrops has enabled (1) improved understanding of patterns and controls of facies architecture in net transgressive sandstone reservoirs, (2) construction of predictive templates of facies architecture in interwell volumes, and (3) quantification of geobody dimensions and spatial distribution patterns. In combination, these data provide appropriate qualitative and quantitative conditioning for reservoir models.

Bivalve trace fossils in an early Miocene discontinuity surface in Patagonia, Argentina: Burrowing behavior and implications for ichnotaxonomy at the firmground–hardground divide

Ichnofossils developed in a firmground at the contact between the middle Eocene–lower Miocene continental Sarmiento Formation and the lower Miocene marine Chenque Formation, in central-eastern Patagonia, Argentina, are assigned to Gastrochaenolites ornatus Kelly and Bromley [Kelly, S.R.A., Bromley, R.G., 1984. Ichnological nomenclature of clavate borings. Palaeontology 27, 793–807.], and interpreted as dwelling structures of suspension-feeding bivalves. These record the activities of pholadids, most likely belonging to the Pholadinae. Bivalves are usually preserved as casts and external molds within the biogenic structures. The details of the internal ornamentation in G. ornatus allow identification of two sets of scratch patterns, permitting comparison with the boring behavior of modern pholadids. In terms of ichnotaxonomy, when the same organism is able both to excavate and to bore, and the excavation technique is identical to the mechanical perforation technique, the same name should be used. Accordingly, Gastrochaenolites, whilst more commonly assigned to bioerosion structures in hard substrates, is herein regarded as available also for bivalve burrows in firm, but unlithified substrates. The firmground studied here represents a co-planar surface of lowstand erosion and transgressive erosion that produced exhumation of deposits belonging to the Sarmiento Formation, providing appropriate conditions for the development of the Glossifungites ichnofacies. Differential erosion of the apertural necks, the heterogeneity of the available casting material, and the wide size range of ichnofossils suggest a complex history of colonization and erosion for this surface.