THE BLACK HAND SANDSTONE MEMBER OF THE CUYAHOGA FORMATION:AN EARLY MISSISSIPPIAN LARGE RIVER DEPOSIT

The Black Hand Sandstone (Tournasian/Osagean) is a quartz to lithic arenite that varies in thickness from 0 – 90 m and occurs as three separate lobes in central and northern Ohio. The southern lobe, the Hocking Valley lobe (HVL), overlies finer-grained, marine sandstones and mudstones of the Fairfield and Raccoon Members of the Cuyahoga Formation and is capped by the distinctive, thin quartz-pebble Berne Conglomerate. At six HVL locations the coarse-grained, pebbly sandstone of the Black Hand exhibit cosets of decimeter-scale trough, and to a lesser extent planar-tabular, crossbedding separated by 3^rd-order bounding surfaces that are at least 8 m high and dip 9° - 22° toward the north and northeast. The cosets indicate paleoflow to the north within five of the large structures and to the northwest in one instance. Thin (< 5 cm) beds of iron-rich, rippled, fine-grained sandstone to mudstone cap the lower portions of the 3^rd-order surfaces. The combination of cosets and bounding surfaces are interpreted to be the product of downstream and lateral accretion of mid-channel and lateral bars that can be traced for up to 180 m downflow. The size of the bar forms is comparable to those reported from very large braided systems, which has implications for drainage basin area and provenance.

Along the southern margin of the HVL the coarse-grained arenite thins to the south from > 60 m to less than 3 m over a distance of 4 km and overlies two amalgamated beds of medium-grained, well-sorted sandstones up to 10 m representing the top of the Fairfield Member of the Cuyahoga Formation. Both beds contain parallel to low-angle laminations with scattered quartz pebbles toward the top of the upper bed. Rare small cephalopod molds and trace fossils indicate that the sandstones are shallow marine in origin. Where the coarse arenite is absent the Berne Conglomerate rests directly on the Fairfield Member sandstones. This facies pattern is interpreted to represent the shallow marine to shoreface deposits of a delta platform that was deposited outboard of a prograding coarse-grained delta.