Patch Reefs in the Carters Limestone (Middle Ordovician) in Tennessee, and Vertical Zonation in Ordovician Reefs

The Carters Limestone (Middle Ordovician) of the Central Basin of Tennessee contains several biohermal patch reefs that contrast sharply with the bedded limestone surrounding them. The most impressive of these reefs is located along the Elk River near the Tennessee-Alabama state line. Here the reef and adjacent limestones are composed of (1) stromatoporoid/coral/receptaculitid boundstone, (2) pelmatozoan grainstone, (3) pelmatozoan pack-stone/wackestone, (4) algal packstone/wackestone, (5) coated-grain grainstone, and (6) pelletal grainstone. The reef contains algae, bryozoans, pelmatozoans, corals, stromatoporoids, mollusks, trilobites, and receptaculitids.

The basal beds of the reef are developed on pelmatozoan grainstone, some beds of which contain a unique community of upright and branching receptaculitids, incrusting bryozoans, and incrusting and entangling pelmatozoans. This community played an important role in initiating reef growth. The middle part of the reef has the greatest taxonomic diversity and the closest association between coral and stromatoporoids. This organic diversity gives way upward to more abundant stromatoporoids and a decrease in corals and other organisms. The reef's stages of development, in ascending order, are designated as (1) stabilization, (2) colonization, (3) diversification, and (4) domination. Similar developments occurred in Ordovician reefs in New York, western Texas and eastern Tennessee, and although taxonomic composition and developmental stages differ, the functional development is quite similar from reef to reef.