A COMPOUND, HOLOCENE STROMATOLITE/THROMBOLITE FROM THE BAHAMAS

A large specimen from the much-studied Adderly Channel locality (Exuma Islands, eastern Bahamas) is an alternation of stromatolitic and thrombolitic sections with intervals of non-deposition marked by bioerosion. Radiometric dating of boring bivalves from two bioeroded intervals will give maximum growth rates of both fabrics.

The specimen some 1.2 m tall comes from 5.5 m below mean sea level where it and others are surrounded by mobile sand waves of oolitic sand. It consists of cemented fine-grained ooid sand. In section, three intervals are evident: 1) a basal interval of 0.4 m consisting of two separate stromatolites (ca..0.15 in diameter) ”bookends” of an area of well-cemented ooid sand, which locally has a clotted fabric; 2) a middle interval, 0.60 m thick of marble-hard cemented ooid sand with a clotted fabric like that ascribed to thrombolites; and 3) a capping interval of 0.2 m of well-developed stromatolitic lamination. These three intervals are separated by two hiatuses, zones of bioerosion produced by boring bivalves and sponges.

It seems unlikely that the alternation of stromatolitic and thrombolitic fabrics was produced by major changes in sea level, sediment supply or current regime. Instead, the close association of the two fabrics supports previous interpretations, proposing that differences in the mode of trapping and binding are responsible for the two fabrics. The stromatolitic laminations are produced by mats of cyanobacteria and the thrombolitic fabric is the product of trapping of ooid sand by shrub-like growths of macro-algae of which Batophora oerstedii and Sargassum spp. may be the most effective. Initial cementation of the stromatolitic laminations is considered the result of microorganisms, but what triggers lithification of the thrombolitic fabric is unknown. Susequently, internal precipitation of carbonate produces limestone,hard enough to polish.