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.

Galveston Island and Bolivar Peninsula have experienced a well-documented history of shoreline and bay shoreline change ranging from +3.63 m/yr to −1.95 m/yr. By integrating core, Light detection and ranging (LIDAR), and coastal change data, we develop a sand budget that attempts to quantify long-term sand sources (e.g., fluvial sand cannibalization through transgression) and sinks (washover fans, offshore sand bodies, and flood-tidal deltas). These results are then considered in light of anthropogenic influences (e.g., beach-nourishment projects, coastal engineering structures, and dredging operations) in an attempt to relate natural versus human influence on coastal change. Our findings suggest that hurricane washover (Galveston Island = 0.4 m/100 yr; Bolivar Peninsula varies from 0.154 m/100 yr to 0.095 m/100 yr) and offshore sand deposits are minimal long-term sand sinks. Flood-tidal deltas, however, appear to be major locations for natural sand sequestration. We also conclude that damming of rivers has had minimal impact on the upper Texas coast, although hard structures, such as the Galveston seawall and its groins, have exacerbated erosion along a shoreline that is currently sand starved. The impact of hard structures has mainly been one of trapping sand in locations where that sand would not have naturally accreted. Sand supply is minimal, so understanding and developing a better sand budget for the Texas coast are vital to sustainability.

No abstract available.

A composite Holocene sea-level curve for the northern Gulf of Mexico coastal region was constructed using basal or bayline peat and swash-zone deposits determined from careful facies analysis. Initial work involved an assessment of published sea-level datums for the region, which show wide scatter. Reevaluation of individual data points based on sea-level–specific criteria required exclusion of most of these data from the composite curve. Additionally, variations in the age of the radiocarbon reservoir across the region were determined, and site-specific corrections were applied to the sample ages, further increasing the accuracy of the final sea-level curve, which is used in our reconstruction of the evolution of the bays of the region in companion papers herein. The refined regional sea-level curve indicates that site-to-site variations in relative sea-level rise across the coastal plain spanning central Texas to Alabama are minimal. The data indicate rapid and possibly episodic rise during the early Holocene, followed by slower and more continuous rise during the middle and late Holocene. In addition, the new composite regional curve unequivocally plots sea level from −10 to −3 m below present from 8000 cal B.P. to 4000 cal B.P., which precludes the occurrence of a middle Holocene highstand above present sea level in this region. Last, a comparison of our composite curve to Caribbean sea-level curves suggests that coastal subsidence within the study area over the past several thousand years has been minimal. A comparison of the late Holocene record of sea-level rise to satellite altimetry and tide-gauge records indicates that rates of rise have increased by an order of magnitude over the past century.

The rate of creation of sediment accommodation and the rate of sediment accumulation are the primary variables that define the evolution of depositional environments, but they are seldom quantified in studies of coastal evolution. From a detailed map of antecedent topography, a sea-level curve, and measured and modeled sedimentation rates, we quantify these variables for Mobile Bay, Alabama, throughout the Holocene. The timing of recorded rapid changes in depositional environments is compared to the calculated changes in sediment accommodation and sediment accumulation to infer causality. A comparison of cumulative changes in sediment accommodation to cumulative sediment volume changes shows that the estimated volume of sediment accumulation in the estuary kept pace with and slightly exceeded sediment accommodation until around 8.2 ka. Prior to 8.2 ka, Mobile Bay was an intertidal to supratidal marsh, likely part of the delta-plain environment. Remnants of this environment are preserved discontinuously across the bay directly above the exposure surface as peat. At 8.2 ka, parts of the study area were initially submerged, and the central-basin depositional environment was created. Between 8680 and 8200 cal yr B.P., the bay shoreline transgressed up the axis of the study area at a rate of ~100 m/yr, while the extensive delta plain and marsh were eroded and replaced by a central-basin environment. After 8200 cal yr B.P., the locations of the depositional environments of Mobile Bay were relatively static and aggraded throughout the remainder of the Holocene rise in sea level. This record suggests that a threshold was crossed during the early Holocene in which low-gradient antecedent topography was rapidly flooded by sea-level rise. This promoted sediment accretion landward of the study area and facilitated erosion in the study area through inundation and ravinement, which are reflected as a dramatic decrease in the rate of estuarine-basin sediment accumulation between 8680 and 8400 cal yr B.P. Some estuaries and coastal areas are inherently unstable and prone to threshold responses to forcing mechanisms, particularly those characterized by low-gradient and low-elevation topography.

