Abstract

We investigate the roles of lithologic heterogeneity and spatial patterns of bedrock fracturing within the Susquehanna-Shale Hills Critical Zone Observatory and their feedbacks to groundwater flow and weathering processes. Based on detailed field observations and a series of down-hole geophysical logs from 8 boreholes, we characterize the local structure and geology within the catchment and explore variations in fracture density and orientation across the catchment and with depth, allowing us to look at relationships between geology and fracture distribution. While the geologic template of a catchment system often creates fractures, fractures in turn affect that template by influencing rates of erosion and weathering. The most prevalent fracturing occurs along bedding planes, setting the dominant fracture orientation in the catchment. Variation in rock type and strength control fracture distribution and ultimately groundwater flow and weathering front locations. The prevalence of fracturing within the bedrock likely serves as an important control on the “active depth of flow,” the subsurface zone that responds to annual recharge and climatic variability and has groundwater residence times that become older as a particle moves from recharge to discharge areas.

We find systematic differences in hydraulic conductivity beneath ridgelines and valleys that map fracture density; both were higher beneath the valley floor when compared to the ridgetop. While we see higher fracturing near the surface and in the valley, the orientation of fracturing is not significantly different in terms of location within the watershed. Borehole gamma-ray emission measurements, XRD, and major element analysis suggest a loss of clay minerals near the surface, with slightly greater weathering within the valley when compared to the ridge top. These data demonstrate a potential weathering-fluid flow feedback driven by fracture abundance: valleys may act as “drains” for shallow groundwater flow through watersheds, potentially driving a positive feedback in which water penetrates deeper and faster into deeper valleys, accelerating the weathering that breakdown the bedrock, and enhancing the growth of valley relief. Our results suggest that fractures likely control groundwater flow within the deeper watershed, but patterns in the observational data provide evidence of fairly consistent fracture orientation and rock structure throughout the watershed.

Citation

Singha, K., Clarke, B.A., Sullivan, P.L., Chattopadhyay, P.B., Brantley, S.L. (2013): Geologic controls on fracture distributions within the Shale Hills Critical Zone Observatory. Abstract H43L-08 presented at 2013 Fall Meeting, AGU, San Francisco, CA, 9-13 Dec..

This Paper/Book acknowledges NSF CZO grant support.