POSITION OF KINDERHOOKIAN-OSAGEAN BOUNDARY IN NORTHEASTERN KENTUCKY AND SOUTHERN OHIO

A detailed study of conodonts and ammonoids demonstrates that the Kinderhookian-Osagean boundary lies only ~50 cm above the base of the Henley Bed (or Member) of the Borden and Cuyahoga Formations. Our study, based on residues obtained by processing 10–200 kg clay shale and mudstone samples, has yielded large, diagnostic conodont faunas from conodont-poor rocks previously dated only by ammonoids. These faunas have enabled recognition of the standard sequence of Lower Mississippian conodont zones and precise correlations with stratigraphic units in adjacent areas. The Henley overlies the Sunbury Shale, the lower part of which is dated as uppermost Upper duplicata Zone. The basal 10 cm of greenish-gray phosphatic claystone in the Henley is recognized as the Jacobs Chapel equivalent and is dated as lowermost Lower crenulata Zone. This interval is unconformably overlain by a thin bed of yellowish-orange-weathering argillaceous dolomicrite equivalent to the Rockford Limestone. The Rockford interval straddles the boundary between the youngest Kinderhookian isosticha-Upper crenulata Zone and the earliest Osagean Lower typicus Zone. The overlying 3.5 m of Henley in Ohio is dated as Lower typicus Zone. The upper part of the Henley in Ohio and Kentucky and the overlying Farmers and Nancy Members of the Borden Formation in Kentucky are dated as Upper typicus Zone. The lower Nancy contains, in association with Upper typicus Zone conodonts, the Cave Run Lake ammonoid fauna, comprising Kazakhstania colubrella, Muensteroceras oweni, and “Karagandoceras” n. sp. This ammonoid fauna, associated with Upper typicus Zone conodonts, was recovered in the basal part of the Portsmouth Member of the Cuyahoga Formation along Ohio Highway 32 at the Pike-Adams County line. Thus, the entire Cuyahoga in southern Ohio, except for the lowest ~50 cm, is now dated unequivocally as Osagean.