High-Resolution Seismic Stratigraphy of a Late Pleistocene Submarine Fan Ponded by Salt-Withdrawal Mini-Basins on the Gulf of Mexico Continental Slope
Authors: Winker Charles D. 
Published: October 2008

The late Pleistocene Brazes-Trinity Fan, a structurally ponded fan completely exposed and undisturbed on the seafloor, was mapped with a combination of conventional and high-resolution seismic data, This fan occupies three salt-withdrawal mini basins (I, II, IV) and a graben (111), each filled with an On lapping package consisting of alternating bedded and non-bedded units evident on high-resolution data. Basins 1-111are filled to their topographic spill point. the or dap-fill succession of each is incised by a channel system which bypassed sediment to the next basin(s) downdip. Seismic continuity generally increses distally in the system and within individual basins, believed to reflect the increasing prevalence of turbidity currents over high-density sediment gravity flows.

Introduction

A late Pleistocene submarine fan system linked to a shelf margin delta of the Brazes and Trinity Rivers (Fig. 1) was ponded on the upper Texas continental slope during the last glacio-eustatic lowstand of sea level by three salt-withdrawal mini-basins (Fig. 2, 3). Such ponded fan systems are abundant in the subsurface of the Gulf of Mexico continental slope, but only the Brazes-Trinity fan is still exposed intact on the seafloor. Shell Development Company and Shell Offshore, Inc. conducted a high-resolution seismic stratigraphic study of theBrazes-Trinity fan to provide an analog for more deeply buried and less well-imaged ponded fans.

The Brazes-Trinity fan was previously recognized by Gardiner' and Satterfield and Behrens2on the basis of seafloor fan channels seen on seismic profiles. Earlier studies34 had depicted this system simply as chaotic fill, similar to the chaotic facies comprising the East Breaks Slides associated with the late Pleistocene Colorado River and shelf-margin delta (Fig, l). Published seafloor maps are insufficiently detailed to show these channels, but they are apparent on recent NOAA multibeam data (Fig. 4).

Paleogeography of the eastern Texas continental shelf and slope during the last glacio-eustatic lowering of sea level was compiled from several sources and summarized in Fig. 1, From the Ingleside strandline of the preceding highstand?, the shoreline migrated 75-200 miles southward to the shelf edge. The Trinity, Neches, Sabine, and Calcasieu (not shown) Rivers became entrenched and joined on the inner shelf to form a single river which extended southward toward the shelf margin. shelf margin, this ‘greater Trinity’ river was joined by the Brazes River'4. This combined river system ultimately built a deltaic complex at the shelf margin and Berryhill mapped two discrete shelf margin deltas (‘A’ and ‘B’ in Fig. 1), whereas this study recognized a single, continuous shelf-margin delta complex whose maximum progradational extent occurs at the head of the Brazes-Trinity fan system (Fig. 1).

Prior to high-resolution seismic acquisition, gross features of the fan system Fig. 3) were mapped using conventional 2D seismic profiles (Fig. 5, 6).