ABSTRACT: Differences in Late Pleistocene-Holocene Transgressive Systems Tracts of the East and Central Texas Shelf Due to Differences in Sediment Supply and Shelf Gradient

ANDERSON, JOHN, KEN ABDULAH, and FERNANDO SIRINGAN, Rice University, Houston, TX, and MARK THOMAS, Shell Western Exploration and Production, Houston, TX

Approximately 7200 km of high-resolution seismic data and several hundred sediment cores and platform borings provide the basis for mapping late Quaternary strata of the east-central Texas shelf. The study area is divided into two depositional regimes. The eastern shelf gradient averages 0.5 m/km and is occupied by the ancestral Trinity/Sabine fluvial valley, a relatively low sediment yield fluvial system.The central shelf gradient averages 1.0 to 2.0 m k/m and is occupied by the ancestral Brazos/Colorado valley system. The two regions are characterized by very different late Pleistocene-Holocene transgressive systems tracts.

The incised Trinity and Sabine rivers have occupied the same incised valley during the last few eustatic lowstands, and the sands delivered to this valley were delivered to the shelf-edge and upper slope through repeated incisions. The valley fill stratigraphy is characterized by backstepping parasequences. Only during prolonged stillstands were appreciable quantities of sand delivered to the coast to nourish coastal lithosomes, and these were preserved only when stillstands were followed by rapid rises. The result was a series of sand banks whose upper portions have been reworked during the transgression. The occurrence of these sands on the shelf is predictable from the incised valley-fill stratigraphy.

During the late Pleistocene-Holocene transgression, the Brazos and Colorado rivers back-filled their fluvial valleys and cut new and progressively shallower valleys. Reincision of the same valley is rare. As a result, much of the sand these rivers deliver to the sea is sequestered on the shelf. During transgression, sands from the shallow portions of these valleys and associated wave-dominated deltas are reworked through shoreface ravinement to produce widespread transgressive sand bodies.

The efficiency of ravinement is related to sediment supply, rates of eustatic rise and pre-transgressive topography. Most sand bodies on the shelf are the products of reworking of coastal lithosomes and fluvial deposits. Shoreface deposits and storm beds are only locally preserved.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)