WINKER, CHARLES D.
Shell E&P Technology Company, Houston TX

Abstract: Trinity-Brazos Salt-Ponded Submarine Fan Compared with Other Late Pleistocene Sediment Gravity Flow Depositional Systems ("Fans") of the Texas Continental Slope

Three late Pleistocene "fans" of the Texas slope are geographically linked with shelf-margin deltas of the larger Texas rivers (Trinity-Brazos, Colorado, and Rio Grande), but lack submarine canyons and fan morphology. Each shelf-margin delta contains "chaotic clinoforms" suggesting repeated mass wasting of the prodelta during progradation. Each slope system consists in part of turbidite systems, with multiple sinuous slope channels emanating from the shelf edge. Each also consists in part of submarine landslide deposits; two discrete slides emanate from large slump scars indenting the shelf edge. These observations suggest a common model, in which (1) turbidity currents originate at distributary mouths of the shelf-margin delta, and (2) higher-density sediment gravity flows originate by mass wasting of the shelf-margin prodelta and upper slope.

The Trinity-Brazos fan, the only salt-ponded fan exposed intact on the Gulf of Mexico seafloor, consists of onlap-fill successions in three salt-withdrawal mini-basins (I, II, IV) and a small graben (111). The downdip basins (II-IV) are connected to the shelf margin by a partially buried channel system with local levee development. Each onlap-fill succession consists of alternating layered units (turbidites) and chaotic/transparent units (higher-density flows); the former are increasingly prevalent downdip. Some of the onlap fill was derived locally from the basin margins, but most was supplied to the downdip basins by the channel system. This channel system underwent repeated grade adjustments as basins were successively filled (causing aggradation) and bypassed (causing incision).

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas