Abstract: Mixing-Zone Dolomitization of Holocene Tidal-Flat Sediments, Southwest Andros Island, Bahamas

Conrad D. Gebelein

Pellet-mud tidal flats on southwest Andros have prograded over 25 km westward to a maximum thickness of 8 m. Periodic seaward migration of the shoreline has led to the development of shoreline ridges, with a spacing of 3 to 5 km. Each ridge is 0.5 to 1.0 km wide and up to 2 m above mean tide level. These shoreline ridges act as input areas for rainwater and serve as the localizing topography for the development of freshwater lenses within the tidal-flat sediments.

Quarterly analyses from over 100 well pipes inserted to known depths in the Holocene wedge yield a quantitative picture of the dynamics of interstitial-water chemistry. Low chlorinity (less than 2,500 ppm Cl^-) lenses are present year-round under the shoreline ridges. Flow across interstitial isohalines averages 0.5 m/day; size of the lenses fluctuates seasonally with rainfall. Below 2,500 ppm Cl^-, waters are undersaturated with respect to all major carbonate minerals, and active dissolution occurs. In the mixing zones (2,500 to 15,000 ppm Cl^-) around the lenses, only calcium-magnesium carbonate phases are above saturation, and data indicate the precipitation of huntite or protodolomite. Hypersaline interstitial waters (derived from the open-marine plat orm) surrounding the mixing zones undergo either no reaction or minor dissolution of aragonite. Sediments under the older of the Holocene shoreline ridges are most highly flushed by fresh water and have the highest protodolomite concentrations.

Subsurface dolomitization is driven by mixing of high-alkalinity (derived from dissolution of carbonate and organic decomposition) meteoric waters with low alkalinity, high-ionic-strength seawaters. Dolomitization is massive and cuts across sedimentary facies boundaries.

AAPG Search and Discovery Article #90968©1977 AAPG-SEPM Annual Convention and Exhibition, Washington, DC