Layered Rocks beneath the Phanerozoic Platform of the U.S. Midcontinent

HAUSER, ERNEST C., Cornell University, Ithaca, NY

A thick sequence of layered rocks lies hidden beneath the Phanerozoic cover of the central U.S. over large regions. A thick sequence of Precambrian layered rocks is imaged on the COCORP transect across southern Illinois and Indiana. The thickness of this layered sequence varies from 1-3 times the thickness of the overlying Phanerozoic section of the Illinois basin. The layered sequence is observed for close to 200 km in an east-west direction. Similar layered reflections are seen on the COCORP data from Hardeman Co., TX, and neighboring southwest Oklahoma. Both of these known occurrences lie within the region of the middle Proterozoic Granite/Rhyolite province of the U.S. midcontinent, an area within which scattered wells to basement commonly encounter 1.3-1.5 Ga undeformed granite an /or compositionally similar rhyolite. Therefore, these layered assemblages may comprise a thick sequence of silicic volcanic and sedimentary rocks (perhaps also injected by mafic sills) between scattered volcanic-intrusive centers, such as exposed in the St. Francois Mountains of southeast Missouri. However, in places such as Illinois and Indiana, the near absence of deep wells leaves the possibility that the upper portion of these layered rocks may locally be of late Proterozoic or earliest Paleozoic age. The reprocessing of available industry data, analyzed in conjunction with the existing COCORP data, includes extended vibroseis correlation. These industry data are invaluable in our effort to expand the known distribution of these layered rocks (e.g., into north-central Illinois) and o map their structures.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)