ABSTRACT An extensive grid of high-resolution seismic data, hundreds of sediment cores, and a robust radiocarbon-age data set acquired over nearly four decades allows detailed analysis of Holocene coastal evolution of western Louisiana and Texas, USA. Results from this study provide a framework for assessing the response of a myriad of coastal environments to climate change and variable sea-level rise. Climate varies across the region today, spanning four climate zones from humid to semi-arid, and has fluctuated during the Holocene. The most notable changes were alterations between cool/wet and warm/dry conditions. Sea-level records for the northwestern Gulf of Mexico indicate an average rate of rise during the early Holocene of 4.2 mm/yr, punctuated by rates exceeding 10.0 mm/yr. After ca. 7.0 ka, the rate of rise slowed, and by ca. 4.0 ka, the average rate decreased from 0.6 mm/yr to 0.3 mm/yr. The current rate of sea-level rise in the region is 3.0 mm/yr, marking a return to early Holocene conditions. Despite its incomplete stratigraphic record of coastal evolution during the middle and early Holocene, it is still the most complete record for the Gulf Coast. Bay evolution, as recorded within the offshore Trinity and Sabine incised valleys, was characterized by periods of bayhead delta and tidal delta expansion, followed by episodes of dramatic landward shifts in these environments. The ancestral Brazos, Colorado, and Rio Grande river deltas and coastal barriers also experienced landward stepping during the early Holocene. The widespread nature of these flooding events and their impact on multiple coastal environments suggests that they were caused by episodes of rapid sea-level rise. Similar methods were used to study modern bays, including the acquisition of seismic lines and drill cores along the axes of the bays to examine the magnitudes and timing of transgressive events. Results from Lake Calcasieu, Sabine Lake, Galveston Bay, Matagorda Bay, Copano Bay, Corpus Christi Bay, and Baffin Bay reveal that landward shifts in bayhead deltas, on the order of kilometers per century, occurred between 9.8 ka and 9.5 ka, 8.9–8.5 ka, 8.4–8.0 ka, and 7.9–7.5 ka. These results are consistent with those from offshore studies and indicate that punctuated sea-level rise dominated coastal evolution during the early Holocene. By ca. 7.0 ka, the average rate of sea-level rise in the northern Gulf of Mexico decreased to 1.4 mm/yr, and there was considerable sinuosity of the coastline and variability in the timing of bay and coastal barrier evolution. The diachronous nature of coastal environment migration across the region indicates that sea-level rise played a secondary role to climate-controlled oscillations in river sediment discharge to the coast. At ca. 4.0 ka, the average rate of sea-level rise decreased to 0.5 mm/yr. During this period of slow sea-level rise, coastal bays began to take on their current form, with the exception of changes in the sizes and locations of bayhead deltas caused by changes in sediment supply from rivers. There were also significant changes in the size and configuration of tidal inlets and deltas as a result of barrier growth. The late Holocene was also a time when coastal barriers experienced progradation and transgression on the order of several kilometers. The timing of these changes varied across the region, which is another indication that sea-level rise played a minor role in coastal change during the late Holocene. Instead, barrier evolution during this time was controlled by fluctuations in sand supply to the coast from rivers and offshore sources. Historical records indicate a dramatic reversal in coastal evolution marked by increased landward shoreline migration of chenier plains and coastal barriers across the region. The main cause of this change is accelerated sea-level rise during this century and diminished sediment supply to the coast. Wetlands are also experiencing rapid change due to their inability to keep pace with sea-level rise, especially in areas where subsidence rates are high. Although direct human influence is a factor in these changes, these impacts are more localized. Coastal change is expected to increase over the next several decades as the rate of sea-level rise increases, the climate in Texas becomes more arid, and more severe storms impact the coast.

