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.

Abstract A 2,700-km high-resolution seismic-reflection data set, acquired in recent years, has helped resolve some old problems concerning the age of Quaternary formations along the east Texas coast, and has resulted in mapping of the Trinity/Sabine incised valley. The seismic data were used in conjunction with oil company platform borings and recently acquired sediment cores to examine the stratigraphy of the incised-valley fill and to map the distribution of sand bodies on the shelf. The Trinity/Sabine valley has experienced at least two episodes of incision and infilling. The earliest incision occurred during δ 18 0 substage 5d and the latest reincision occurred during δ 18 0 stage 2. The late Wisconsinan-Holocene transgressive deposits that now fill the valley include the following facies (from bottom to top): fluvial; upper estuary/bayhead delta; middle estuary; lower estuary/tidal inlet/tidal delta; and offshore marine deposits. Backstepping parasequences, indicating an episodic rise in sea level, characterize the valley fill. Sandbody formation and preservation on the shelf also has been influenced strongly by the episodic nature of the late Wisconsinan-Holocene sea-level rise. Sabine Bank, the largest of the sand bodies and the only one studied in detail, is a reworked coastal lithosome that rests on the ravinement surface. Inner-shelf muds contain few discrete storm beds. Relatively thick (<75 cm) amalgamated storm deposits are restricted to sand banks and the incised valley. The modern Sabine Lake and Galveston Bay estuaries formed initially by flooding of the Sabine and Trinity valleys approximately 8 ka. The subsequent flooding event, which inundated the broad, shallow meander portions of the valleys, occurred approximately 4 ka and appears to have been rapid. Extant coastal systems of the study area incorporate a wide range of environments, including barriers, strandplains, chenier plains, and tidal inlets. The systems formed predominantly during the stillstand of the past 3,500 years. Galveston Island and Bolivar Peninsula were derived from offshore sand sources. Progradation of the coastal barriers ceased with the exhaustion of the sand supply.