The American Upper Ordovician Standard. VII. Stratigraphy and Petrology of the Cynthiana and Eden Formations of the Ohio Valley
M. P. WEISS, W. R. EDWARDS, C. E. NORMAN, E. R. SHARP, 1965. "The American Upper Ordovician Standard. VII. Stratigraphy and Petrology of the Cynthiana and Eden Formations of the Ohio Valley", The American Upper Ordovician Standard. VII. Stratigraphy and Petrology of the Cynthiana and Eden Formations of the Ohio Valley, M. P. Weiss, W. R. Edwards, C. E. Norman, E. R. Sharp
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The lower Cincinnatian and subjacent beds are suitable for study by lithostratigraphic methods, and have been studied and classified on that basis. Logs of the clastic ratio (shale-plus-siltstone to limestone) are most useful, but logs of the stratigraphic index and of abundance of the kinds of limestone are helpful and particularly significant in paleogeographic considerations.
The Cynthiana Formation of the Ohio Valley is represented by the Point Pleasant Limestone Member, lying directly beneath the classic Eden and grading northwestward into the shalier Bromley tongue. Nicholas, River Quarry, and Rogers Gap are all rejected as stratigraphic units. The Point Pleasant is a thin- to medium-bedded biogenic limestone with shale having a clastic ratio of about 1 and always less than 5. The limestones are rather free of clay and silt, and some contain intraformational conglomerates, submarine-slump features, and a few mudcracks. The member is at least 75 feet thick within the Ohio Valley outcrop belt. The basal part contains a conodont assemblage of principally European forms; the rest contains long-ranging indigenous species.
The Eden Formation of the type region is interbedded shale and limestone lying between limestones of the Point Pleasant Member and the Fairview Formation. It contains no subordinate lithic units; Fulton, Economy, Southgate, and McMicken are rejected as stratigraphic units. The clastic ratio throughout the base of the formation is very high; above the base the lower beds have a ratio of about 2 at eastern localities. From that value the ratio generally increases upward at any locality and northwestward at any level to as much as 15 in mid-formation west of Cincinnati. The Eden limestones are thin- or medium-bedded, but the shales range from laminae to very thick beds. The macrofauna is dominated by Onniella, Thaerodonta, and several bryozoans. Conodont Zone I is coextensive with the formation, which increases in thickness from 150 to 280 feet northwestward across southwestern Ohio.
The shales and siltstones of these formations are massive or laminated and fissile and consist mostly of fine and medium silt. About 15 per cent is calcite; illite, chlorite, and quartz are the most abundant acid-resistant minerals. The limestones fall into a few coherent groups that differ in texture, state of fossil debris, and “insoluble” content; all are at least partly biogenic.
A shoal, lying southeast of the crest of the Cincinnati “arch,” that served as a partial barrier to detritus from the east and southeast, and was locally crowded with benthic organisms, controlled the petrography of the shales and limestones and the stratigraphy of the Cynthiana and Eden Formations. Intermittent changes in the conditions of mechanical energy on the bottom—through shifts of position or direction of currents, perhaps augmented by waves—damaged or destroyed certain colonies of organisms. The positioning of lenses and layers of limestone in the intervening shales was dependent on the direction and distance of transport of calcareous debris; and the kind of limestone formed was controlled by the material available in the colony, the degree of destruction, and the perfection of the sorting.