Carbonate hardgrounds are surfaces of synsedimentarily cemented carbonate layers that have been exposed on the seafloor. A hardground is essentially, then, a lithified seafloor. Ancient hardgrounds are found in limestone sequences and distinguished from later-lithified sediments by evidence of exposure to normal marine waters. This evidence can consist of encrusting marine organisms, borings of organisms produced through bioerosion, early marine calcite cements, or extensive surfaces mineralized by iron oxides or calcium phosphates. Modern hardgrounds are usually detected by sounding in shallow water or through remote sensing techniques like side-scan radar. hardground from Texas with encrusting oysters and Gastrochaenolites borings. The scale bar is 1.0 cm. Carbonate hardgrounds often host a unique fauna and flora adapted to the hard surface. Organisms usually cement themselves to the substrate and live as sessile filter-feeders. Some bore into the cemented carbonate to make protective domiciles for filter-feeding. Sometimes hardgrounds are undermined by currents which remove the soft sediment below them, producing shallow cavities and caves which host a cryptic fauna. The evolution of hardground faunas can be traced through the Phanerozoic, from the Cambrian Period to today. hardground with encrusting oysters and borings. intervals are plotted on the time axis. Carbonate hardgrounds were most commonly formed during calcite sea intervals in Earth history, which were times of rapid precipitation of low-magnesium calcite and the dissolution of skeletal aragonite. The Ordovician-Silurian and the Jurassic-Cretaceous Systems have the most hardgrounds and the Permian-Triassic Systems have the least. This cyclicity in hardground formation is reflected in the evolution of hardground-dwelling communities. There are distinct differences between the Paleozoic and Mesozoic hardground communities: the former are dominated by thick calcitic bryozoans and echinoderms, the latter by oysters and deep bivalve and sponge borings. Stratigraphers and sedimentologists often use hardgrounds as marker horizons and as indicators of sedimentary hiatuses and flooding events. Hardgrounds and their faunas can also represent very specific depositional environments such as tidal channels and shallow marine carbonate ramps
