PENNSYLVANIAN LIMESTONES AND THEIR SIGNIFICANCE IN SOUTHEASTERN OHIO

Recognizing condensed sections and correlating them biostratigraphically are key to applying sequence stratigraphic concepts to shallow marine deposits, and are particularly important in high-shelf areas of low accommodation space where sequences are thin and have less preservation potential. Within the Late Pennsylvanian Conemaugh Group of southeastern Ohio in the distal Appalachian basin, most condensed sections are limestone beds that contain most of the marine fossils in the succession. Earlier workers relied on slight variations in macrofauna among the limestone beds and particularly on the assumption of an undisturbed layer-cake stratigraphy to correlate these beds. Their application of the cyclothem concept to the Conemaugh Group resulted in many names being applied to strata based largely on where they were expected to occur in a vertical section, rather than on unequivocal paleontological criteria. For example, the name Cambridge Limestone was applied to lithologies varying from cherty limestone to unfossiliferous sandstone that occur from 16 to 31 m below the Ames Limestone in Athens County. This is because the Ames Limestone is the most extensive marine unit and best marker bed in the region, and lies 27 m above the Cambridge Limestone at the Cambridge type section 50 miles to the north. Conodont data recently recovered from Athens County show that exposures previously reported as Cambridge are instead at least two different limestones that were deposited at nearly the same stratigraphic position by different glacioeustatic marine incursions. High resolution biostratigraphic control is essential before meaningful sequence stratigraphic analysis can begin in a region where sequence preservation was limited due to low total accommodation space, and where lowstand incision could either remove earlier limestones or confine later ones to paleovalleys.