REINTERPRETATION OF THE OF THE BLACK HAND SANDSTONE (LOWER MISSISSIPPIAN) OF OHIO AS INCISED VALLEY FILL

The Black Hand Sandstone Member of central Ohio comprises about 60 m of coarse-grained, conglomeratic sandstone within the generally shaly units of the Cuyahoga Formation. The Black Hand has been interpreted as a Gilbert-type delta, as a shoreline or spit deposit, and as a distributary deposit. Each of these interpretations assumes that the Black Hand is laterally gradational with the surrounding Cuyahoga, a relationship that has not been directly observed in outcrop.

Cross bedding in the Black Hand consists of both trough and tabular bedding. The trough cross bedding ranges between one meter and 300 m in width and .5 m to 20 m in thickness. The tabular bedsets range in size from 15 cm - 8 m. Marine fossils have been reported only from the uppermost Black Hand. In the lower Black Hand the only fossils observed or reported are plant fossils, also indicative of a terrestrial influence. The combination of grain size, cross bedding styles, and the absence of marine influence indicate deposition in a braided stream system and not a marine, deltaic system. The trough cross beds were probably formed along temporary channels associated with braided streams and the tabular bedsets were deposited as a series of point or channel bars.

In central Ohio, the Black Hand is observed to be in direct, vertical contact with thin, interbedded siltstones and shales that contain marine trace fossils. The juxtaposition of fluvial sediments with marine sediments is one characteristic of incised valley deposits. The contact between the two units is scoured, and the overlying sandstone contains clasts of the underlying marine sediments. This suggests that the Black Hand was deposited as fluvial valley fill following incision of the marine Cuyahoga Formation. The lower Black Hand is predominantly fluvial suggesting sediment bypass of the marine system, concentrating the coarse-grained sediment in the valley. The presence of marine fossils in the uppermost Black Hand suggests the return of marine influence as the valley was filled and transgression proceeded.