EARLY PENNSYLVANIAN (MORROWAN-EARLY ATOKAN?) CONODONTS FROM THE UPPER KERBER AND LOWER SHARPSDALE FORMATIONS, ARKANSAS RIVER VALLEY, CENTRAL COLORADO

The Kerber and Sharpsdale formations comprise the lower 650 meters of the thick Pennsylvanian section that fills the Central Colorado Trough in the Arkansas Valley area east of Salida, Colorado. The Central Colorado Trough developed during the early Pennsylvanian as a narrow northwesterly trending structural and depositional basin flanked by the Umcompahgre Uplift on the west and by the Ancestral Rockies on the east. The Kerber and Sharpsdale formations are dominated by terrigenous clastics that record the unroofing of the adjacent rising highlands. In the upper Kerber and lower Sharpsdale, gray shallow water limestones and thin marine shales occur interbedded with terrigenous clastics through an interval of nearly 150 meters. Henbest (1946) reported the Morrowan fusulinid Millerella in the uppermost Kerber. The Kerber and Sharpsdale formations thin to the northwest and are replaced by the Belden Shale over a distance of about 50 kilometers. The Belden Shale comprises Morrowan to Desmoinesian deeper water black marine shales with thin limestone interbeds that occur over a large area of central and western Colorado.

Low diversity conodont faunas were obtained from many of the limestone beds in the upper Kerber and lower Sharpsdale. CAI values are approximately 4 to 5, a result of contact metamorphism from a nearby Cretaceous igneous intrusion. Elements of Adetognathus dominate most samples. Idiognathoides and Idiognathodus elements occur frequently, but often only elements of one genus or the other. Idiognathoides sinuatus characterizes the lowermost limestones and a few specimens of Id. ouachitensis are present. Higher in the section, Idiognathodus sinuosus becomes common and I. suberectus occurs. In the uppermost limestones in the lower Sharpsdale, small Pa elements that can be assigned to I. parvus occur with a few large Pa elements like those that have been placed in I. klapperi by Grayson et al. (1990). This conodont succession indicates that the limestone-bearing interval ranges from the mid-Morrowan into possibly the earliest Atokan, depending on the level of the base of the Atokan. Belden conodont faunas are poorly known, but the upper Kerber-lower Sharpsdale conodont fauna is similar to that illustrated by McLaughlin (1947) from the Glen Eyrie Shale, near Colorado Springs.