Late Pleistocene Fluvial-Deltaic Deposition, Texas Coastal Plain and Shelf: ABSTRACT

Abstract

End_Page 1611------------------------------Deposition during the last Pleistocene glacial cycle has been interpreted in Texas from topography and bathymetry, drillers' logs, engineering test borings, offshore sparker profiles, and by detailed drilling in Brazoria County. Major controls on deposition were found to be glacio-eustatic sea-level fluctuations and drainage-basin characteristics. The lower coastal plain is essentially a mud-rich alluvial plain of coalescing low-gradient fans. An older Pleistocene alluvial plain (Lissie Formation) is tilted and overlapped by a younger one (Beaumont Formation). By repeated avulsions, each river deposited a branching network of discrete meander-belt sand bodies, shown by both soil maps and net-sand maps. Detailed drilling of a meander belt of the ancestral Brazos River shows the channel to be 5 to 7 m deep, but substantially greater sand thicknesses developed by stacking of point-bar sequences during fluvial aggradation. Crevasse-splay deposits are virtually absent and fluvial-sand bodies can terminate abruptly against overbank clay. Clay of the Brazos and Colorado alluvial plains is predominantly red, apparently derived from Per ian and Triassic red beds in west Texas. Transition downdip into deltaic and paralic deposits is recognized by brackish and marine fauna, by strike-oriented sand bodies, by changes in clay color, and by clinoform reflectors on sparker profiles. In response to a slow decline in sea level the Brazos, Colorado, and Rio Grande deltas prograded, increasing in thickness from less than 30 m near the present shoreline to as much as 130 m at the shelf edge. Between these major delta systems, barrier-island sand up to 20 m thick was deposited at high stand. At a lower sea level, patch reefs grew at the shelf edge between major delta lobes. Sea-level fluctuations resulted in three depositional phases: an aggradational phase (ca. 120,000 years ago) during late rise and stillstand, dominated by fluvial and barrier systems; a progradational phase (120,000 to 20,000 years ago) during a gradual fall, dominated by deltaic systems; and a rapid transgressive phase (20,000 to 4,000 years ago), with little deposition other than filling of interdeltaic lows. The Texas coast is now in an early aggradational phase. End_of_Article - Last_Page 1612------------