STRATIGRAPHY OF A MISSING, BACK-STEPPING, TRANSGRESSIVE REEF IN THE SILURIAN KEYSER FORMATION, MUSTOE, VIRGINIA

The exposure of the latest Silurian upper Keyser Formation at Mustoe, Highland County, Virginia contains a stromatoporoid and coral framestone reef and associated flank debris that show stratigraphic evidence of accumulation during transgression. The exposure has been well studied and is about 200 meters long with two small, massive cores that are exposed at either end. The exposed portion of the larger core is about eight meters thick and 40 meters long at the western, lower end of the outcrop and the exposed portion of the smaller core is about three meters thick and 20 meters long at the eastern, higher end of the exposure. Strata are divided into eight sigmoid beds that can be traced laterally into the cores. Off reef beds are largely rudstone to packstone composed of broken reef fragments up to several meters thick with sharp contacts. The lowest four layers can be traced into the larger, massive core, then thin eastward. The younger layers lap onto or thin westward above the core, but thicken, then thin eastward. The thickest part of each bed is stacked progressively higher in elevation and eastward of the preceding bed between the two reefs before gradually thinning eastward. The topmost layer gradually thins to the east before it rises and swells into the exposed portions of the eastern core. The reef cores are interpreted to be relatively close to undeformed horizontal, as indicated by geopetal sediments filling cavities observed in thin section and outcrop. Deposition of core facies must have been very close to sea level because of the presence of several types of green algae, especially dasyclads. Basinward has been interpreted as being to the west and strata progressively stack upward and to the east, indicating aggradation and retrogration of reef and rudstones. The rudstones of most of the beds that are younger than the western core, but older than the eastern core are interpreted as the fore-reef or flank of core facies that were backstepping during transgression, but out of the plane of view.