A COMPLEX GUTTER CAST FROM THE UPPER ORDOVICIAN (CINCINNATIAN) KOPE FORMATION, NORTHERN KENTUCKY

An unusual gutter cast was collected from the Cincinnatian (Upper Ordovician) Kope Formation in northern Kentucky. Gutter casts in the Kope are interpreted to have formed on a paleo seafloor by scouring and infilling during large storms, possibly hurricanes. This gutter cast contains a second (inner) gutter cast, which contains two cephalopods with their long axes oriented parallel to the long axis of the structure and to a hypothesized sea floor current. Although such “composite gutter casts” have been reported in the literature, we believe oriented cephalopods in a gutter cast are unreported. Brachiopods, echinoderms, crinoid stems, and trilobite fragments with minor amounts of fine clastics are found in the lag deposits of both the inner and outer gutter casts. Both cephalopods are infilled with a bladed calcite rim overlain by calcite spar with the larger cephalopod partially infilled with lime mud at the anterior end, hypothesized to have faced up current. Staining with potassium ferrocyanide indicates both generations of cement and lime mud are iron-rich. Oxygen and carbon isotopic data for lime mud surrounding the cephalopods in the inner gutter cast (δ¹³C, -0.85 to -0.97‰ PDB; δ¹⁸O, -5.82 to -6.01‰) and calcite cement (δ¹³C, +0.42 to +0.75‰; δ¹⁸O, -4.93 to -5.95‰) infilling the intraparticle porosity in the cephalopods indicate different sources of carbon and oxygen with the cement generally more positive in both isotopes than the lime mud. High iron content in the bladed cement likely negates a submarine origin for it. Instead, the bladed rim is interpreted to have formed in a freshwater phreatic environment in which iron-rich groundwater flowed through the structure. The overlying blocky calcite spar is interpreted to have formed in a deep burial environment. δ¹⁸O of the calcite cement may reflect a mix of the two cement types with the less negative values representing cement precipitation in the freshwater phreatic. Perhaps the lime mud recrystallized in the deep burial environment as suggested by its iron content and δ¹⁸O, which is approximately equivalent to the most negative δ¹⁸O in the cement. δ¹³C indicates at least two different sources of carbon for the lime mud and cement.