ABSTRACT

Stromatolitic structures of two types are forming in the subtidal zone (3-25 ft) in Bermuda. Small headshaped structures (biscuits) and large ellipsoidal structures (domes) form by the agglutination and binding of detrital sediment onto the filaments of blue-green algae. These forms are composed of convex-upward laminations produced by the alteration of thick (1 mm) sediment-rich layers and thin (0.1 mm) algal-rich layers. Algal stromatolites accrete at the rate of one complete lamination (sediment-rich layer plus algal-rich layer) per day, in response to a day-night algal growth cycle.

The distribution, abundance and morphology of stromatolitic structures are controlled by current velocity and rates of sediment movement over the bottom. Algal mats occur in areas of less than 60-80 gm/hr/foot bottom sediment movement (less than 15-20 cm/sec surface current velocity); mat thickness is inversely proportional to current velocity. Algal biscuits and algal domes are restricted to the range 8-60 gm/hr/foot bottom sediment movement (1-11 cm/sec current velocity). Algal domes form in areas of rapid sediment accumulation. Algal biscuits are restricted to areas protected from high rates of sediment movement; the degree of streamlining, that is, the approach to ellipsoidal form, is directly proportional to current velocity and sediment movement. Elongate biscuits and domes are riented parallel to the axis of current flow. Stromatolites with asymmetric laminations have the thickest side of the laminations facing upcurrent or toward the sediment source.