PALEO GHOST-ROCK KARSTIFICATION AT WATER SINKS CAVE REEF, LATEST SILURIAN KEYSER FORMATION, VIRGINIA

Latest Silurian reef exposures of the Keyser Formation at Water Sinks, Highland County, Virginia show evidence of early to intermediate stage ghost-rock karstification. Reef strata are characterized by horizontal beds several meters thick, composed of cobble-sized heads of stromatoporoid and coral. The large heads are separated by degraded, fossiliferous grainstone rich in disseminated limonite, clay minerals, and chert, as well as void filling, limonite-rich travertine that contains abundant fossil residue. Large, impermeable stromatoporoid heads are the only fossil components left that may have originally been composed of aragonite. Smaller fossil particles within the residue between stromatoporoids were all originally calcite. This differs from exposures of a reef at a similar stratigraphic position at Mustoe, located 7 miles northwest of Water Sinks which is interpreted to be farther basinward. Mustoe reef displays aggradational growth and cavities are filled with normal marine skeletal grainstones. Calcareous algae are present, and although no longer composed of original aragonite, were likely preserved because of recrystallization and cementation within a buffered system rather than an open system with abundant flushing of meteoric waters. Karsting of the Water Sinks reef indicates that its shallower position resulted in exposure during periods of sea level fall. If both reef systems were deposited during the same sea level cycle, one would interpret that the Mustoe reef nucleated at sea level earlier and seaward and was not exposed by subsequent sea level fall. Water Sinks reef nucleated later and higher on the platform, but was exposed during sea level fall and was planed off, resulting in the residual tabular geometry. The characteristics consistent with ghost rock karstification are that undissolved materials were not physically removed to form open voids. The residual material containing insoluble minerals as well as more stable and larger carbonate grains remains in the areas between the more robust, impermeable stromatoporoid and coral heads. Small spaces created by the loss of rock volume by dissolution were filled with travertine and then all pore spaces were filled by later burial cementation.