The evolution of depositional environments is strongly controlled by the rate at which sediment accommodation is created; however, this factor is seldom quantified in studies of estuarine evolution. From a detailed map of the exposure surface, or bay line, and a precise sea-level curve, we calculated sediment accommodation over the last 8.4 k.y. in Weeks Bay, Alabama, and compared sediment accommodation with the late Quaternary evolution of the bay derived from seismic and lithologic data. The stratigraphy of the Fish and Magnolia paleovalleys, the two fluvial systems that discharge into the bay, is composed of multiple sequences. Due to data limitations, only the upper sequence boundary was mapped regionally. This surface formed in response to the oxygen isotope stage 2 lowstand, and it is overlain by alluvial and estuarine deposits that are separated by an exposure surface. Estuarine sediments were first deposited at 7200 cal yr B.P., and no bayhead delta sediments are recognized above the bay line, indicating that the Fish and Magnolia bayhead deltas back-stepped across the bay. Bayhead delta back-stepping occurred when the rate of sea-level rise was decreasing. A map of the exposure surface shows it to be broad and relatively flat. When this antecedent topography was flooded, the rate at which sediment accommodation was created increased rapidly, forcing the bayhead delta to step back. The morphology of the land being inundated is the most significant control on changes in sediment accommodation and, therefore, the most important factor to quantify when predicting the future response of coastal systems to sea-level rise.

Calcasieu Lake records a Holocene history of dramatic environmental change. The changes resulted in up to 20–30 km of landward translation of deltaic and estuarine depositional environments, and intervening periods of delta progradation, from 9600 cal B.P. to 1600 cal yr B.P. The sediments preserved beneath the lake record a series of back-stepping events. Seven events that affected the entire estuary are recognized: at 8900–8500 cal yr B.P., 8300–8000 cal yr B.P., 8000–7900 cal yr B.P., 7200 cal yr B.P., 5800–5600 cal yr B.P., 2000–2800 cal yr B.P., and 2500–1800 cal yr B.P. Only one of these events (8200 cal yr B.P.) can be linked to a known eustatic event that is also associated with climate change. The Calcasieu system maintained a bay-head delta within the present-day lake during a period of relatively rapid eustatic sea-level rise (10,000–6000 cal yr B.P.). It was only after 3000 cal yr B.P. that a bay-head delta was not present in the modern Calcasieu Lake. This strongly suggests a climate–sediment supply control. Lower estuary tidal delta and inlet facies indicate restricted estuarine circulation since ca. 2800 cal yr B.P.

The Sabine-Neches fluvial-estuary system is composed of deposits that represent fluvial, deltaic, central-basin, tidal inlet/delta, and chenier depositional systems. The Holocene deposits and associated environmental changes preserved in the drowned Sabine-Neches alluvial valley provide a valuable analog for present and future environmental changes. These deposits are bounded by flooding surfaces that record episodes of dramatic environmental reorganization during the Holocene. The most dramatic environmental changes are manifested as stratigraphic back stepping in which central-basin sediments overlie deltaic sediments. The magnitude of flooding varies from a few tens of kilometers to less pronounced back stepping followed by rapid progradation. Initial flooding of the onshore Sabine-Neches incised valley occurred around 9800 cal yr B.P., and by ca. 8900 cal yr B.P., a vast bayhead delta occupied the southern half of the valley. This delta backstepped up the valley during the relatively rapid sealevel rise of the early to middle Holocene, and by ca. 7100 cal yr B.P., it occupied the entire valley. After ca. 7100 cal yr B.P., the bayhead delta shifted up the valley again, and a central-basin setting existed in the lower half of the valley. The middle basin expanded episodically between ca. 5500 cal yr B.P. and 1700 cal yr B.P., and a brief period of delta growth occurred ~300 yr ago. Controlling mechanisms for flooding surface formation include sea-level rise, changes in the antecedent topography of the incised valley, and sediment supply variations. Antecedent topography was influential in controlling estuarine evolution between ca. 7800 and 7500 cal yr B.P., when an extensive fluvial terrace was inundated. The fact that some flooding surfaces appear to be synchronous, within a few centuries, in several estuaries across the northern Gulf of Mexico suggests a eustatic rather than local control. Flooding events at ca. 8900 cal yr B.P. and ca. 8400–8000 cal yr B.P. were likely caused by rapid, sub–meter-scale sea-level rise events. Sediment supply variations controlled by climatic forcing appear to have been the main cause of other flooding events. Unfortunately, the Holocene climate record for the east Texas–west Louisiana coastal region is poorly documented, and a direct relationship to central and western Texas climate records may be complex. So the exact nature of climate control on sediment flux to the estuary system remains elusive.