Global sea-level rise increased during the twentieth century from 1.5 to 3.0 mm/yr and is expected to at least double over the next few decades. The Western Louisiana and Texas coast is especially vulnerable to sea-level rise due to low gradients, high subsidence, and depleted sediment supply. This Memoir describes the regional response of coastal environments to variable rates of sea-level rise and sediment supply during Holocene to modern time. It is based on results from more than six decades of research focused on coastal and nearshore stratigraphic records. The results are a wake-up call for those who underestimate the potential magnitude of coastal change over decadal to centennial time scales, with dramatic changes caused by accelerated sea-level rise and diminished sediment supply.

Abstract A robust collection of seismic and geomorphic data is used to examine the evolution of the Antarctic Ice Sheet within the Ross Sea Embayment. We use geomorphic data to reconstruct Last Glacial Maximum and post-Last Glacial Maximum ice sheet drainage and demonstrate retreat behaviours for the East Antarctic and West Antarctic sectors of the ice sheet. Using this framework, we then use seismic data and chronostratigraphic information from drill cores to reconstruct the long-term evolution of the ice sheet. Early ice sheet evolution during the Late Oligocene was characterized by isolated ice caps on bathymetric highs, followed by an interval of sediment infilling of rift basins and the development of more subdued relief in the eastern Ross Sea than in the western Ross Sea. Both ice sheets have experienced multiple episodes of expansion across the continental shelf since the Middle Miocene, with the frequency increasing during the Plio-Pleistocene. We conclude that seafloor bathymetry has been the principal control on ice sheet palaeodrainage and retreat behaviour since at least the middle Miocene, demonstrated by broad West Antarctic ice streams loosely guided by south to north cross-shelf troughs, whereas East Antarctic ice streams were funnelled through troughs that merge and converge around banks.

ABSTRACT The northern Puget Lowland of Washington State, USA, provides an exceptional opportunity not only to examine grounding line processes associated with marine-based ice sheets, but also to relate subaerial outcrop to marine geological observations of grounding line landforms and sedimentary processes in Antarctica and the deglaciated Northern Hemisphere. During this trip, we visit outcrops that record the interaction of the Cordilleran Ice Sheet and its bed, starting with locations where the ice sheet slowly flowed across crystalline bedrock. We also visit locations where the ice flowed across unconsolidated deposits, allowing discussions of subglacial bed deformation and grounding zone wedge development. Evidence shows that grounding line retreat across Whidbey Island was punctuated by periods of grounding line position stability and local ice advance during the growth of multiple grounding zone wedges. We will discuss the criteria for identifying grounding zone wedges, including diamicton units with foreset bedding that downlap onto a regional glacial unconformity at the base, and are truncated at the top by localized unconformities indicative of ice advance across the foreset beds. Grounding zone wedge foreset beds are composed of debris flows sourced from a deformation till and from sediment transported to the grounding line by subglacial meltwater. The overlying surface unconformity is associated with a laterally discontinuous till and pervasive glacial lineations. Other field stops focus on iceberg scouring and evidence of subglacial meltwater drainage, as well as the transition from marine to subaerial conditions during retreat of the Cordilleran Ice Sheet from the northern Puget Lowland.

Abstract Hurricanes annually threaten the Atlantic Ocean margins. Historical hurricane records are relatively short and palaeohurricane sedimentary archives provide a geological and climatic context that sheds light on future hurricane activity. Here we review palaeo-trends in hurricane activity elucidated from sedimentary archives. We discuss dating methods, site selection and statistics associated with previously published records. These archives have been useful for understanding the long-term evolution of coastal systems and the response of intense hurricane activity to climatic changes. Regional shifts in hurricane overwash on centennial to millennial timescales have been linked to various climatic modes of variability, including El Niño/Southern Oscillation and the North Atlantic Oscillation, but could also reflect regional-scale controls on hurricane activity.

