Geology of Basement in Midwestern United States

Abstract

A thick sedimentary cover over the midwestern United States has restricted geologic investigation of the basement complex in the past to a slow and fragmentary study based on a few deep wells. Recent basement studies have been stimulated by State-wide geophysical surveys now available for regional interpretation. Modern investigations have included mapping of basement configuration, lithology, orogenic trends, and geochronology. A map showing basement configuration reflects the basin and arch provinces recognized in the Paleozoic rocks of the Midwest, and thus supports the usual premise that the basement forms a regional structural framework for the overlying sedimentary strata. Cross sections based upon seismic surveys and extrapolations from deep well data show that Paleozoic structures, such as the La Salle anticline and the Rough Creek and Kentucky fault systems, are underlain by basement ridges and scarps. Thickening and thinning of Lower Ordovician and Cambrian strata over basement scarps and ridges demonstrate basement influence on early Paleozoic sedimentation. The relation of basement relief to oil accumulation is a possibility that has not yet been demonstrated fully. Lithologic studies of well samples reveal that the basement in the Midwest is predominantly granitic. Linear gravity and magnetic anomalies in Michigan, Indiana, Ohio, and Kentucky may be caused by Keweenawan-type basalts superposed on the granite. A study of basement lithologies and isotopic age determinations helps in the recognition of extensions of structural provinces of the Canadian shield into this region. A geochronological map based on sparse age data shows, for example, a southwestern extension of the Grenville front through Michigan and Indiana.