ABSTRACT: Contrasts in Incised Valley Geometry and Facies Architecture of Quaternary High and Low Sediment Yield Fluvial Systems of the East-Central Texas Continental Shelf

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

An investigation of the fluvial valley systems of the east-central Texas continental shelf (Sabine, Trinity, Brazos, Colorado, and Lavaca valleys) examines how fluvial systems with different sediment yields respond to the same eustatic changes. Presently, the low sediment yield valleys (Sabine, Trinity, and Lavaca) are occupied by bays. In contrast, high sediment yield valleys (Brazos and Colorado) are occupied by meander belts. The database for this study consists of approximately 7200 km of high-resolution seismic reflection data and several hundred cores and platform boring descriptions.

All of the valleys were incised to approximately the same depth (40 m subbottom on the inner shelf) during the maximum lowstand, implying that sediment load has no influence on river adjustment to base-level change. In general, the valleys widen in a downdip direction and become difficult to image seismically.

The Trinity, Sabine, and Lavaca rivers have occupied the same valleys for several glacial eustatic cycles, resulting in a complex facies architecture in which valley-fill deposits of different ages are juxtaposed. Furthermore, the episodic nature of glacial eustatic rises has resulted in a discontinuous downdip distribution of valley-fill facies. These valleys have served as a point source of sediment at the shelf edge and slope during lowstands.

Numerous valleys with varying levels of incision characterize the offshore Brazos-Colorado area, implying fluvial switching and renewed incision during a single transgression. During maximum lowstands, new incised valleys were formed. On the inner shelf, the valley-fill sequence is dominated by fluvial sands; middle and outer shelf valley facies range from fluvial to offshore marine. This style of valley fill and shifting has resulted in more widespread (line source) sediment supply to the outer shelf and slope. The locations of valleys on the outer shelf have been influenced strongly by diapirs.

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