Subtidal oolitic stromatolites are forming in normal marine waters (1-5 m) in the high energy oolitic sand environment on Eleuthera Bank, Bahamas. Penecontemporaneous marine cementation transforms these stromatolites into hardgrounds, some of which may localize subsequent reef development.
Accretion of oolitic stromatolites results from trapping and binding of ooids by various algae. Direct precipitation from seawater of aragonite and/or high magnesium calcite, calcification of algal filaments by high magnesium calcite, or commonly a combination of both processes lithify these stromatolites to create hardground substrates. Degree of marine cementation increases downward from stromatolite surfaces. Stromatolites themselves are localized on other low-relief oolitic hardgrounds.
Morphologies of oolitic stromatolites are strikingly similar to some Shark Bay algal stromatolites. Bahamian stromatolites originate as small pinnacles which can evolve into mounds over a meter in height. Individual pinnacled stromatolites also coalesce laterally into continuous elongated ridges which develop preferred orientations in response to local hydrographic conditions. Internal crude algal laminations often are destroyed by macroborers.
Oolitic stromatolite growth is ephemeral, apparently controlled by the rate of burial by shifting oolitic sand. This physical stress, therefore, sufficiently excludes grazers and encrusters, permits algal binding of ooids and explains stromatolite development in normal marine waters. Buried stromatolites that become exposed are recolonized by algae and begin accreting upward. Where physical stresses are removed for longer periods of time, oolitic stromatolites become susceptible to colonization by coralgal organisms and represent an early stage of reef development.