Shelf-margin deltas and linked downslope depositional systems are in most cases fed by alluvial valleys that serve to deliver sediment eroded from the hinterland. Accordingly, alluvial valleys provide the link between processes that control sediment flux to the continental margin and processes that control dispersal into the basin. Current research shows the volume of sediment delivered to the margin will reflect hinterland drainage areas and large-scale relief. Superimposed on this background rate will be an unsteadiness that reflects climate change in hinterland source regions, but the rates and directions of change in sediment supply will vary regionally. Alluvial valleys modulate unsteadiness in sediment supply through changes in sediment storage. However, regional variability in the rates and directions of change in sediment supply insures that responses to climate change are regionally circumscribed, and alluvial valley systems in different regions may respond in opposite ways to the same global climate change. Sea-level change has little effect on the total volume of sediment delivered to the margin, but instead forces channel extension and shortening, which plays a major role in determining the proximal to distal location of the river mouth point source through which sediment is dispersed to the shelf and beyond. Moreover, the widely used concept of incision and complete sediment bypass within incised valley systems during periods of relative sea-level fall should be abandoned. Instead, falling stage to lowstand fluvial deposition is actually common in well-studied Quaternary analog systems, and falling stage sand bodies may comprise a significant proportion of reservoir-quality sands within many incised valley fill depositional sequences. Models for falling stage and lowstand systems tracts should therefore incorporate significant fluvial channel belt deposits that are likely connected to, and feeding, the offlapping shore faces, shelf-margin deltas, and linked downslope systems.

Abstract Along the shelf margin of the northern Gulf of Mexico, numerous late Quaternary deltaic systems occur where ancestral rivers encountered the shelf-slope break. These shelf-margin deltas are products of deposition during glacioeustatic fluctuations resulting from expansion and contraction of continental ice sheets. Lowered sea level shifts paralic environments seaward and creates widespread subaerial unconformities, well-defined drainage networks (incised valleys), and deltaic systems that prograded to the shelf margin. Shelf margin deltas are primary mechanisms for shelf margin and upper slope progradation, and serve as important conduits of sediment to deeper water environments.

Abstract An ultra-high resolution, short-offset 3D seismic survey (EBHR3D) has been used to study the sedimentary fill of an intra-slope basin in the East Breaks area of the Gulf of Mexico. The site chosen for the seismic program is the fourth and southernmost basin (Basin 4) in the Brazos-Trinity Slope System. The Brazos-Trinity Slope System is a set of latest Pleistocene salt-withdrawal basins that are connected by channels in the upper to middle portion of the Texas continental slope. They are filled with sediment delivered to the slope by the ancestral Brazos and Trinity rivers and associated shelf edge deltas. Together, the linked shelf and slope depositional systems form a late Pleistocene lowstand systems tract. The seismic survey has been designed to target a large submarine fan at the top of the basin-filling succession (the Upper Fan), but imaging of the entire 250 m (maximum) of basin fill is excellent. The results are providing detailed information regarding deep water deposition far surpassing what is possible from outcrop or conventional subsurface studies. The data provide unprecedented images of the three-dimensional geometry and internal architecture of these deepwater deposits. The fill of Basin 4 records a stratigraphic evolution that includes a “ponded” fill stage followed by a “perched” fill stage. Contrasting deposit geometry and stacking patterns occur during these two stages of evolution. The perched fill of the basin contains the Upper Fan, which is located in the shallowest portion of the subsurface beneath an extensive Holocene drape. The Upper Fan represents the terminal, distributive complex of the lowstand system tract. It is a basinward-tapering wedge of sediment that contains both channel-form and sheet-like depositional elements. The prominent stratigraphic features interpreted from the Upper Fan are: (1) off-lapping, clinoform reflection patterns; (2) distributary channel systems linked to channel mouth lobes; (3) down-fan progression in architecture from channel-form elements to more sheet-like elements; and (4) down- and across-fan decrease in sand percent and/or grain size inferred from seismic attributes. In these and other ways, the stratigraphy of the Upper Fan is similar to that commonly observed for modern and ancient river deltas.

