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Starting around 2009, a greater number of earthquakes than anticipated have occurred in the midcontinent region of the United States. These earthquakes have been linked to increased rates and volumes of wastewater injection at several km’s depth into the Earth’s crust near a contact between crystalline metamorphic or igneous rock and overlying sedimentary rock, known as a nonconformity. While much is known about why these new earthquakes occur, comparatively little is known about the physical and chemical rock properties because the nonconformity contact is primarily buried under km’s of sedimentary rock in the midcontinent region. These rock properties are important because they influence rock strength and therefore the likelihood of earthquake activity. In this study, we examined two drillcores from southeastern Minnesota and document the presence of a ~6-22m horizon at the nonconformity which has been extensively altered due to low-temperature chemical weathering and high-temperature hydrothermal interaction. This distinct horizon is intensely damaged, semi-permeable, and is composed of mechanically weak minerals. We provide a comparative set of detailed mineralogy, structures, geochemistry, porosity values, and lab permeability measurements that can be used to model impacts of fluid injection and migration near the nonconformity interface. We also examined rock properties of the nonconformity interface in surface exposures of the contact near Gunnison, Colorado. The drillcore and outcrop observations and analyses demonstrate complex variabilities in rock properties that may exist along and/or across nonconformity contacts at depth. These small-to large-scale rock variations can be used to understand how high-pressure injected fluids may move along or across the rock contact. An additional goal of this work is to provide the background geologic information for a proposed USU-USGS collaborative scientific drilling project. This project targets previously identified magnetic and gravity anomalies associated with an underground igneous rock body called the Northeast Iowa Intrusive Complex. We observed an intensely fractured and faulted metagabbro overlying a layered intrusive complex with a weathered and hydrothermally altered contact at the nonconformity.
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