MODERN MARINE STROMATOLITES IN THE EXUMA-CAYS, BAHAMAS - UNCOMMONLY COMMON

Modern stromatolites in open marine environments, unknown until recent ly, are common throughout the Exuma Cays, Bahamas. They occur in three distinct settings: subtidal tidal passes, subtidal sandy embayments a nd intertidal beaches. These stromatolites have a relief of up to 2.5 m and occur in water depths ranging from intertidal to 10 m. Surfaces near the sediment-water interface are typically colonized by cyanobact erial mats, whereas high relief surfaces are commonly colonized by alg al turf and other macroalgae such as Batophora, Acetabularia, and Sarg assum. The internal structure of the stromatolites is characterized by millimeter-scale lamination defined by differential lithification of agglutinated sediment. In thin section, the lithified laminae appear a s micritic horizons with distinct microstructures: they consist of thi n micritic crusts (20-40 mu m thick) overlying layers of micritized se diment grains (200-1000 mu m thick); the micritized grains are cemente d at point-contacts and are truncated along a surface of intense micro boring. The Exuma stromatolites are built by cyanobacterial-dominated communities. These laminated prokaryotic structures grade to unlayered thrombolites built by eukaryotic algae. The variety of sites, setting s and shapes of stromatolites in the Exuma Gays present excellent oppo rtunities for future studies of stromatolite morphogenesis.