Abstract During the previous glacial-eustatic fall, the ancestral Brazos and western Louisiana rivers, which flowed across low gradient coastal plains and shelves, constructed large fluvial-dominated deltas that extend to the shelf margin. These rivers shifted to new locations prior to the lowstand, resulting in shelf-margin deltas that have no associated down-dip lowstand deltas or fans. The Trinity and Colorado rivers remained fixed in their locations throughout the eustatic fall and lowstand, resulting in linked valley/delta/fan complexes. Re-incision of lowstand valleys by these rivers over several eustatic cycles resulted in significant sediment bypass to the slope. Factors that influenced the response of rivers to falling sea level include long-term sediment supply, diapiric controls on channel location, and the physiography of the shelf over which the rivers flowed.

Abstract Lithologic, biostratigraphic and isotopic data from cuttings provides calibration of three sigmoidal, clinoform packages separated by regionally continuous, parallel seismic facies. This depositional geometry is interpreted as a shelf-margin, deltaic wedge deposited within the East Breaks 160-161 minibasin. A chaotic seismic facies package extending over more than 84 mi 2 (218 km 2 ) occurs between two clinothem packages of Ericson Zone Y (71 ka BP to 12 ka BP). The chaotic package is interpreted to be a mass-transport complex that failed during the accelerated rate of sea level fall during late Oxygen Isotope Stage 3 (approximately 30 to 20 Ka). Clinothem foresets, bottom sets and the mass-transport complex are predominantly clay with minor siltstone. Sands are restricted to proximal topsets and fluvial channels. The mass-transport complex consists of three subfacies: rotated-block, hummocky-mounded, and disrupted. Distribution and volume of these facies suggests that only the rotated-block subfacies has been significantly transported, and that the hummocky-mounded and disrupted subfacies result from different degrees of disruption of ponded, clay-prone layered sediments by compression and dewatering triggered by the submarine slide of rotated blocks.

Abstract Linked shelf edge deltas and slope channel systems are observed in the eastern Gulf of Mexico. The slope channels are characterized by deep incision into the substrate and moderate sinuosity nearly to the shelf-slope break. Channelized flows were not fully confined as evidenced by well-developed levees up to 90 m thick. This sinuosity suggests that turbulent flow within the channel was likely nearly from the uppermost slope. With apparent turbulence characterizing these channels nearly to the shelf-slope break, the dominant mode of sediment delivery to the slope and basin beyond probably was in the form of density underflow ( i.e. , hyperpycnal flow) rather than shelf edge slump and/or slide progressively transformed into turbidity flow. The stages of evolution of these slope channels are (1) clustering of small slope gullies on the slope at the initiation of lowstand deposition, (2) dominance of one of these slope gullies and formation of one significant channel, formation of a frontal splay fed by the dominant channel, (3) abandonment of frontal splay deposition in favor of leveed channel deposition across the entire slope, and (4) entrenchment of the leveed channel into the earlier deposited leveed channel and frontal splay.

