The Response of a High Sediment Yield Depositional System to Episodic Rises in Sea Level: The Record from the Brazos Fluvial System, Central Texas Coast

BARTEK, LOUIS R., J. B. ANDERSON, and K. C. ABDULAH, Rice University, Houston, TX

High resolution seismic data, cores, and platform borings have been utilized to investigate the response of the Brazos fluvial system, a high sediment yield depositional system, to late Pleistocene-Holocene episodic rises in sea level. This investigation was conducted in order to provide control for a related study of the Trinity-Sabine fluvial system, a low sediment yield depositional system. The Brazos incised valley, which was carved during the Wisconsinan eustatic lowstand, was

abandoned during an interval of rapid eustatic rise. The fluvial-deltaic system occupied the adjacent interfluve following abandonment of the incised valley. This behavior contrasts sharply with the responses of the Trinity-Sabine low sediment yield depositional system to rapid base level rises. Stream piracy, triggered by the episodic eustatic rises, played an important role in diverting the fluvial system of the high sediment yield system out of the lowstand entrenched valley and onto the interfluve. The system stabilized during eustatic still stands and nucleated deltas in an area that lies approximately 15 km offshore of the modern Brazos delta, 14 km southwest of San Luis Pass, and offshore of Oyster Creek. These deltas were transgressed during ensuing eustatic rises and were pre erved as shelf sand bodies depending upon the rate of eustatic rise. Deltas that were transgressed during intervals when the rate of eustatic rise was high (2 cm/year) have the highest preservation potential.

The research presented in this paper suggests that exploration geologists working with up-dip portions of high sediment yield depositional systems in the transgressive systems tract may find prospects (slightly modified wave-dominated delta deposits encased in marine shales) by searching along strike, away from the abandoned incised valley, at positions along depositional dip that correspond to eustatic stillstands. Accretionary structures observed in recently acquired seismic data indicate that the down-dip portion of the high sediment yield incised valley also contains a large volume of sand.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)