ABSTRACT

A prominent sandstone hardbottom, herein termed the DeSoto Canyon rim feature (DCRF), is present along the western rim of the DeSoto Canyon. The DCRF occurs as a northeast-southwest trending, isobath parallel ridge presently located in approximately 55 m of water. The ridge is over 12 km in length and ranges from 50 to 120 m in width. Relief varies from less than one to over seven meters. The DCRF varies along strike from a broad, low-relief mound to a prominent ridge with a steep, 7-meter high 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 DCRF is composed of fine 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. Terrigenous grains range from subangular to subrounded. The sand is lithified by Mg-calcite cement that makes up from <5% to >20% 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 crusts on carbonate allochems, bladed cements occur as irregular crusts on quartz grains, and drusy cements are present as pore fills and irregular coatings on terrigenous grains. The cements have ¹⁸O values that range from +0.1 to +0.8 and ¹³ C values that range from +0.7 to +2.5. ¹⁴C dating of the cements provides average ages from 5,630 ± 70 to 10,560 ± 70 years B.P.

Morphology, sedimentary structures, and sediment texture suggest the DCRF is composed of sediments transported by fluvial processes to near the shelf margin during a sea level lowstand and subsequently reworked during the Holocene transgression. Cement precipitation accompanied the Holocene transgression and may have occurred during periods of sea-level stillstand or short-term reversals in sea level.