REPEATED FORMATION OF FAVORABLE NEW SUBSTRATES DETERMINES ABUNDANCE AND POPULATION STRUCTURE OF BAHAMIAN STROMATOLITES

We believe the remarkable abundance and large variation in height (age) of closely spaced, Holocene columnar stromatolites is the result of frequent renewal of firm or hard substrates. In about half a square kilometer within the Adderly oceanic tidal channel adjacent to Lee Stocking Island of the Exuma Cays (Dill, 1991) the many hundreds of specimens that range in height from several cm to 2.5 m consist of ooid sand trapped and bound by the cyanobacteria Schizothrix gebleini (Golubic and Browne, 1996). We consider it unlikely that the variations in height are the result of variations in the strong tidal currents that sweep the entire habitat twice-daily or in the readily available supply of ooid sand. Instead we found these microbial structures growing on four different kinds of repeatable substrates: syndepositional hardgrounds of skeletal and ooid sand; large molluscan shells, a storm-generated conglomerate of skeletal debris and intraclasts of mud; and a firmground of mud several cm thick deposited by a hurricane (Shinn, et al, 1993). The repeated development of these favourable new substrates explains the large variations in age of closely spaced stromatolites. We suggest that similar substrates and especially mud firmgrounds may explain the common occurrence of repeated horizons of stromatolites in Precambrian and Phanerozoic strata.