Unravelling end-Cretaceous paleobathymetric dip-profiles along strike in the northern Gulf of Mexico continental margin (nGoM) is important because it provides us with the initial framework in which to assess the evolution of Cenozoic clastic systems and burial history. Light can be shed on the problem by integrating seismic and potential field data with analytical basin modelling techniques and palinspastic-kinematic paleo-geographic analysis. Progressive palinspastic reconstruction of the Gulf is critical to setting up the appropriate lithospheric models to assess subsidence history. Kinematically, the opening of the GoM involved an early rift stage of northwest–southeast stretching (with minor counterclockwise [CCW] rotation) between North America and Yucatán (Triassic–Early Oxfordian), followed by a drift stage of seafloor spreading between the opposing rifted margins that entailed significant CCW rotation of Yucatán Block. This Stage 2 rotation was accommodated by transform motion of Yucatan/Chiapas Massif along the foot of the very narrow eastern Mexican margin, but transform motion stopped once the Central Gulf Spreading Ridge passed any point along this margin. Thus, the crustal boundary along eastern Mexico is ultimately an igneous, constructional contact that is overlain by the entire stratigraphy visible on seismic, and no transform faults are to be expected ( Pindell, 1985 ). The rift stage was asymmetric ( Pindell et al. , 1986 ; Marton and Buffler, 1994) such that Yucatán collapsed off the mainly northwest-vergent Alleghenian Orogen of the southern USA. The nGoM margin (foot-wall) underwent large and rapid tectonic subsidence during the rift stage (because the crust was highly stretched), but little thermal subsidence during the drift stage and thereafter (because the lithosphere was NOT stretched much). In contrast, the Yucatán hanging wall underwent little syn-rift subsidence (because the crust was not stretched much), followed by considerable postrift (Late Jurassic and younger) thermal subsidence (because the lithosphere was stretched). During the rift stage, thick red beds, possibly with lacustrine or even marine intervals, effectively buried basement in most nGoM areas and, toward the end of the rift stage (Callovian–Early Oxfordian), gave way to salt deposition across much of the half-open Gulf basin. Original salt thickness is generally considered to exceed what could have been achieved by thermal subsidence alone (~2km) during Callovian–Early Oxfordian time (presumed agespan of salt deposition); thus, salt accumulation was coeval with Stage 1 tectonic subsidence (syn-rift) and/or involved the marine inundation of pre-existing, isolated, sub-sea level accommodation space, which, by Oxfordian time, was almost definitely filled to sea level with salt. Together, red beds and salt probably are 5–10km thick beneath much of the nGoM rifted margin. Oxfordian onset of CCW rotational seafloor spreading in the Gulf split the pre-existing red-bed/salt basin into the Louann and Campeche halves. Backstripping shows that ocean crust was generated near its typical 2.6km depth below sea level, and not at an Icelandic-setting near sea level. The continent-ocean boundary typically is marked by a large step up from continental to oceanic crust ( i.e. , the rifted continental crust was already buried by red beds and salt far thicker than 2.6km ocean-generation depth, so the basement step to ocean crust is UP). As spreading ensued, a central, widening “chasm” was produced that was floored by oceanic crust and that, once Smackover open marine conditions were established, received no new depositional salt. To our knowledge, truly autochthonous salt cannot be shown to overlie definite oceanic crust; thus, initial spreading was effectively coeval with the onset of Smackover open marine conditions, and there may have been a causal relationship between the onset of spreading and the breaking of the evaporitic sill, wherever that was (Florida Straits or Veracruz Basin are equally viable guesses). As seen south of the Middle Grounds margin, the immediately adjacent shoulders of these salt walls halo-kinetically collapsed into the widening chasm, but, given the enormous width of the nGoM rifted margin, an important question is to assess how far north into the salt basin such early collapse occurred. The Red Sea analogue shows that the salt could have supported shelf platforms at least into the Cretaceous; we typically observe minor (<20km) extrusion of salt across the step up onto oceanic crust, but locally, such as at Sigsbee Escarpment, salt may have collapsed much farther (100km) onto the ocean crust, possibly as early as Late Jurassic–Cretaceous times. In such places, the term “parautochthonous salt” applies, because the salt still underlies most stratigraphy although it acquires a tapered-wedge cross-sectional geometry as it collapses. Because the nGoM margin was the footwall during Jurassic asymmetric rifting, thermal subsidence had far less influence on paleobathymetry than is commonly believed. Thus, determination of paleobathymetry can be roughly gauged by structural analysis of halokinesis. Thus, for large areas of the nGoM margin, we propose (1) that a relatively shallow, “supra-salt platform” persisted until the Late Paleocene onset of the well-known Wilcox growth faulting, and (2) that the Upper Jurassic–Cretaceous supra-platform section remained shallow, and was never deeply buried until halokinetic collapse began. This contrasts sharply with the Campeche Salt margin of Mexico, which was drowned to truly basinal depths in the Late Jurassic-Early Cretaceous due to far higher rates of post-rift thermal subsidence and weak clastic sediment supply. Thus, in the north but not in the south, we envision a very broad, relatively shallow supra-salt platform with a thinner-than-often-assumed Upper Jurassic-Cretaceous section that extended well beyond much of today's coastline. In this case, the true continental slope and rise would have been located much farther out than the Stuart City carbonate trend (which is often inferred as the paleo-shelf edge). This platform may have been stepped due to early halokinesis, particularly at Sigsbee, and along its outer reaches probably sloped or ramped down to the area of oceanic crust. Given this scenario for the paleobathymetry, it should not be surprising to find early Paleogene sands at the foot of that platform slope ( e.g. , Perdido area). The sands could have been transported there from (1) the north or northwest by shelf bypass across the suprasalt platform, or (2) the west, out of a proto-Rio Grande river system, or both. By the end of the Paleocene, salt collapse in updip areas of the supra-salt platform began due to differential burial by prograding clastics, producing syn-depositional counter-regional faults and basin-facing half-grabens at the Wilcox and younger fault trends, which controlled the [new, syntectonic] position of the paleo-shelf edge. Such collapse fed downslope shortening behind (landward of) the Paleogene sands at the foot of the true continental slope. We infer detachment on salt, such that the Mesozoic marine supra-salt section was cut both updip and downdip by at least some of the faults. Apparent rafting of the Mesozoic shelf section at the landward limit of the Wilcox trend ( e.g. , Anderson and Fiduk, 2003 , and also observed in NE Mexico on seismic by the authors) demonstrates that the salt (and inferred Upper Jurassic–Cretaceous shallow shelf) was mobile during end-K/early T time. The concept of the Mesozoic supra-salt platform in the nGoM: (1) requires significant changes to commonly-accepted Late Jurassic through Paleocene paleobathymetric and paleogeographic maps of nGoM , and therefore of reservoir and source rock distribution; (2) indicates the need to develop maturation models for the inner shelf areas that do not assume a pre-existing deep-water setting outboard of the Sligo/Stuart City “reef trends” prior to Tertiary clastic deposition; and (3) provides a new paleogeographic context for assessments and models of Cenozoic deltaic and progradational depositional systems along the northern Gulf of Mexico.

