Knox Unconformity in Subsurface of Northern Ohio

Alan H. Coogan, Mark U. Maki

The 10 m.y. Early Ordovician erosional period produced a folded and faulted karstic surface on the Late Cambrian carbonate rocks of the upper Knox (or Beekmantown) and lower Knox (or Lee Valley) Group in northern Ohio. The unconformity surface is much more complex than H. P. Woodward's reconstruction of the Waverly arch as a broad, south-dipping, symmetrical anticlinal nose. In northern Ohio, two major north-south anticlinal folds--the Findlay arch and the Wooster arch--are expressed at the Knox unconformity as valleys. The major intervening synclinal fold, the Huron-Lorain swale, lies in the position of Woodward's "Waverly Arch," which we do not recognize as a positive structural feature.

In northeastern Ohio, the Wooster arch trend is the site of a valley 10-20 mi wide, in which the lower Copper Ridge Dolomite (Lee Valley Group) subcrops at the unconformity surface. The valley is bounded on the east by the cuesta of the Rose Run Sandstone Member and overlying Chepultepec Dolomite (Beekmantown Group). The valley is bounded on the west only by upper Copper Ridge Dolomite and Beekmantown dolomite, inasmuch as the Rose Run Sandstone Member is missing west of the valley, probably owing to nondeposition. In northwestern Ohio, there is a similar valley (approximately 30 mi wide) over the position of the Findlay arch in which the lower Copper Ridge Dolomite subcrops. The Huron-Lorain swale is a low, broad, synclinal area with smaller superimposed folds. These folds contribute to the development of hills and valleys on the karstic eroded carbonate rocks of the so-called "B-Zone" (a quartzose, glauconitic dolomite) and the lower Copper Ridge Dolomite in the north and on the B-Zone and upper Copper Ridge Dolomite farther south where the Cambrian oil fields of Morrow County are located.

Smaller scale hills and valleys appear on the unconformity surface map. Large residual hills and outliers are also mapped. Major drainage patterns have a southerly trend. Secondary streams partly flowed from the east into the south-oriented anticlinal valley. Earlier work showed the unconformity surface to have 150 ft of erosional relief locally. Overall, at least 200 ft of Upper Cambrian carbonate rocks were removed in northern Ohio during the Early Ordovician. The Knox unconformity surface dipped to the south at about 2-3 ft per mile. Some of the steepest slopes are along the sides of the Wooster arch valley where they range from 8.5 to 27 ft per mile or between 0.1 and 0.5%.

AAPG Search and Discovery Article #91041©1987 AAPG Eastern Section Meeting, Columbus, Ohio, October 7-10, 1987.