Holocene cementation along the central Texas coast.
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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 mixed 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.