The Borden Formation crops out in Kentucky around the Cincinnati  Arch in an arcuate pattern
           to the north.  Predominantly a fine-grained clastic sequence with minor carbonate layers of the top, the
           formation averages slightly more than 200 m in thickness.  This formation is interpreted as the marine part
           of an ancient deltaic complex, which prograded southwestward during the Early Carborniferous, infilling
           a shallow epicantinental sea.

              In northeastern Kentucky, the area of our principal studies, almost the entire formation is 0sagean
           (late Tounaisian to early Viséan in age, as indicated by conodonts.  The Kinderhookian-Osagen boundary
           lies within the Iowermost 0.5 m of the Borden, and conodonts of late rniddle to early late Meremecian
           (early Viséan) age are found in the uppermost 1 m of  the formation.  Witmn the Osagean part,
           distinct ammonoid assemblages characterize three separate faunal  zones.

             East-to-west progradation of the deltaic complex is demonstrated by successively younger
           assemblages near the base of  the Borden.  The easternmost assemblage, in northeast Kentucky, includes
           Protocanites lyoni, Imitoceras rotatorium, I. sp., Karagandoceras sp., Kazakhstania colubrella, Muensteroceras
           oweni, and M. parallelum and begins about 20 m above the base of the formation.  In south-central Kentucky,
           Merocanites drostei and Muensteroceras? sp. occur in a similar stratigraphic position.  In western Kentucky,
           Merocanites sp., Amonellipsites n. sp., and Beyrichoceras sp. are found a few meters above the base.
           This is a progression in age for the ammonoids of the lower part of the Borden from earliest 0sagean to
           late middle Osagean (late Tournaisian to earliest Viséan).