STRUCTURAL DIAGENESIS AND MICRO- TO MACRO-SCALE STRUCTURAL ARCHITECTURE OF BASAL NONCONFORMITIES

The structural and hydrological properties of sedimentary and crystalline rock are relatively well understood on their own; however, there is a knowledge gap associated with nonconformities. Nonconformities involve a diversity of lithologies, and exhibit a range of alteration and deformation features. Past fluid/rock interactions as inferred from mineral paragenesis and geochemistry can help in understanding modern day fluid flow. We are particularly interested in fluid flow occurring at the contact associated with fluid injection into basal reservoirs, which may trigger seismicity kilometers below the contact resulting from pressure perturbations along crystalline basement faults. These pressure perturbations and earthquake hypocenters in crystalline basement indicate that fluid pressures have to communicate over large distances over relatively short times; the paths might be through the basement rocks, and/or along the contact. Our data are also relevant to applications in carbon sequestration, and exploration of crystalline basement-rock hosted hydrocarbon and unconformity-type uranium deposits.

We use a multifaceted approach of core and field study on rocks that encompass a range of scenarios encountered in the subsurface. We use mineral exploration cores from Michigan that intersect the nonconformity, including cores from an unconformity-type uranium exploration project, with Jacobsville and Bessemer Formations on Archean Michigamme Slate, and iron mining core through the Cambrian Mt. Simon Sandstone on greenstone-BIF rocks. Our main field site is located west of Las Vegas, New Mexico in the Sangre de Cristo Mountains. The well-exposed nonconformity includes an undulating contact and regolith horizon, a variety of deformation and alteration structures, and a range of protoliths. Several sites are located in southern Colorado between Durango and Silverton in the San Juan Mountains, including an unweathered and relatively undeformed interface outcrop. Our preliminary work indicates the nonconformity zone consists of a range of structural, sedimentological, and geochemical features that result from alteration, deformation, paleoweathering, and mineralization. Our data support the idea that the nonconformity is a distinct hydromechanical unit.