Seismic data and sediment cores from the Galveston estuary complex were used to reconstruct the evolution of the estuary during the Holocene. These data show that the estuary complex has had a history of rapid and dramatic change in response to (1) sea-level rise across the irregular topography of the ancestral Trinity River valley and (2) changes in climate, which regulated sediment supply to the estuary. In general, the valley morphology consists of a deep incision near the center and broad, terraced flanks. As sea level rose during the Holocene and flooded the valley, the shape of the estuary changed from narrow and deep to wide and rounded. As sea level rose to the elevation of the relatively flat fluvial terraces, these areas were flooded rapidly, resulting in expansions in bay area and dramatic reorganization of bay environments. The most notable changes were up-valley shifts in the bayhead delta of tens of kilometers in a few centuries. Radiocarbon ages indicate that these events took place ca. 9600, ca. 8500, and between ca. 7700 and 7400 yr B.P. The early flooding events occurred when sea level was rising rapidly (average 4.2 mm/yr; Milliken et al., this volume, Chapter 1), perhaps episodically. The ca. 8200 yr B.P. flooding surface corresponds to a prominent terrace at −14 m. The ca. 7700–7400 yr B.P. flooding episode was the most dramatic in terms of its impact on the estuary setting. Following this event, the area of the estuary increased by ~30%. This flooding event occurred as the rate of sea-level rise was starting to decrease. The level of this flooding surface also corresponds to a terrace at ~–10 m, but the magnitude of flooding is too large to be explained entirely by flooding of this terrace. At the same time, Matagorda Bay to the west and Sabine Lake to the east experienced similar dramatic flooding events. This event occurred when the climate of east-central Texas was in transition from cool and moist to warm and dry, and the vegetation cover of the region was undergoing a reduction in forest and an increase in grasslands. Hence, the ca. 7700–7400 event was likely amplified by a reduction in sediment supply to the estuary triggered by this regional climatic change coupled with an increase in sediment accommodation space caused by flooding of a terrace. Following the ca. 7700–7400 yr B.P. flooding event, the estuary setting changed relatively little as the rate of sea-level rise decreased to less than 2.0 mm/yr. By ca. 2600 cal yr B.P., the modern Trinity bayhead delta had begun to form. Circa 1600 cal yr B.P., the delta experienced a phase of rapid growth. This more recent episode of delta growth may have resulted from an increase in the rate of sediment supply, perhaps associated with human occupation and agriculture in the drainage basin. More recent anthropogenic changes include accelerated subsidence due to groundwater and hydrocarbon extraction. These changes are occurring at rates that approach those that occurred during prior flooding events of the Holocene. Thus, the Galveston estuary complex could be on the verge of another flooding event that would mainly impact the Trinity bayhead delta and low-lying areas around the delta.

