Late Pleistocene fluvial-deltaic deposition, Texas coastal plain and shelf




Winker, Charles David

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Deposition on the Texas coastal plain and shelf during the last Pleistocene glacial cycle has been interpreted from topographic and bathymetric maps, from borehole data including results of a detailed drilling study in Brazoria County, and from offshore sparker-profiles. The stratigraphic unit deposited during the last glacial cycle is bounded above by a largely undissected topographic surface (Beaumont), and below by a buried paleosol of stiff gray clay that correlates offshore with a persistent seismic reflector. The lower Texas coastal plain is essentially a clay-rich alluvial plain made up of coalescing low-gradient fans. The Beaumont alluvial plain onlaps an older surface (Lissie) which was tilted seaward prior to Beaumont deposition; the Lissie in turn onlaps remnants of older surfaces. During Beaumont deposition, each major coastal river deposited a branching network of meanderbelt sand-bodies by repeated avulsions. Borehole data for a meanderbelt of the ancestral Brazos River indicate that the channel was 5 to 7 m deep but that substantially greater sand thicknesses developed by stacking of point-bar sequences during fluvial aggradation. Downdip transition of fluvial deposits into deltaic and paralic sediments is inferred from shell beds, strike-oriented sand bodies, beach ridges, changes in clay color, and clinoform reflectors on sparker profiles. The updip limit of marine influence is delineated by the distribution of shell beds; the downdip limit of deltaic pro gradation is indicated by a paleobathymetric break in slope. Strike-oriented sands (Ingleside), including barrier islands and strandplains, developed contemporaneously with Beaumont fluvial aggradation. Sand thicknesses and multiple levels of sand suggest that beach-shoreface sequences are multistoried, similar to the fluvial sands. The thickest and widest strike sands formed in bights between the larger, more prominent deltaic systems. In response to falling sea level, deltas prograded from the Ingleside shoreline to the shelf edge. Sparker profiles show that deltaic thicknesses and offshore slopes increased gradually during progradation, then rapidly near the shelf edge, where deltaic sequences became stacked or imbricated. Major growth faulting and salt flowage near the shelf break were associated with the thickest deposits. Between the large Colorado and Rio Grande delta systems, reefs grew near the shelf edge. Late Pleistocene sea-level fluctuations resulted in three depositional phases: an aggradational phase (ca. 120,000 B.P.) during late rise and stillstand, dominated by fluvial and strike systems; a progradational phase (100,000 to 20,000 B.P.) during a gradual fall, dominated by deltaic systems, and a rapid transgressive phase (20,000 to 4000 B.P.). The Texas coast is now in another aggradational phase. Average rates of late Pleistocene sediment influx were similar to historic rates, and show a decrease in sediment production toward the arid southwest. Post-depositional deformation of the Beaumont and Lissie alluvial plains and Ingleside shoreline can be explained largely as an isostatic response to sedimentary loading