LATEST SILURIAN (PRIDOLI) REEFS IN THE APPALACHIAN BASIN

Carbonate organic buildups occur commonly in uppermost Silurian (upper Pridoli) rocks in the Appalachian Basin from Virginia and West Virginia northeast to New York; however, only a few are bioherms, representing latest Silurian reefs. These are found in the Keyser Formation of west-central Virginia and western Pennsylvania.

The best developed of these buildups is found in a roadcut at Mustoe, Virginia, in the Jersey Shore Member of the Keyser. This was originally thought to be in the lowermost Devonian, but conodont-based dating in nearby localities indicates a latest Pridoli age. Here the Jersey Shore Member is divided into three subunits, in ascending order: 1) a stromatoporoid-dominated bioherm at least 8.8 m thick (lower contact not exposed); 2) a stromatoporoid-dominated biostrome 2.3 m thick; and 3) a second stromatoporoid-dominated biostrome 1.8 m thick. Stromatoporoids dominate the Jersey Shore Member at Mustoe to the extent that they constitute over half the volume of the rock. The bioherm contains massive stromatoporoids at least 50 cm in diameter, and well over 1 m high. The biostromes contain small laminar and bulbous stromatoporoids. Also occurring in the Jersey Shore are favositid and cladoporid tabulate corals, and pelmatozoan echinoderm fragments. Less common are rugose corals, brachiopods, and ostracodes. Several smaller bioherms are located in the Keyser Formation within 27 km to the southwest and southeast.

Bioherms are present in the Keyser Formation at two localities in Altoona, Pennsylvania. The first are at the old Eldorado Quarry, which is now a roadcut adjacent to Interstate 99. The Jersey Shore Member exhibits stromatoporoid bioherms that also contain brachiopods, bryozoans, pelmatozoan echinoderms, and favositid and cladoporid tabulate corals in lesser numbers. The second locality is known locally as the Allegheney Furnace section, where there is a small bioherm at the top of the Byers Island Member. Corals are most abundant here, with bryozoans and stromatoporoids also present.