GEOLOGICAL CONTROLS ON CAVE DEVELOPMENT IN HIGHLAND COUNTY, VIRGINIA

Highland County, Virginia contains 260 mapped caves formed in folded and faulted carbonate-rich strata of the Valley and Ridge province. Surveys conducted by the Highland County Cave Survey indicate some caves stretch over 11 km long and can descend more than 158 m deep. Most of the caves exist in Ordovician formations exposed along the breached crests or exposed limbs of 3 vertical or overturned tight anticlines, or in Silurian-Devonian strata along the troughs and limbs of 2 broad synclines. Occasional thrust faults dip east and have traces generally parallel to regional strike (N40E). Pattern analyses indicate strong control by orthogonal joint sets (NE-SW; NE-SW) with some influence of 60-120-degree shear fractures. Fissure caves less than 30 m long and slot fissure caves less than 60 m long are most numerous, and the maze networks hold the most passage length. More than 90% of the mapped cave passages lie within the present vadose zone, and less than 5% show active phreatic development. Many relationships between cave pattern, recharge, stratigraphy, and structure conform to trends recognized by workers elsewhere, but some differ. Branchwork caves exist, but are less numerous than expected. Approximately half of the caves that formed halls and narrows have halls parallel to dip, instead of parallel to strike as expected. Vadose cave passages usually form in down-dip fractures as expected, although a quarter of the passages now in this zone trend along strike. Caves in the phreatic zone developed along strike, as found elsewhere. Overall these patterns indicate that fold-related fractures and faults primarily control speleogenesis, with secondary control by planar bedding fractures sometimes apparent. Diffuse recharge appears to form most caves (e.g. fissure and maze patterns) rather than recharge from concentrated surface flow (e.g. curvilinear and rectilinear branchwork patterns). Along anticline crests in formations with thick carbonate sequences, vertical fractures dominate cave development even in areas of extensive sinkhole plains. Strata exposed in the broad synclines have numerous thin clastic beds that promote diffuse recharge patterns. The scarcity of large surface drainage basins in these steep terrains and the abundance of clastic beds present on fold limbs also foster diffuse recharge conditions.