UNDERSTANDING BRYOZOAN BIOHERMS IN THE TANGLEWOOD AND GRIER MEMBERS, LEXINGTON LIMESTONE (MIDDLE ORDOVICIAN), THROUGH MODERN, SMALL, PATCH-REEF ANALOGUES

Meter-scale bryozoan bioherms occur at a correlative horizon in the Grier and Tanglewood members of the Lexington Limestone. The bioherms sit on a hardground, surrounded by coarse, crossbedded, skeletal calcarenites. The bioherms are dome-shaped accumulations of platy, encrusting bryozoans with more delicate, ramose bryozoans fringing the base on one side.

Small, red algae-coral, patch reefs on the eastern, oceanward side of San Salvador Island, Bahamas, were used as partial analogues. The reefs occur in the surf zone and in deeper shoreface waters and sit on a hardground surrounded by coarse skeletal sands. Reef shape is largely related to position relative to shoreline, with those close to shore being flat-topped due to exposure and surf action, while those in deeper, energetic waters are more dome-shaped. Surrounding these reefs are shallow moats where currents have scoured away skeletal sands. On the oceanward side of the reefs, red algae form a strong, encrusting lip, whereas more fragile, branching corals and sea fans are found on protected, lee parts of the reefs. In the deeper water reefs, red algae are absent and branching corals and sea fans are ubiquitous across the reefs.

Although differing by an order of magnitude in size, comparison with the Bahamian patch reefs suggests that the bioherms developed on a hardground and persisted in the midst of partially enveloping, mobile, skeletal sands; evidence of surrounding moats is also present. Bioherm shape suggests development in deeper, energetic, shoreface waters, but below the surf zone. Comparison with paleocurrent directions indicates that more delicate, ramose bryozoan thickets grew on the protected lee side of the bioherms as observed in the Bahamian reefs. Hence, modern, small, patch-reef analogues, even though of different sizes and involving different organisms, were able to help in understanding these unusual bioherms.