BAHAMIAN SUBMARINE HARDGROUNDS ARE A RESEARCH FRONTIER

We discovered submarine hardgrounds of unprecedented extent and thickness on ooltic sands bars around Tongue of the Ocean, Bahamas. The hardgrounds form in place by interstitial precipitation of fibrous aragonite within ooid and peloidal sand grains at water depths of 4-10 m. Many occurrences are less then 20cm thick, similar to other Bahamian examples. Some however, are well over a half-meter thick and multi-layered examples over a meter thick occur. Continuous pavements of hardgrounds tens of meters in dimensions are common; they crack spontaneously and are eroded to subrounded and tabular intraclasts up to several dcm. The hardgrounds form at or near the surface in highly porous clean carbonate sands. We propose that the fiberous aragonite cement as well as the oolitic rings are unlikely the result of modified carbonate chemistry.

These hardgrounds provide a model for the in situ formation of breccias and are preferential sites for the establishment of domical stromatolites and coral reef communities. They are an elegant example of the feedback of early diagenesis on depositional processes.