DEVELOPMENT OF CAVERNOUS POROSITY IN THIN, FLAT-LYING CARBONATES: THE VANPORT LIMESTONE, PENNSYLVANIA

The Vanport Limestone (Pennsylvanian) is a 0 to 6 m thick unit occurring in portions of the Appalachian Plateau of western Pennsylvania. It is underlain by shale and variously overlain by buhrstone iron-ore, sandstone, shale, and coal. The limestone is notable for the presence of extensive conduit mazes, including Harlansburg Cave, which has a mapped length of 6.6 km. Previous workers have hypothesized that these conduits formed from overlying diffuse infiltration, or from floodwater injection by adjacent streams. To understand the development of these features, we compared cave maps to local and regional measurements of joint directions in the Appalachian Plateau, calculated porosities, evaluated relationships between the caves and the land surface, reviewed drilling records, and made observations in quarries and caves.

All of the substantial conduits are joint controlled and occur along valley margins. The orientation of 1656 conduit segments shows no concordance between caves, regional joint orientations, or 2675 locally measured joints. Rough alignment of the conduits with local valley walls suggests that stress relief fractures are exploited for conduit formation. Although vertically constrained due to small bed thickness, the networks exhibit high local porosities (average 35% for the 4 major caves). Information from quarries shows that extensive conduits are found during excavations. They are frequently clay filled, and occur near hillsides. Smaller enlarged joints are found throughout the rock mass, and may be free of clay. Water well drilling records for a 4 county area show only a single bit drop (1 meter) from drilling through this unit. That well is on the nose of a topographic ridge. Enterable caves are characterized by clay and water fills. Morphology and sediment character indicate extensive development in the phreatic zone. Some of the conduits contain free-flowing streams and alluvial deposits, but this appears to be a secondary process. Extensive conduit development appears limited to near surface exposures of the limestone where diffuse infiltration possibly mixes with lateral groundwater flux. Enlargement of locally conductive joints can be accommodated by mischungskorrosion or acids from oxidation of minerals in the overlying shale or coal.