Abstract The upper Miocene (late middle to early late Miocene) depositional episode (UM depisode) records a long-lived family of sediment dispersal systems that persisted for nearly 6 Ma with little modification. In the central Gulf of Mexico basin, this depisode records extensive margin offlap, primarily centered on the paleo-Tennessee River and Mississippi River dispersal axes, that began immediately following the Textularia W/Textularia stapperi flooding and is terminated by a regional flooding event associated with the Robulus E biostratigraphic top. Thickest sediments are deposited in the paleo-Tennessee River delta beneath modern southeast Louisiana, where three major depocenters are recognized. These depocenters have migrated in both strike and dip directions, and margin progradation is very prominent. The composite fluvial-dominated paleo-Tennessee and Mississippi delta system rapidly built beyond the subjacent middle Miocene shelf margin to construct a sandy delta-fed apron. Margin outbuilding was locally and briefly interrupted by hyper-subsidence due to salt withdrawal and consequent slope mass wasting. Sediments also continuously bypassed into the Mississippi Canyon, Atwater Valley and Green Canyon OCS areas throughout the entire upper Miocene, forming two secondary depocenters composing the McAVLU submarine fan system at the base of the paleo-continental slope. A broad, but relatively thin, sandy strandplain and clastic shelf succession, supplied by reworking of the deltaic deposits, extended eastward and westward from the delta system. Abundant strike-reworked sediment locally prograded the strand plain to the shelf edge, and slope offlap exceeds 30 mi (50 km). The presence of extremely large volumes of high-quality shelf margin delta and deep-water fan sandstone reservoirs results in the great productivity of the central Gulf of Mexico upper Miocene, and upper Miocene production is dominated by a major deltaic oil and gas trend straddling the southeast Louisiana coast.