Print this pageAbstract: Holocene Marine-Cemented Sands, Joulters Ooid Shoal, Bahamas
During the last 5,000 years, the Joulters ooid shoal formed north of Andros Island on the margin of Great Bahama Bank. Sediment probing, coring, and observations of the seafloor have revealed the widespread occurrence of grainstone and packstone crusts (hardgrounds) and intraclasts within otherwise uncemented Holocene sands. The marine-cemented sands occur in fine-peloid, ooid, and skeletal facies and commonly are continuous across facies boundaries. Multiple cemented crusts are most common in a mobile shoal fringe, but the single most continuous crust occurs 15 km bankward extending across the shoal's western border.
Crusts are continuous for meters to several kilometers and range in thickness from millimeters to tens of centimeters. The lower surface of most crusts is gradational into underlying uncemented sands and the upper surface is sharply defined, more indurated, bored, and encrusted. Intraclasts range from poorly cemented platy chips of coarse-sand to medium-pebble size 1 to 2 mm thick, to well-cemented cobbles that are millimeters to centimeters thick. The crusts and intraclasts are cemented with acicular aragonite, rimming grains in a fringe 10 to 50µ thick, or with micrite (aragonite and magnesian calcite) enveloping grains and completely filling voids in a patchy distribution.
Syndepositional marine cementation in the Joulters ooid shoal punctuates Holocene sands with widespread less porous and permeable layers. Where exposed, the layers provide a stable bottom for colonization by hardground fauna. Recognition of crusts may prove a valuable clue in deciphering facies and diagenetic patterns in carbonate reservoirs and in understanding the timing of porosity-reducing diagenesis.
AAPG Search and Discovery Article #90965©1978 GCAGS and GC Section SEPM, New Orleans, Louisiana