ABSTRACT: Sandstone Hardbottoms Along the Western Rim of DeSoto Canyon, Northeast Gulf of Mexico
BENSON, D. JOE, W. W. SCHROEDER, and A. W. SHULTZ
A prominent sandstone hardbottom is present along the western rim of the DeSoto Canyon. The feature occurs as a northeast-southwest trending, isobath parallel ridge presently located in approximately 60 m of water. The ridge is a discontinuous feature over 11 km in length and from 50 to 120 m in width. Relief varies from less than 1 to over 7 m. The feature varies along strike from a broad, low-relief mound to a prominent ridge with a steep, 7-meter high, near-vertical face on the seaward side. The feature displays strong orthogonal jointing. Erosion and undercutting has produced a prominent debris field along the seaward margin. The size of talus blocks indicates lithification extends several meters into the sediment.
The ridge is composed of medium to coarse grained, moderately to well-sorted sublitharenite. Quartz is the dominant framework component with lesser amounts of potassium feldspar and metamorphic and sedimentary rock fragments. Carbonate skeletal grains comprise from 2% to 15% of the lithologies. Grains range from subangular to well rounded. Mg-calcite cement makes up from <5 to >30% of the lithologies. Cement content is greatest toward the crest of the feature and on the seaward side. Fibrous, bladed, and drusy cements are all present and there is a marked substrate control on cement morphology. Fibrous cements occur as isopachous rims on carbonate allochems, bladed cements occur as irregular rims on quartz grains, and drusy cements are present as pore fills. The cements have deltaO18 values that range from +0.1 to +0.8 and deltaC13 values that range from +0.7 to +2.5. C14 dating of the cements provides average ages from 8,880 to 10,550 years B.P.
Composition and texture along with morphology of the ridge suggests the feature formed as a shoal-type deposit during sea-level lowstand. The age and isotopic composition of the cements suggest precipitation from marine waters as sea level rose following the lowstand.
AAPG Search and Discovery Article #90941©1997 GCAGS 47th Annual Meeting, New Orleans, Louisiana