RECONNAISSANCE-LEVEL INVESTIGATION OF LATE PLEISTOCENE INCISED CHANNEL-FILL SEQUENCES AS POTENTIAL BORROW AREAS FOR NOURISHMENT OF EAST TEXAS GULF COAST SHORELINES

Rapidly retreating shorelines along the Texas Gulf coast now threaten the structural, aesthetic, and environmental function of native beaches. Periodic beach nourishment is one technically feasible alternative to mitigate the chronic erosion, however, economically viable potential borrow areas are rare. One promising source of beach compatible sand are the late Pleistocene incised fluvial systems now well-documented on the east Texas continental shelf.

This reconnaissance-level investigation was undertaken in search of channel-fill sands located beneath the Brazoria County shoreline (a.k.a. Follet’s Island) of the Texas Gulf coast. The investigation was conducted in two phases: (1) geophysical surveys along the inner shelf, lower shoreface, and Intracoastal Waterway to locate potential coarse-grained channel-fill deposits and (2) rotary drilling through the lower backshore of Follet’s Island to validate geophysical interpretations and quantify sedimentology and stratigraphy.

CHIRP survey output yielded seventeen incised channel signatures. The channels are typically 10 to 15 meters in width and 4 meters deep. These dimensions are one order of magnitude smaller than those late Pleistocene channels previously identified on the Texas continental shelf. Rotary drilling was conducted at 57 sites and penetrated to an average depth of -37 feet MSL. Analysis of 526 meters of core log and 237 sediment samples revealed a sediment succession broadly classified into five major litho-facies (in ascending order): (a) mottled, organic-rich, calcareous stiff clay - deltaic, (b) red, organic-rich plastic clay - fluvial, (c) gray, organic-rich plastic clay - estuarine, (d) fine- to medium-grained quartz sand - back-barrier, and (e) fine-grained, shelly quartz sand - shoreface/foreshore.

The clay-rich sediment succession is clearly transgressive and generated by erosional shoreface retreat of the east Texas coastal plain and associated barrier island system. No fining-upwards sandy intervals were identified that could be confidently interpreted as a channel-fill succession. It is probable the CHIRP images of small-scale incised-channel fill sequences reflect the presence of mud-filled channels within a clay-rich back-bay environment analogous to flood tidal landscape of San Luis Pass.