LOWER ORDOVICIAN COTTER AND POWELL TRANSITION IN THE OZARK REGION OF NORTHERN ARKANSAS

Conodonts from the Cotter and Powell Dolomite transition in north-central Arkansas have not been previously described, and the age-relation between these two formations in Arkansas is not known in terms of conodont biostratigraphy. Also, it is not known if there is a recognizable conodont biofacies difference between these formations.

A total of 3,023 conodont elements were collected from three different locations. The faunas are coniform-dominated and consist of twenty-five species representing 17 genera. The Cotter Dolomite is characterized by its abundance of Diaphorodus delicatus and Oepikodus communis, and also contains Aloxoconus staufferi, Colaptoconus quadraplicatus, Diaphorodus russoi, Drepanodus arcuatus, Drepanodus concavus, Drepanoistodus aff. D. forceps, Eucharodus parallelus, Fahraeusodus marathonensis, Parapanderodus striatus, Scandodus ethingtoni, Scolopodus aff. S. rex, Scolopodus quadratus, Toxotodus amphigyus, Tropodus comptus, and Ulrichodina abnormalis. The fauna in Powell Dolomite is characterized by abundant Diaphorodus delicatus, Drepanoistodus concavus, and Colaptoconus quadraplicatus, and also contains Aloxoconus staufferi, Diaphorodus russoi, Drepanodus arcuatus, Drepanodus concavus, Drepanoistodus aff. D. forceps, Drepanoistodus suberectus, Eucharodus parallelus, Eucharodus toomeyi, Fahraeusodus marathonensis, Oepikodus communis, Oistodus sp., Paraoistodus parallelus, Parapanderodus striatus, Pteracontiodus bransoni, Scandodus ethingtoni, Scolopodus cornutiformis, Scolopodus inconstans, Scolopodus aff. S. rex, Scolopodus quadratus, Toxotodus amphigyus, Tropodus comptus, and Ulrichodina abnormalis.

These faunas indicate that the upper part of the Cotter and the entire Powell are within in the Oepikodus communis zone, and support the idea that these units were deposited in intertidal to open shelf environment.