42-21 Relationship Between Fractures, Faults, Karst Features, and Groundwater Flow in Sinking Valley, Pennsylvania

Abstract:

The orientations of fractures, faults, and bedding planes can play a critical role in controlling the development of secondary permeability within karst aquifers. We investigated the relationship between geologic structure, groundwater flow, and the development of karst features in Sinking Valley, a karst valley hosting middle Ordovician carbonates in Blair County, Pennsylvania.

Mapping of geologic structures was conducted in Tytoona Cave, Field Trip Cave, and an adjacent roadcut and documented thrust faults, strike-slip faults, joints, veins, and en echelon vein arrays consistent with the established deformation sequence found within the Appalachian Valley and Ridge.

To evaluate groundwater flow pathways, sodium fluorescein dye was injected into a swallow hole within Sinking Run, a sinking stream that loses its flow into the subsurface upstream of its confluence with the Little Juniata River. Water samples collected over 2.5 days in the Little Juniata River and were analyzed using a spectrofluorophotometer. The dye discharged from a single source, a riparian spring on the bank of the Little Juniata River.  Dye was first detected 24 hours after injection. This rapid travel time and localized discharge indicate that the spring is fed by a karst conduit.

The structural and hydrologic data together indicate that geologic structures influence conduit development within Sinking Valley. Cave passage orientations in Tytoona Cave correspond to regionally extensive joint sets and veins, suggesting that speleogenesis occurred by preferential dissolution along previously existing discontinuities. Furthermore, the inferred groundwater flow path must crosscut the Coburn and Loysburg limestones into the Bellefonte Dolostone before discharging at the riparian spring, indicating that lithologic solubility alone does not control conduit development. Instead, the orientation of fractures may drive conduit formation even in less soluble lithologies. These results suggest that dye tracing combined with the evaluation of geologic structures such as fractures and faults can improve models of groundwater flow in karst aquifers, where heterogenous permeability makes fluid flow difficult to characterize.

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