NEW VIEW OF SEGMENT OF THE EAST CONTINENT RIFT BASIN, OHIO BASED ON GRAVITY, GEOMAGNETIC, AND SEISMIC SURVEYS

At GSA Denver 2002 Fidler and Noltimier reported analysis of gravity and geomagnetic anomalies due to Precambrian crystalline basement topography beneath the Bellefontaine Outlier, Ohio, situated within the East Continent Rift Basin (ECRB), SE of the Fort Wayne Rift. In this study we consider the dimensions of the down-faulted rift valley, the probable age of its development and possible nature of clastic sedimentary infill.

The area of our study (circa 2,500 square km) lies between 40.13 and 40.50 degrees N. latitude, 83.50 and 83.88 degrees W. longitude. Crystalline basement depth below the surface varies from 5.7 km to 7.5 km in the rift valley. The rift valley width ranges from 13 to 15 km and trends NE for 30 km in the southern half of our study area, NW for 20km in the northern half. The volume of clastic infill is at least 850 cubic km.

The age of infill sediment ranges from rifting onset (1.1 b.y.) to post-Grenville Orogeny. Grenville foreland basin sediments and Middle Run Fm represent conclusion of Precambrian ECRB depositional sequences. Arkose, red sandstone, and black limestone have been reported in deep borehole cores reaching the Middle Run Fm. Volcanoclastic lenses within sandstone are interpreted from microseismic data in Allen Co. to the north of our study area. Equivalents to the Nonesuch Shale and Portage Lake Volcanics are potential stratigraphic units based upon seismic interval velocity analyses by Dean and Baranoski, Oil and Gas Journal, 2002.