Abstract Sediment cores and high-resolution seismic and side-scan sonar data were collected from four shelf banks on the east Texas inner continental shelf. Sabine, Heald, Shepard, and Thomas banks all have similar sediment facies, structure, and genesis. The banks are composed of three facies (top to bottom): A) an interbedded shell hash and sand unit; B) a muddy-sand unit characterized by a seaward-prograding and chaotic seismic facies, and C) an interbedded sand and mud unit characterized by landward-dipping seismic reflectors. These three sediment facies represent amalgamated storm beds, lower-shoreface or ebb-tidal delta, and back-barrier/flood-tidal delta environments respectively. Facies B and C were deposited during a time of relatively slow sea-level rise and were stranded on the shelf during a rapid sea-level rise. Facies A is the result of storms and wind-driven currents reworking the paleoshoreline deposits on the shelf. The banks are drowned paleo-shorelines restricted to the area above and immediately adjacent to the Trinity and Sabine incised fluvial valleys. This association is explained by the greater thickness of sands within the valleys (larger sources of sands), greater accommodation space, and greater subsidence rate within the valleys (greater preservation potential).

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.

ABSTRACT The sediments that now fill the incised Trinity River Valley record the evolution of the Galveston Bay estuary during the last transgression. High resolution seismic data and sediment cores have been used to study this record. The most significant outcome of this study is the observation that dramatic landward shifts in the position of the estuary occurred on several occasions in response to rapid sea-level rises. These shifts caused complete reorganization of the bay environments. If glaciologists are correct in their predictions, mass wasting of the West Antarctic Ice Sheet will cause another catastrophic change in coastal and estuary evolution within the next millennium.

ABSTRACT High resolution reflection seismic (uniboom and 3.5 kHz) data and a high density core coverage and engineering borings provide the data base for a three-dimensional study of coastal lithosomes on the north Texas coast. Seismic and core data from Galveston Bay and Sabine Lake indicate that the present bay and lake subenvironments were rapidly established around 4,000 years ago, after a rapid flooding event inundated the shallow flanks (-5 to -6 meters) of the Trinity River and Sabine River incised valleys. Bolivar Roads tidal inlet evolved about 3200 years ago in response to bay mouth constriction as Bolivar Peninsula grew by southwestward spit accretion. The peninsula was later breached by storm overwash channels which served as ephemeral tidal inlets activated by hurricanes. Widening of the peninsula through accretion made it less susceptible to breaching. The Bolivar Roads tidal inlet initially was situated 5 kilometers further to the east of its present position, at the eastern edge of the Trinity River incised valley. It then shifted to the western edge of the valley. The proto-Sabine Lake estuary mouth was restricted at least 2,500 years ago by the rapid development of sand spits. The extant shoreface is characterized by dip-oriented, completely healed, cross-cutting channels of varying sizes (a few to several meters in relief and width). These may be storm return flow channels. Though these avenues of sand to the inner shelf exist, thick amalgamated storm beds are scarce offshore of Galveston Island and virtually absent off Bolivar Peninsula and that overall, Modern marine sediments are scarce on the lower shoreface and inner shelf. This scarcity of sediments and storm deposits may be partly due to the predominant southwestward longshore drift direction in the region. Few preserved Holocene coastal lithosomes exist on the north Texas inner continental shelf. Those that do, occur within the incised valleys of the Trinity and Sabine rivers in the form of discrete pods of tidal inlet and flood-tidal delta deposits. Sand banks in the study area are interpreted as barrier/strandplain deposits that were reworked during the transgression. For example, Sabine Bank rests on the ravinement surface and was reworked obliquely (to the WNW). Thick (> 75 cm), possibly amalgamated storm deposits on the inner shelf are associated with the shelf sand banks and the incised valley. The latter are possible lower shore-face deposits that escaped ravinement. The occurrence of preserved coastal lithosomes within the incised valley strongly suggest that pre-transgressive topography is a major factor for preservation. However, a rapid and large sea-level rise is responsible for the landward and vertical jump of the ravinement surface and the increase of accommodation. The low gradient on the north Texas inner continental shelf could also be another factor that contributed to their preservation. At the present rate of sea level rise, none of the extant coastal lithosomes might be preserved.