Present coastal systems along the east Texas coast evolved during the past 3.5 ky of relative sea-level stillstand following a rapid sea-level rise 4 ka. Closure of proto-Galveston Bay, caused by spit accretion of Galveston Island and Bolivar Peninsula, formed Bolivar Roads approximately 3.3 ka. Increased tidal prism and entrenchment over the Trinity River incised valley led to inlet stabilization and intensification of tidal influence.
The present shoreface and inner shelf package is characterized by a paucity of storm deposits. Strong along-shelf storm currents, low sediment supply, and low effective accommodation space in the region are unfavorable for the preservation of storm beds. Higher sand supply during the early establishment of the present coastal lithosomes resulted in a greater occurrence of storm beds lower in the section. Amalgamated storm deposits on the east Texas shelf are associated with reworked coastal lithosomes.
Pods of tidal-inlet, tidal-inlet/spit, and tidal-delta deposits mark previous shoreline positions on the continental shelf. Their distribution mimics the along-strike variation of the present coastal systems, defines six relative sea-level stillstands, including the present, during the past 10.2 ky, and supports the model of a step-like sea-level rise. The seismic architecture of pre-8 ka coastal lithosomes provide evidence for greater tidal influence, greater accommodation space, and higher sedimentation rates compared to the present.
The preserved coastal lithosomes indicate that the depth of shoreface ravinement decreases with decreasing shelf gradient, increasing rates of sea-level rise, and increasing sediment supply. Better preservation within incised valleys results from greater accommodation space and the soft valley-fill that allows incision of the inlets beyond the depth of shoreface ravinement.
The mechanism of shoreline translation (discontinuous erosional shoreface retreat, transgressive submergence, or in-place drowning), is a function mainly of shelf gradient and rate of sea-level rise. Gentle shelf gradient and rapid sea-level rise favor transgressive submergence. In regions with steep shelf gradient, aggradation may produce stratigraphic signatures consistent with in-place drowning and discontinuous erosional shoreface retreat.