ABSTRACT
A stratigraphic framework is necessary to an understanding of the stratigraphic distributions of upper Richmondian bryozoans. For more than 150 years, the paleontologic and stratigraphic relations in the upper part of the Upper Ordovician Cincinnatian Series have continued to raise a number of questions as the various stratigraphic approaches to the study of its rocks and fossils have changed and been modified. In part, this situation results from subtle, but rapid, facies changes, limited continuous outcrops, and the prevailing stratigraphic philosophy of the geologists at the time. For example, until the 1960s or the 1970s, the occurrences of distinctive fossils were considered to mark identifiable (time) horizons (Nickles 1903, Cumings 1908, 1922, Cumings & Galloway 1913, Caster et al. 1955). Beginning in the late 1960s and continuing through the 1990s, major and minor lithologic facies changes began to be emphasized and most fossils became interpreted as representative of biofacies in the various stratigraphic sections (Hay et al. 1981, Karklins 1984, Tobin 1986). This combined lithologic-biofacies approach, supplemented to some extent with fossil occurrences, was carried out over a
larger area, as in the U.S. Geological Survey mapping program in Kentucky in the late 1960s, 1970s, and 1980s, and was successful in introducing new ideas into the stratigraphic interpretations of the region (Weir et al. 1984). This involved defining mappable lithologic units, considering major lithologic changes as facies changes, using arbitrary lateral cut-offs of members and formations, and even having the same named members in different formations (Weir et al. 1984). Although this approach was common in many of the studies throughout the region in the 1970s and 1980s, this seems particularly true of the U.S. Geological Survey’s stratigraphic and mapping interpretations in central Kentucky, where the middle to upper Cincinnatian formations and members are mapped as a series of lithologic lateral facies and arbitrary stratigraphic cut-offs. Because Weir et al. (1984) did not consider their stratigraphic units to be separated by unconformities, but rather as lithologic recurrences, the same member names were retained for similar lithologic units even if they appeared in different formations and in different stratigraphic positions. Their stratigraphy (Weir et al. 1984) was correlated so that a named lithologic member may be a member in two or more formations (and each occurrence may be a
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June R.P. Ross Department of Biology, Western Washington University, Bellingham, Washington, USA
Charles A. Ross Department of Geology, Western Washington University, Bellingham, Washington, USA
ABSTRACT: Bryozoan diversity in the upper part of the Richmondian Stage was much reduced and consisted of one to only a few species in the genera Rhombotrypa, Parvohallopora, Constellaria, Bythopora, Escharopora, Heterotrypa, Homotrypa, Peronopora, Batostoma, Gortanipora, Tarphophragma, Amplexopora, Monticulipora, Crepipora and Graptodictya. Except for the large species of Rhombotrypa, Parvohallopora, and Heterotrypa, they are generally fragmentary specimens in fine-grained calcarenitic debris facies that form thin, sheet-like, shallow subtidal sediments. The Drakes Formation, in central and northcentral Kentucky, and the correlative Whitewater Formation in southeastern Indiana form the upper part of the Richmondian Stage of the Cincinnatian Series (Upper Ordovician). Within this succession the number of bryozoan-bearing beds are relatively few. In central Kentucky, the Drakes Formation rests with regional unconformity on a number of different members within the middle Cincinnatian Bull Fork and Ashlock formations and has at least 60 m of topographic relief across the Cincinnati Arch (Central Kentucky Middle Richmondian hiatus).The upper part of the Whitewater thickens into the Sebree Trough and represents the youngest Richmondian strata present in outcrop.
