MIDDLE CAMBRIAN TIDALLY-INFLUENCED, SILICICLASTIC SHELF DEPOSITS, WESTERN OHIO (U.S.A.): SUBSURFACE FACIES ANALYSIS OF THE EAU CLAIRE FORMATION AND CONASAUGA FORMATION

During the Middle Cambrian, the present State of Ohio was split N-S by a topographic high, with a sandy unit (Eau Claire Fm.) deposited on the western side, a carbonate unit (Conasauga Fm.) deposited on the eastern side, and a muddy unit (Conasauga Fm.) deposited along the southern margin. A middle-late Cambrian transgression submerged the topographic high, and recent studies (Banjade, 2011) reinterpreted the deposits in north-central Ohio as a transgressive barrier sequence. This study focuses on western Ohio, using cores from Allen County (135-m), Warren County (184-m), and Scioto County (33-m). Many of the deposits are tidalites. These deposits are heterolithic, sandstone-mudstone or siltstone-mudstone couplets, displaying planar lamination with double-mud drapes, lenticular bedding, wavy bedding, or flaser bedding. Thin, brecciated limestones within the tidalites sections may represent carbonate beachrock. Bioturbation is pervasive in the sections. The tidalites are interbedded with thicker intervals of massive/bioturbated mudstone and siltstone, interpreted as muddy inner shelf deposits. The shelf sequences contain probable condensed sections (glauconite grain accumulations) and probable flooding surfaces (tidalites overlain by a thin lag surface overlain by shales). The coarser deposits consist of hummocky stratified sandstones, overlain by planar bedded and ripple laminated sandstones, siltstones, and mudstones. These are interpreted as tempestites (storm deposits) which form above storm-weather wavebase. The tempestites are interbedded with lag deposits consisting of winnowed sandstones and siltstones containing shell debris, scattered glauconite grains, or brecciated carbonate clasts. The overall succession observed in the cores is (from bottom to top): tidalite deposits overlain by successions of tempestites, overlain by muddy shelf deposits. This is a characteristic transgressive, deepening-upward succession. Ongoing work includes microfacies analysis, sandstone point counts, evaluating depositional geometry by correlating between cores using 20 additional wells that have geophysical log datasets, and XRD/SEM analysis of the shales.