Rotary-drill cores from the Matagorda and Lavaca estuary complex sampled six distinct lithofacies, and high-resolution seismic data reveal four main seismic facies. The facies architecture of the bay indicates a history of Holocene flooding punctuated by major landward shifts in estuarine environments. Radiocarbon ages help to constrain the rate and timing of these flooding events and indicate significant reorganization of the estuarine environments at a decadal to century time scale. The Holocene evolution of the Matagorda and Lavaca estuary complex began ca. 11,600 yr B.P. with initial flooding of the portion of the ancestral Lavaca River incised valley that is now occupied by lower Matagorda Bay. Early flooding of the lowstand valley was punctuated by landward shifts in the river mouth, followed by episodes of fluvial aggradation. The most pronounced phase of aggradation occurred between ca. 11,600 and 9600 yr B.P., when sea level was rising rapidly, and a step in the valley floor slowed the rate of flooding. By ca. 9500 cal yr B.P., the deep, narrow part of the Lavaca incised valley had been flooded and was occupied by a bayhead delta. The river mouth shifted landward within Lavaca Bay between 8500 and 8200 cal yr B.P., followed by another landward shift in the delta between 7900 and 7700 cal yr B.P. These changes were mainly caused by variations in the rate of sea-level rise. The most dramatic change in the bay setting occurred between 7300 and 6700 cal yr B.P., when the bayhead delta stepped landward at least 30 km, establishing most of modern-day Matagorda Bay. This flooding event is attributed to a combination of rapid sea-level rise and to a reduction in sediment supply to the estuary that was associated with a change in the regional climate from cool and moist to warm and dry conditions. During the past 6700 yr, as the rate of sea-level rise decreased from an average rate of 4.2 mm/yr to an average rate of 1.4 mm/yr, the bayhead delta of the Lavaca River retreated slowly up the valley as the present-day outline of Lavaca Bay formed. The most significant change in the estuary setting during the past 6700 yr was the development of the present coastal barrier system, which led to gradual restriction of tidal exchange and associated changes in salinity.

Over 400 km of high-resolution seismic data and 53 sediment cores up to 30 m in length were collected from Corpus Christi Bay along the central Texas coast in order to study the impact of sea-level and climate change on coastal environments over the last 10 k.y. Although coastal environments experienced a general landward migration as relative sea level rose over the last 10 k.y., this retreat was punctuated by three, possibly four, major flooding events. These flooding events are marked by abrupt changes in lithologic and seismic facies interpreted to represent rapid landward shifts of bay environments. Such changes include the back stepping of bayhead deltas, tidal deltas, oyster reefs, and other bay environments. Flooding events occurred at 8.0, 4.8, 2.6 ka, and possibly at 9.6 ka, and lasted only a few hundred years. The 9.6 ka flooding surface represents the initial drowning of the ancestral Nueces River valley and may or may not have been rapid in nature. The flooding surface that formed around 8.0 ka is interpreted to record either an increase in the rate of relative sea-level rise or the flooding of relict fluvial terraces that formed by the Nueces River during the stepped fall in sea level since marine isotope stage (MIS) 5e (120 ka). The 4.8 ka flooding event is thought to have formed as a result of either a climatic change during the mid-Holocene, characterized by warmer and drier conditions compared to present, and/or the flooding of another fluvial terrace. The most recent flooding event (2.6 ka) is thought to have resulted from a decrease in sediment delivery to the bay associated with a return to more mesic conditions similar to those of the present climatic regime.

Abstract Modern and ancient progradational shoreface-shelf deposits contain a complex physical stratigraphy that is below parasequence scale. This stratigraphy is interpreted to reflect a threefold hierarchy of geomorphic elements: (1) beach ridges, approximating to units bounded by minor facies-discontinuity surfaces, (2) beach-ridge sets, bounded by surfaces across which there is a distinct offset in shoreline trajectory, and (3) progradational wave-dominated shoreline systems, which correspond to parasequences and are bounded by flooding surfaces. All three geomorphic elements and their bounding surfaces are readily reproduced in simple process-response numerical models. A synthesis of modern and ancient datasets and numerical-modeling experiments indicates that the three geomorphic elements and associated stratigraphy can be produced by a number of mechanisms, including changes in wave climate, temporal and spatial variations in sediment supply, and relative sea-level fluctuations. Models of high-resolution, intra-parasequence stratigraphy can be used to guide correlations in subsurface wireline-log and core datasets, thus improving the definition of reservoir facies architecture and rock-property distributions. The key to robust application of these models is the consistent identification of subtle, high-order stratigraphic surfaces, and their subsequent correlation as shoreface-shelf clinoforms. Data from the Rannoch Formation, Brent Field, U.K. North Sea, are used to illustrate the application of such models, which provide a mechanism to explain anomalous fluid distributions and drainage patterns in the reservoir.