FRACTURE STRATIGRAPHY LINKED TO MULTILEVEL BOREHOLE MONITORING IN LOWER PALEOZOIC BEDROCK OF THE CENTRAL MIDCONTINENT, NORTH AMERICA

Outcrop-based mapping remains the best technique to provide insight into field scale sub-surface fracture characteristics such as connectivity, spacing, and orientation. Recent fracture mapping in exposures of relatively undeformed lower Paleozoic bedrock in the central midcontinent of North America has led to the recognition of stratigraphically controlled, and thus predictable, preferential termination horizons (PTHs) that can serve as barriers to vertical groundwater flow. In Minnesota, our fracture mapping at several outcrops of carbonate and sandstone bedrock document PTHs at correlatable stratigraphic horizons. Each of the PTHs have one or more features that limit propagation of vertical fractures, including relatively weak rock strength compared to adjacent strata, the presence of ductile shale, or bed parallel partings.

Concurrently, we have monitored boreholes with multilevel systems (MLSs) constructed across stratigraphic intervals equivalent to those studied in outcrop. MLSs with meter-scale spacing of monitoring zones provide hydrogeologic data (such as hydraulic head, hydraulic conductivity and chemistry) at the resolution necessary to recognize flow-related characteristics dictated by the fracture patterns expressed in outcrops. In one such MLS project, we are attempting to overcome the common difficulty of acquiring borehole data from sites close to outcrops suitable for fracture analysis. This project targets the Ordovician Platteville Formation, a fractured carbonate rock with widespread contamination in Minnesota’s Twin Cities Metropolitan area. We collected temperature and pressure data at discrete (meter-scale) intervals in Platteville wells at distances less than 500 meters from well-exposed outcrops where fractures are being characterized in three dimensions. Preliminary results show promise for directly linking the hydrologic characteristics in the boreholes to fracture attributes. For example, hydraulic head deflections in wells correspond to PTHs and/or bed parallel partings identified in nearby outcrops. The results exemplify how combining outcrop data with borehole data, each with their own inherent limitations, can provide insights that will improve prediction of contaminant transport and lead to more effective remediation strategies.