THE SILURIAN BRASSFIELD-ROCHESTER SHALE SEQUENCE IN THE SUBSURFACE OF EASTERN OHIO

ABSTRACT The Early-Middle Silurian Brassfield-Packer Shell-Rochester Shale interval in the subsurface of eastern Ohio thickens ten-fold 'from west to east and southeast toward the clastic source area. The carbonate units in the interval also thicken six-fold. The subsurface carbonate units, the Packer Shell limestone, Casing Shell limestone and unnamed carbonate rock units axe the equivalent of the Brassfield Limestone of the outcrop in southern Ohio. The Brassfield splits in the subsurface into many carbonate rock units which are interbedded with the Rochester Shale. The base of the Brassfield-Packer Shell is a disconformable surface. The basal carbonate unit of the interval onlaps the underlying clastic deposits of the Clinton sandstone interval (Medina Group of New York). The lower surface is a satisfactory marker horizon for subregional correlation and structural mapping. The whole interval between the Clinton sandstone and the Lockport (Niagaran) Group dolomites is time-transgressive from west to east.