FRACTURE STRATIGRAPHY IN LOWER PALEOZOIC BEDROCK OF THE CENTRAL MIDCONTINENT, NORTH AMERICA

Outcrop-based fracture mapping remains the best technique to provide insight into subsurface fracture characteristics and their hydrologic significance. 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 impact groundwater flow.

Fracture mapping at several outcrops of the carbonate-dominated Platteville Formation (Ordovician) in the Twin Cities Metropolitan area (TCMA) of Minnesota identified PTHs at the upper and lower contact of a 1.5 m shale and silt-rich dolomitic mudstone unit. The Jordan Sandstone (Cambrian) in the eastern TCMA has PTHs along contacts between 0.5-1m thick sandstone beds at a consistent stratigraphic position within the lower part of meter-scale parasequences. Reconnaissance work at scattered Cambrian outcrops elsewhere in the region also suggests the presence of a PTH in the uppermost 2m of sandstone and shale in the Tunnel City Group. Fractures above and below PTHs in these examples typically cross multiple beds, with heights of a few to several meters. Each of the PTHs contain one or more features known to limit propagation of vertical fractures, including relatively weak rock strength compared to adjacent strata, the presence of ductile shale, or bed parallel macropore networks.

Some hydrologic characteristics of these stratigraphic units appear to reflect the subsurface hydraulic expression of PTHs. Our most comprehensive dataset links the uppermost PTH in the Platteville Formation to the preferential stratigraphic position of perched groundwater (commonly expressed as spring lines), and to significant head deflections in boreholes.

Our results are consistent with earlier fracture mapping of Silurian carbonate bedrock in eastern Wisconsin by Cooke et al. (2006) who also documented thick units with through-going fractures separated from one another by PTHs. These collective results indicate the lower Paleozoic bedrock in this region has a generally consistent, predictable fracture character across a wide spectrum of rock types and age. The correspondence to measurable hydraulic properties suggests promise for improved predictability of groundwater flow.