Facies Architecture And Evolution of Coastal Lithosomes On The North Texas Gulf Coast and The Occurrence Of Preserved Analogues On The North Texas Inner Continental Shelf
Authors: Siringan Fernando P. , Anderson John B. 
Published: October 2019

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 (&gt; 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.