Carbonate hardgrounds

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 (especially bryozoans, oysters, barnacles, cornulitids, hederelloids, microconchids and crinoids), borings of organisms produced through bioerosion, early marine calcite cements, or extensive surfaces mineralized by iron oxides or calcium phosphates (Palmer, 1982; Bodenbender et al., 1989; Vinn and Wilson, 2010; Vinn and Toom, 2015).

Sometimes hardgrounds are undermined by currents which remove the soft sediment below them, producing shallow cavities and caves which host a cryptic fauna (Palmer and Fürsich, 1974).

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, deep bivalve (Gastrochaenolites) and sponge (Entobia) borings (Taylor and Wilson, 2003).

Stratigraphers and sedimentologists often use hardgrounds as marker horizons and as indicators of sedimentary hiatuses and flooding events (Fürsich et al., 1981, 1992; Pope and Read, 1997).