Coquina

Coquina (koʊˈkiːnə) is a sedimentary rock that is composed either wholly or almost entirely of the transported, abraded, and mechanically sorted fragments of the shells of mollusks, trilobites, brachiopods, or other invertebrates. The term coquina comes from the Spanish word for "cockle" and "shellfish".English Wiktionary: concha mussel, shell." For a sediment to be considered to be a coquina, the particles composing it should average or greater in size. Coquina can vary in hardness from poorly to moderately cemented. Incompletely consolidated and poorly cemented coquinas are considered grainstones in the Dunham classification system for carbonate sedimentary rocks. A well-cemented coquina is classified as a biosparite (fossiliferous limestone) according to the Folk classification of sedimentary rocks. Coquinas accumulate in high-energy marine and lacustrine environments where currents and waves result in the vigorous winnowing, abrasion, fracturing, and sorting of the shells that compose them. As a result, they typically exhibit well-developed bedding or cross-bedding, close packing, and good orientation of the shell fragments. The high-energy marine or lacustrine environments associated with coquinas include beaches, shallow submarine raised banks, swift tidal channels, and barrier bars. Coquina is composed mainly of the mineral calcite, often including some phosphate, in the form of seashells or coral. Coquinas dating from the Devonian period through to the much more recent Pleistocene epoch are a common find all over the world, with the depositional requirements to form a coquina being a common thing in many marine facies. Adjacent to Shark Bay Road southeast of Denham is an approximately long stretch of coastline composed of billions of tiny shells of the Shark Bay cockle (Fragum erugatum), averaging less than in length. The shell deposit, between thick, has compacted and cemented in some areas into solid masses of limestone that formerly was quarried and cut into blocks used in local construction.