ABSTRACT

Twenty-seven rock samples were collected using SCUBA from a small, northwest-trending ridge with 5.5 m of relief located 74 km south of the northern entrance to Padre Island National Seashore and 3.2 km offshore from Padre Island (26°51N, 97°18W) in 14 m of water. All are massively bedded subarkoses and sublitharenites cemented by low-Mg micritic calcite. The acid insoluble residue, which averages 73 percent, is a subrounded, muddy, fine sand (M_z = 3.06) that is poorly sorted (_I = 1.74), extremely leptukurtic (K_G = 4.32), and strongly fine skewed (SK_I = +0.65). Most contain mesovugs and channels that are lined with sparry calcite, clay, or fibrous chalcedony. Irregular shaped lumps and clots of iron and manganese oxides are common. Land snails (Helicina orbiculata tropica, Polygyra septemvolva febigeri) and freshwater snails (Helisoma trivolvis, Physa sp.) have been extracted from the rock. Teeth and bones of Pleistocene mammals (Mammuthus columbi, Mammut americanum, Bison sp.) have also been found in crevices in the ridge.

The ridge is interpreted as an intermittent lake deposit that formed on a late Pleistocene extension of the south Texas eolian sand sheet. Mud and fine sand were blown into the lake from surrounding dune fields and mixed with accumulating carbonate. During dry periods, the sediments developed soils that were vegetated and later calichified. Because of their superior hardness, the lake sediments survived the Holocene transgression and have become a submarine prominence.