The Petrology of a Composite Vertical Section of Cincinnatian Series Limestones (Upper Ordovician) of Southwestern Ohio, Southeastern Indiana, and Northern Kentucky

Abstract

ABSTRACT The majority of Cincinnatian limestone strata in the Kope to Saluda Formations were formed under low energy, subtidal, offshore environmental conditions. The upper part of the rock section (Saluda, Whitewater, and Drakes Formations) was deposited under nearshore subtidal, lagoonal and intertidal conditions. Bryozoans and brachiopods, nearly equal in abundance, form about 60% of the allochems of the limestones. Crinoids form about 20%, and trilobites nearly 10% of the allochems. Brachiopods and trilobites decrease appreciably in abundance upward in the section and algae, ostracodes and corals, which are uncommon through most of the sequence, are very common in the upper part. Dolomite forms less than 1% of the limestones of the lower part of the sequence but more than 5% of those of the upper part. The most abundant limestone is a coarse, poorly sorted rock containing 30 to 40% fossil allochems which were originally deposited with calcareous ooze. The ooze matrix has been largely converted by neomorphism to microspar and pseudospar. The average limestone is approximately a borderline rock between a biomicrosparrudite and a biopseudosparrudite. It is postulated that for most of the subtidal limestones, bryozoans and crinoids forming meadow-like patches stabilized the muddy bottom sediment and created a wave-current baffle for each biotic area. The quiet water conditions thus produced were favorable for filter-feeding organisms and for the concomitant accumulation of allochems and ooze. Sediment generated in the interior of the biotic communities formed coarse coquinoid limestones. Accumulations of ooze with few allochems and the uncommon, better sorted arenites were probably developed under higher current energy conditions along the margins of the biotic patches in areas of less dense growth. With this model, development and association of allochems and ooze is independent of energy levels of topographic relief of the depositional surface and of the accumulation of the surrounding terrigenous mud. Aperiodic destruction of the benthic communities by suffocation resulted when largely terrigenous bottom sediments were thrown into suspension under storm conditions and swept over the communities by bottom currents.