BASAL SAUK SEQUENCE IN THE SUBSURFACE OF OHIO-- MT. SIMON SANDSTONE

In western Ohio, the Cambrian Mt. Simon Sandstone (MSS) ranges in thickness from 0-120 m, and is found about 1060 m below the ground surface. There is little subsurface data about the MSS in Ohio, despite the fact the unit hosts a dozen Class I hazardous waste injection wells. This study focused on core and well logs from Well DGS 2627, and correlated the results to four other exploratory wells in the MSS.

The MSS disconformably overlies the Precambrian Middle Run Formation. The contact is readily apparent in cores as a transition from the dark reddish brown, highly indurated and well cemented, poorly sorted, angular, fine- to medium-grained lithic arenite and quartz-feldspar-rhyolite-quartzite-chert pebble conglomerate of the Middle Run Formation to the tan, friable, moderately sorted, rounded, coarse- to very coarse-grained siliceous quartz arenite and quartz granule conglomerate of the MSS. Gamma-ray and neutron-porosity logs notably decrease across the contact.

The MSS contains tidal rhythmites, lenticular-, flaser-, and wavy-bedding, herringbone cross-bedding, mud-drapes, tidal bundles, reactivation surfaces, intraclasts, and bioturbation. The unit generally coarsens and thickens upwards. The MSS is interpreted as a transgressive barrier sequence in which the barrier (the bulk of the MSS) migrated across a basal estuarine/lagoonal sequence. The upper progradational, shoaling- and fining-upward parts of the MSS observed in Wisconsin and the Michigan Basin are absent in western Ohio. Instead, the MSS is conformably overlain by tidal flat and off-shore deposits of the Eau Claire Formation.

The Eau Claire Formation is readily recognized in cores as the transition to gray, fine-grained, feldspathic arenite and shale, typically intensely bioturbated and mottled, with marine macrofossils. The geophysical signature of the contact is a pronounced increase in gamma-ray and neutron-porosity logs.