ABSTRACT

Sandy biosparite and shelly biomicrite gravel thinly covers a significant portion of the inner shelf from the Rio Grande to Sabine Pass, covering an area of at least 3,000 km², and similar gravel is also a conspicuous component of beach sediment along rapidly eroding Holocene deltaic headlands of the central Texas coast. Although gravel clasts occur in a normal marine setting, most include shells of brackish to freshwater fluvial-deltaic and estuarine molluscs. Contained cements record lithification in two distinctly different diagenetic settings. Biosparites contain intergranular and intragranular cement that precipitated from meteoric fluids during subaerial exposure, whereas biomicrite cements precipitated in the presence of marine and marine-derived fluids, possibly mix d with connate fluids from depth. Most meteoric cement consists of equant crystals of low-Mg calcite spar precipitated during the partial dissolution and calcitization of molluscan aragonite. Marine cements are predominantly equant to elongate crystals of highMg calcite, but they also include acicular aragonite and radial barite. Unlike most normal marine calcite cements, Texas shelf highMg calcite is enriched in iron and manganese, is luminescent, and is isotopically depleted in carbon. Marine cementation, including baffle precipitation, took place at shallow depths below the sediment/water interface.

The presence of brackish to freshwater molluscs in sediment lithified by both meteoric and marine fluids records a complex sequence of Quaternary cementational and erosional events. Meteoric lithification of Pleistocene shelly, deltaic, and estuarine coquinas took place during the Wisconsinan lowstand, whereas marine cementation of other, as yet unlithified, shelly layers began with the subsequent Holocene transgression of the Gulf shoreline to its present position.