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 molluscs, trilobites, brachiopods, or other invertebrates. The term coquina comes from the Spanish word for 'cockle' and 'shellfish'. 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 molluscs, trilobites, brachiopods, or other invertebrates. The term coquina comes from the Spanish word for 'cockle' and 'shellfish'. For a sediment to be considered to be a coquina, the particles composing it should average 2 mm (0.079 in) 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, which 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 mainly composed 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 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 45 kilometres (28 mi) southeast of Denham is an approximately 110 kilometres (68 mi) long stretch of coastline composed of billions of tiny shells of the Shark Bay cockle (Fragum erugatum), averaging less than 14 millimetres (0.55 in) in length. The shell deposit, between 8 to 9 metres (26 to 30 ft) 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. St. Andrew's Anglican Church, the Old Pearler Restaurant, and parts of the Shark Bay Hotel in Shark Bay were built from coquina shell blocks. The church, built in 1954, has walls infilled with coquina shell blocks between a light steel frame and a shell facing, while the Old Pearler was built in 1974–1977 with buttressed coquina shell block walls. Recently discovered petroleum-bearing formations off the coast of northeastern Brazil hold coquina reservoirs of oil. The coquinas are generally heterogeneous in their porosity and permeability, but like other lacustrine carbonates, they are uncommonly reservoirs. Corbett et al. (2015) in their discussion of the reservoirs say the finding of the Badejo Field (Campos Basin) in 1975 was the first hydrocarbon discovery in the coquinas of the Lagoa Feia, followed by that of the Pampo and Linguado Fields in 1978. The coquinas of the Morro do Chaves Formation were formed by non-marine bivalves and ostracods. The shells of the bivalves, which lived in shallow oxygenated water, were transported and deposited as washout over stream fans and beaches by storms and long-shore drift. The palynological record of coquinas of the Sergipe-Alagoas Basin has been analyzed and the sediments dated to the late Barremian age; the results suggest a marine and/or brackish environment. Daniel Thompson (2013) asserts that the coquinas of the Morro do Chaves Formation include a wide range of marine mollusca characteristic of brackish environmental conditions, suggesting periodic marine ingression during the Early Cretaceous. According to a paper by Senira Kattah published in The Sedimentary Record, the discovery of the Lula Field by Petrobras and partners in 2006 opened petroleum exploration in the Barremian/Aptian pre-salt play in the offshore Santos and Campos basins, and consequently deeper coquina reservoirs have become important targets. He says the two main reservoir targets recognized for the pre-salt within the study areas are: 'late rift coquinas, lacustrine facies deposited at the Late Barremian to Early Aptian, and the younger rift/sag microbial limestones deposited during the Aptian, just before the establishment of the major evaporitic sag basin between South America and Africa.' There are abundant beds of coquina in the Outer High of the Santos Basin, similar to those from the neighboring Campos. Pre-salt stratigraphy of the Santos Basin shows lacustrine sediments composed of coarse pelecypod (bivalve) coquina during the Barremian and Aptian sag phase of the continental crust subsidence. Coquina deposits in Florida occur mostly along the eastern coast of the peninsula. This coquina is named the Anastasia Formation after Anastasia Island, where the Spanish quarried the rock to construct the Castillo de San Marcos, the fortress they built to defend St. Augustine. The Anastasia Formation stretches from just north of St. Augustine in St. Johns County to southern Palm Beach County. The formation and associated sand form part of the Atlantic Coastal Ridge, a Pleistocene barrier island chain that extends from Duval County to Dade County. Other coquina deposits are found in the state, but only in limited areas. The Anastasia Formation is naturally exposed in a number of places along the east coast including Washington Oaks Gardens State Park, Gilbert's Bar House of Refuge, Hutchinson Island in Martin County and Blowing Rocks Preserve, owned by The Nature Conservancy, in Martin County.