Anatomy of a Modern Carbonate Tidal-flat, Andros Island, Bahamas

Abstract

ABSTRACT The carbonate tidal-flat complex of northwestern Andros Island consists of three geomorphic areas: (1) adjacent marine belt, (2) channeled belt, and (3) marsh. The various sedimentary accumulations within these belts are controlled by tide levels. Three tidal zones are recognized: (1) subtidal zone, consisting of sediments deposited below low tide (adjacent marine, channels, ponds); (2) intertidal zone, consisting of sediments deposited between normal low and normal high tide (intertidal-flats); and (3) supratidal zone, consisting of sediments deposited above normal high tide but within the range of spring and storm tides (beach ridges, levees, marsh). The complex may be thought of as a river delta turned inside out. Most sediments that make up this delta have been transported via c annel systems from the adjacent marine zone. Early diagenesis in the form of reduction, oxidation, dolomite formation, and carbonate cementation is also controlled by tide level. Strong odor of H2S in gray subtidal sediments suggests reduction, whereas the absence of this odor in the cream or light tan intertidal and supratidal sediments suggests oxidation. Dolomite formation and carbonate cementation are restricted to the distinctively laminated, algal-coated, and mud-cracked supratidal sediment. Stratigraphically, the channeled tidal-flat complex on northwest Andros Island is transgressive, whereas the nonchanneled flats on the southwestern coast are thought to be regressive. Alternation of transgressive and regressive tidal-flat sedimentation during subsidence or eustatic sea level rise could produce stratigraphic pinch-outs that may also become stratigraphic traps. Although such stratigraphic traps would parallel land, preserved porous channel deposits may produce perpendicular trends within them.