CORRELATION OF SANDSTONES AND FLAT-PEBBLE CONGLOMERATES IN CARBONATES OF THE TONOLOWAY AND KEYSER FORMATIONS (SILURIAN-DEVONIAN), HIGHLAND AND BATH COUNTIES, VIRGINIA, AND A REVISED INTERPRETATION OF THE PRINCIPAL CAVE-FORMING STRATIGRAPHIC HORIZONS

Bedrock mapping of Silurian and Devonian formations in the Williamsville 7½' quadrangle (Virginia) supported by the EDMAP program of the U.S. Geological Survey has led to discoveries in the Silurian-Devonian carbonate sequence that clarify significant regional stratigraphic relationships. Exposures in the Water Sinks (a large, complex karst feature) and nearby areas, both surface and especially subsurface (in caves), have been the key to working out details of stratigraphic relations within the Tonoloway and Keyser Formations. We can now recognize and distinguish several distinct clastic units in the Tonoloway (Silurian) and the overlying Helderberg Group (Devonian, including the Keyser) in this region, with key findings being (1) discovery that certain unnamed sandstones in the Tonoloway are laterally persistent; (2) discovery that the three-part character of the Tonoloway, especially the distinct lower Tonoloway with its interlayered thin-bedded pink/red and gray/black lithologies, is regionally persistent; (3) clear evidence that a well-known facies change in the middle Keyser (Big Mountain Shale into Clifton Forge Sandstone) takes place in this area; (4) discovery that the thinly laminated and thin-bedded Tonoloway limestones are separated in places from the overlying and commonly massively to nodular bedded and reefal Keyser limestones by a flat-pebble conglomerate that varies markedly in thickness (0 cm to 54 cm within a few tens of feet laterally) but which nonetheless can be traced between outcrops in this region, and (5) recognition that this area is where sandier facies of the Silurian and Devonian carbonates (which are well-developed to the south along the James River) grade laterally northward into more muddy or limey facies. These findings allow for a more detailed and thorough understanding of the stratigraphic variations and changes in these carbonate units. This work also supports a revised interpretation of the stratigraphy of at least two of the major caves in the region, Butler and Breathing. Our findings indicate that those caves are not developed in the middle Keyser in association with “upper” and “lower” tongues of the Clifton Forge Sandstone, as suggested since the 1950s, but are instead developed in the lower and middle Tonoloway in association with unnamed sandstones of that unit.