ABSTRACT

During the last 5000 years the Joulters Ooid Shoal formed north of Andros Island on the margin of Great Bahama Bank. Sediment probing, coring and observations of the sea floor have revealed the widespread occurrence of grainstone and packstone crusts (hardgrounds), and intraclasts within otherwise uncemented Holocene sands. These marine-cemented sands occur in fine-peloid, ooid and skeletal facies, and are commonly 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 vary in thickness from millimeters to tens of centimeters. Most have a lower surface gradational into underlying uncemented sands, and a sharply defined, more indurated, bored and encrusted upper surface. Intraclasts range from poorly-cemented platy chips, coarse sand- to medium pebble-size and 1 to 2 mm thick, to well-cemented cobbles, millimeters to centimeters thick. The crusts and intraclasts are cemented with acicular aragonite, rimming grains in a fringe 10 to 50 µ 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 is punctuating 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 understanding the timing of porosity-reducing diagenesis.