Date of Award:

5-2019

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Geosciences

Department name when degree awarded

Geology

Committee Chair(s)

James P. Evans

Committee

James P. Evans

Committee

Kelly K. Bradbury

Committee

Anthony R. Lowry

Abstract

Rising incidents of earthquakes caused by human activity in the United States, known as induced earthquakes, is a growing concern. Induced earthquakes may occur when fluid and/or wastewater is injected several kilometers beneath the Earth’s surface into sedimentary rocks. Fluids and pressures can migrate from the sedimentary rocks, which are typically friendlier to fluid flow, into underlying less friendlier crystalline rocks along fluid pathways weakening and possibly reactivating preexisting faults. Understanding potential fluid pathways and/or barriers from the sedimentary rocks to crystalline rocks is crucial. I investigate the structure, composition, and heterogeneity of rocks near the contact between the sedimentary and crystalline rocks, known as nonconformities, and highlight their possible role in the transmission of fluids and porefluid pressures into the crystalline basement.

To characterize nonconformities, we examined outcrop analogs and drillcore of nonconformities in New Mexico, Colorado, and Michigan. Geochemical, structural, and hydrological techniques were used to analyze the nonconformities at microscopic to megascopic scales. The nonconformities observed in this study consist of variably deformed, weathered, and altered igneous and metamorphic crystalline basement overlain by sedimentary rocks cut by outcrop- and map-scale faults. The nonconformity at the New Mexico sites includes a clay-rich weathered horizon atop deformed and jointed crystalline basement. Heterogeneity observed in the fracture and joint networks within the crystalline basement contributes to permeability heterogeneity and anisotropy. The crystalline basement adjacent to the nonconformity at the Colorado site is relatively fresh and unweathered and overlain by low permeability sandstones. The nonconformity and underlying slates in the Michigan drillcore are overprinted by hydrothermal alteration and carbonate mineralization, which provides evidence of hydrological communication between sedimentary and crystalline rocks. The nonconformities display a range of structural, hydrological, and geochemical styles and characteristics which vary over relatively small spatial extents. The geological and hydrogeological histories and complexities of nonconformity analogs provide valuable information to understand how fluids, past, and present, interact with the contact. This study introduces some of the factors that may control fluid flow adjacent to nonconformities and their possible significance to the interplay of deformation, fluid flow, and induced seismicity.

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