Sedimentary Features of the South Texas Continental Slope as Revealed by Side-Scan Sonar and High-Resolution Seismic Data (1)

Abstract

Sedimentary provinces on the south Texas slope have been identified by their acoustic character on long-range side-scan sonar records and high-resolution seismic profiles. Probable lithofacies within these provinces have been identified by core data and by analogy with previously cored acoustic facies. In the northern part of the study area, the East Breaks Slide is a prominent mass-transport feature. Revised bathymetry shows that the slide originated on the upper slope (200-1000 m), in front of a sandy late Wisconsinan shelf-margin delta, where the gradient is up to 3 degrees. It was deposited in a middle slope position (1000-1500 m) where the gradient is about 0.5 degrees. Side-scan sonar data indicates that the slide is a strongly backscattering feature extending more han 110 km downslope from the shelf edge. It consists of two lobes that are separated by a diapiric high. Diapiric highs on the middle slope have blocked most of the flow. Borehole data shows that the slide deposit contains intercalated sands and contorted bedding. The slide is therefore attributed to failure of sandy deltaic material deposited close to the shelf edge during the last period of low sealevel (late Wisconsinan, circa 11-29 Ka). Core data suggests that the weakly backscattering acoustic facies adjacent to the slide are fine-grained sediments (mudturbidites and hemipelagites) of a slope mud drape. The middle slope in front of the sandy late Wisconsinan shelf-margin delta of the Rio Grande has an intermediate level of backscattering with numerous channels leading to the Sigsbe Deep. These channels are believed to be part of the northern lobe of the Rio Grande Fan. Acoustic facies mapping using long-range side-scan sonar matches well with acoustic facies mapping using 3.5-kHz high-resolution seismic profiles. Within this study area higher levels of GLORIA backscatter are returned from sea floors containing shallow subbottom sand than from muddy sea floors.