THE GEOCHEMICAL AND PHYSICAL NATURE OF THE GREAT UNCONFORMITY, MIDCONTINENT, US

We investigate the nature of physical and chemical rock properties within and across the Precambrian nonconformity zone (the Great Unconformity) in the central U. S. and Wyoming to constrain its hydrogeologic history. We document lithologies, mineralogy, weathering and alteration, and deformation structures at outcrop in Michigan and Wyoming, and in core from Michigan and Illinois. Petrography, large-scale field mapping, core logging, microstructural and whole-rock geochemical analyses are conducted on samples near and across the nonconformity interface characterize the nature of past fluid flow and inform models of fluid flow in the future.

The contact zones consist of a basal conglomerate with a relatively sharp boundary, a weathered/altered zone of crystalline rocks ± a coarse grus, or strongly mineralized contacts, with carbonates, silicates, oxides, ± sulfides. The nonconformity is typically fractured or faulted, resulting in a variety of hydrological implications. Regolith, clast-supported grus or granitic wash, or poorly cemented conglomeratic horizons may act as high permeability conduits, whereas clay-rich horizons, matrix- supported grus, granitic wash, or tightly cemented conglomerates, may act as low permeability barriers.

Core from the mineralized nonconformity in the Michigan basin exhibits hydrothermally deposited carbonate veins in crystalline basement, and serves as an analog for how fluids migrate in crystalline rocks in injection systems. Prior higher-permeable pathways developed within these Precambrian crystalline rocks are identified by pervasive carbonate deposition and feldspar alteration. These now mineralized contacts are presently low permeability barriers due to a reduction of pore space and/or resistance to weathering. The introduction of modern warm brines of complex or mixed geochemistry during injection could result in mineralization or chemical alteration via fluid-rock interactions in the contact zone. This analog study suggests that subsurface physical and chemical rock properties, coupled with locally complex fault geometries, could dynamically impact permeability depending on the mineralogy of the host rock and chemical composition of the injected brine and ultimately lead to transmission of pore-fluid pressures over long distances.