Abstract: Biofacies Relations in Lower Devonian Stromatoporoid Reef Complex

S. M. Warshauer, R. A. Smosna

Although stromatoporoids and corals are abundant throughout the Helderberg Group of the Central Appalachians, accumulations of these taxa commonly do not constitute true ecologic reefs. However, at Mustoe in Highland County, Virginia, a 13-m thick exposure of upper Keyser Limestone is recognized as a true reef and represents the first report of such in these Lower Devonian carbonate rocks.

Hemispheric stromatoporoids which are the major frame builders, accounting for up to 90% of the reef proper, increase in size with upward reef growth. Cladopora, Favosites, and crinozoan columnals are minor constituents between stomatoporoids and within reef-flat pools. The presence of Cladopora is inversely related to that of Favosites and rugosans; Cladopora is primarily near the base of the reef and the other corals are more abundant in the upper part. Crinozoans likewise increase toward the top. Mound growth was terminated locally by a slight drop in sea level as evidenced by a wave-cut erosion surface. Hence, the vertical increase in size and frequency of certain taxa indicates that they preferred shallow, agitated water.

The reef itself most probably continued to grow farther offshore in deeper water, and 6 m of lagoonal sediments prograded the erosion surface. Small, sediment-binding, globular and tabular stromatoporoids dominate minor corals and rare brachiopods in these lagoonal deposits. The uppermost 4 m of the Keyser is a laminated limestone. It contains sparse Leperditia, tabular stromatoporoids, and brachiopods and was deposited in the distal lagoon.

AAPG Search and Discovery Article #90968©1977 AAPG-SEPM Annual Convention and Exhibition, Washington, DC