HARDGROUNDS AND EPIBOLES AS CORRELATION TOOLS: UPPER-MIDDLE TO UPPER ORDOVICIAN (SHERMANIAN-EDENIAN) POINT PLEASANT AND LOWER KOPE FORMATIONS IN NORTHERN KENTUCKY AND SOUTHWESTERN OHIO

A recent investigation of the sequence and event stratigraphy of the Point Pleasant and lower Kope formations utilized key marker horizons, including hardgrounds and epiboles, in correlations over sections of northern Kentucky and southwestern Ohio. Hardgrounds are developed on grainstones that cap storm dominated, meter-scale cycles; they are bored and commonly show phosphate and/or pyrite mineralization. The caps of storm-cycles represent small diastems (flooding surfaces) during which the sea floor became partially cemented and was colonized by epifauna and endofauna. Anomalocrinus and bryozoan holdfasts are often present. An unusual occurrence of a hardground with Anomalocrinus holdfasts and Trypanites borings on the underside of a thick grainstone ledge at the Point Pleasant type section has been correlated to hardgrounds in surrounding outcrops. This horizon shows evidence for erosional scouring and undercutting to form cavities. Another correlation tool essential to this study was the tracing of stratigraphically restricted occurrences of rare faunas or epiboles. Epiboles of Triarthus and Glyptocystites fultonensis occur near the contact of the Point Pleasant Formation in the Fulton beds of the lower Kope Formation. These horizons formed key stratigraphic markers. Triarthrus, a small deep-water adapted trilobite, is located within two thin horizons in the basal Fulton beds of the Kope Formation in the area of Cincinnati, OH. Glyptocystites fultonensis, an uncommon form of rhombiferan cystoid, is only known from a single thin interval, again in the Fulton beds, approximately half a meter above its contact with the Point Pleasant Formation. Although Glyptocystites is very rare overall, in the horizon in which it occurs large numbers of plates and complete thecae, some displaying excellent preservation of detail, have been recovered. The epibole of Glypocystites just overlaps with the Triarthris epibole near Twelve Mile Creek, OH. These two marker horizons, combined with multiple hardgrounds, nodular beds, distinctive shell beds, and other marker horizons have allowed for detailed correlation of the lower Kope and upper Point Pleasant formations over 10,000 km².