DISTRIBUTION OF CALCAREOUS ALGAE FROM MUSTOE REEF, SILURIAN KEYSER FORMATION, HIGHLAND COUNTY, VIRGINIA

The latest Silurian patch reef exposed in the upper Keyser Formation at Mustoe, Highland County, Virginia is composed primarily of a stromatoporoid and coral framestone and rudstone that displays evidence of accumulation during rising sea level. The reef grew on a westward-dipping ramp during a time of widespread carbonate deposition on the eastern margins of the Taconic foreland basin. The main core at the western end of the southward-facing exposure is composed of a series of aggradationally and retrogradationally stacked strata. Characteristics of the sediments and framebuilders have been well documented by previous workers and those data indicate successively lower energy in younger strata. The exposed portion of the main core is about eight meters thick and 40 meters long at the western, lower end of the outcrop. Strata retrograde into the swale behind the main core, with later strata being composed of some in-place, tabular stromatoporoids, as documented by previous workers, but a large proportion is also reef rubble rudstone and packstone. The reef and associated strata are interpreted to be relatively close to undeformed horizontal, as indicated by geopetal sediments filling cavities observed in thin section and outcrop. In situ core facies contain evidence of deposition very close to sea level, including the presence of several types of green algae, especially dasyclads but also including possible recaptaculitids, codiaceans, and calcareous cyanophytes. Biota also include microproblematical tubular encrusters that encrust the rooves of cavities.

New micropaleontological data from the younger rudstones behind and above the main reef exposures shows more of a tendency to include pieces of rubble encrusted by calcareous cyanophytes such as Girvanella, Ortonella, Hedstroemia, Renalcis, and Sphaerocodium. Some algal thalli are loose, broken fragments, some are encrusting broken pieces of stromatoporoid rubble, and some form oncoids around fossil grains. Small fragments of poorly preserved codiaceans and solenoporaceans may also be present. The rudstone and packstone debris do not appear to be derived from the main mound that is below and to the west, but are interpreted to be from nearby, younger mound cores that nucleated in shallower water after sea level rise, but are not exposed.