From Pristine to Diagenetically Altered Skeletal Material: Examples from the South Texas Gulf Coast

Big and Little Shell beaches, Padre Island National Seashore, Texas, are composed predominantly of modern shell hash. Penescal Point, which lies on the mainland shores of Laguna Madre, just west of Big and Little Shell beaches, is a Pleistocene analog, i.e., it consists of well lithified biosparites (beachrock).

Big and Little Shell beaches are composed predominantly of very coarse whole shells and fine abraded fragments as well as a minor amount of siliciclastic sand. The shell debris, although commonly abraded, is still mineralogically fresh. X-ray diffraction data show that the shells along the swash zone are still 100% aragonite whereas some of those from behind the berm and in the dunes have been altered and contain a minor admixture of calcite.

The Penescal Point beachrock predominantly exhibits parallel laminations dipping gently seaward, interrupted rarely by trough-cross stratified deposits. These coarse biosparites are thoroughly lithified and composed mainly of whole and fragmented bivalve shells. In addition, ooids comprise a minor carbonate component. The beachrock contains the same shell fauna found presently on Padre Island's Big and Little Shell beaches. Shells range upward to over 2.5 cm in length and there is a quartz admixture of fine to medium sand.

The shells on Penescal Point display a large diagenetic range, from complete dissolution of the aragonite and creation of secondary porosity to complete fill of the biomolds with calcite. Botryoidal fans composed of radiating fibrous aragonitic rays form pendulous cements. Diagenesis has altered many of these botryoids to bladed and blocky calcite. In addition, equant and bladed isopachous calcite cements surround shell fragments and coated grains. Thus, the beachrock displays aspects of both vadose as well as phreatic diagenesis.