BRYOZOAN REEF-MOUNDS IN THE LEXINGTON LIMESTONE (ORDOVICIAN, KENTUCKY)

Significant new occurrences of bryozoan-built reef-mounds are on northbound (east side) Interstate 75 1.1 mi (1.7 km) south of its exit to KY Hwy 418, southeast of Lexington (Fayette County), which exposes four such bioherms, crust-mounds, in the lower tongue of the Tanglewood Member low in the Lexington Limestone (Shermanian, upper Trentonian, mid-Chatfieldian; classical topmost Middle Ordovician, redefined lower Late Ordovician). The Tanglewood, skeletal calcarenite and calcirudite, represents deposition above wave base on tidally influenced shoals on an extensive carbonate platform. Although the mounds are underlain and overlain by thin-bedded calcarenites (grainstones), the mound horizon developed on a hardground and is truncated by another. The bryoherms' cores range up to 0.8 m high by 2.3 m wide, and unusual for bryozoan mounds, have flank deposits thinning away on either side for another 1.0 m. About 200 peel-sections reveal 18 bryozoan species as reefal participants here.

The cores consist of encrusting to massive colonies of Batostoma chazyensis (incl. campensis) and subordinate Tarphophragma (Hallopora) multitabulata layered upon one another to form cruststones. Both species contain many diaphragms, which strengthened their colonies against destruction by wave action.

The flanks are dominated by large robust branching trepostomes, particularly Heterotrypa foliacea and Homotrypa callosa, whose fragments mingled with the surrounding calcarenite to form rudstones with grainstone to packstone matrix. Their size, numerous diaphragms, and thickened exozonal walls provided considerable strength, so that they grew as sturdy thickets on the sides of the reefs, probably stabilizing the loose sediment around the mounds.

Smaller ramose trepostomes and occasional bifoliates occur rarely as fragments scattered throughout the reefs, having grown as accessory surface, crevice, and thicket dwellers, contributing minor skeletal sediment to the accumulating biohermal mass. A few are found in both cores and flanks (Amplexopora minor, Heterotrypa patera, Homotrypa cressmani), some only in cores (Dekayia epetrima, Homotrypa obliqua, Parvohallopora tolli, Chazydictya chazyensis, Escharopora recta), and others only on flanks (Cyphotrypa acervulosa; Homotrypa exilis, minnesotensis; Parvohallopora inflata, onealli; Eopachydictya gregaria).