AN EARLY MISSISSIPPIAN GILBERT DELTA DEPOSIT WITHIN THE BLACK HAND SANDSTONE OF CENTRAL OHIO

The Black Hand Sandstone Member of the Cuyahoga Formation (Early Mississippian) is a quartz arenite preserved as two separate lobes exposed central Ohio. The thickness ranges from 10-90 meters. The sandstone varies in grain size from medium- to coarse-grained and commonly contains granules and pebbles either dispersed or as lenses within beds. The beds contain parallel, planar-tabular and trough cross-bedding. Large dipping strata reported within the two lobes are sometimes draped by fine-grained sandstone to mudstone, which are wave-rippled and occasionally bioturbated. The large dipping strata were originally interpreted as Gilbert Delta foresets, and more recently, as fluvial lateral accretion within an incised valley fill. Here we report on the analysis of the strata from a road cut extending 180 meters within the southern Hocking Lobe with excellent exposures of large, low angle surfaces. Master bedding planes are up to 18 meters high and dip at an average of 16 degrees. The grain size varies from fine-grained sandstone to conglomerate. Five lithofacies are present ranging from clast-supported conglomerates (maximum clast size 19 mm) in beds 5 cm to 3 meters thick to wave-rippled, very-fine grained sandstone. Cross-bedding is occasionally present and indicates flow down the master bedding planes. Matrix- to clast-supported pebble lenses are interpreted to be chute deposits formed as a result of subaqueous mass wasting. The facies association within this highway cut is consistent with the original interpretation of Gilbert Delta foreset deposits. The implications are that the minimum water depth into which the delta prograded was 18 m and that the delta prograded into the basin after a transgression, not prior to a major sea level rise.