FACIES GRADIENTS IN CINCINNATIAN CYCLES: IMPLICATIONS FOR LATE ORDOVICIAN SEQUENCE STRATIGRAPHY AND PALEOENVIRONMENTS IN EASTERN LAURENTIA

Recent high-resolution correlations of upper Cincinnatian (upper Katian: Maysvillian to Richmondian) strata, exposed on the periphery of the Cincinnati Arch, have been established using a combination of marker beds, epiboles, and carbon isotope stratigraphy. These correlations reveal patterns of facies change along a gently north-sloping paleoramp, as well as widespread regionally angular erosion surfaces leading to a revised third-order sequence stratigraphy. Consistent facies trends occur in four major cycles: lower, and upper Grant Lake Formation, and lower, and upper Rowland Member cycles. Parallel, proximal to distal (S to N) facies belts include: A) greenish gray, argillaceous fenestral lime mudstones (micrites) with glauconite-filled burrows, sparse ostracode and lingulid faunas and scattered desiccation cracks; B) rhythmically interbedded, pale gray, micritic, mollusk-rich wackestones and dark algal-rich shales; C) skeletal grainstone and rudstone, with corals and stromatoporoids; and D) brachiopod and bryozoan-rich mudstones and packstones. This facies spectrum appears to occupy a depth range from supratidal to subtidal ramp environments, about 30 to 40 m deep, with a gradient of a few cm per km. Upramp argillaceous, micritic facies (A, B) show aggradational to retrogradational patterns developed during lowstands to early transgressive portions of cycles, and appear to grade northward into condensed C intervals, recording shallow subtidal low to higher energy shoal environments, about 5 to 15 m deep. Facies A packages appear to overlie regional erosion surfaces, thicken upramp to a maximum before thinning, and to occupy an increasing proportion of depositional sequences southward, while deeper water, late transgressive to highstand facies thin and/or are truncated beneath overlying sequence boundaries. We suggest that the muddy micrites were deposited largely during intervals of rising base level, when increasing accommodation facilitated sequestering of muds in shallow subtidal to intertidal environments. Comparative study of the four cycles shows that shallow A-B facies extend progressively further north through time suggesting ramp steepening. Increase in micrite and biotic changes (e.g. increased abundance of corals) may also reflect an overall warming trend.