| Summary: | Articulate brachiopod communities sensitive to environmental factors, especially depth and distance from shore, are described from the upper Middle Devonian (upper Eifelian and Givetian; Cazenovia, Tioughnioga, and Taghanic stages of eastern North America) of eastern North America. The Subrensselandia, Cupularostrum, Mediospirifer, Tropidoleptus, Devonochonetes, Mucrospirifer, Atrypid-Strophodontid, Ambocoeliid, Pacificocoelia, Truncalosia, and Camarotoechia Communities are described from the Hamilton Group of the Appalachian and Illinois Basins. The Cupularostrum, Rhipido thyris, Orthospirifer, Tropidoleptus, Mucrospirifer, Elytha, Spinatrypa, Hypothyridina-Ambocoeliid, Rhyssochonetes, and Leiorhynchus Communities are described from the Tully Formation of the Appalachian Basin. The Subrensselaxdia, Stringocephalus, Rhipidothyris, Cupularostrum, Atrypid- Gypidulinid, Devonochonetes, Cyrtina-Athyris, Camarotoechia, and Longispina Communities are described from the sub-Taghanic Traverse Group of the Michigan Basin. The Rensselandia, Chonetid, Pentamerella-Orthospirifer-Cranaena, Spiriferid-Atrypid, Mucrospirifer, Emanuella, and Leiorhynchus Communities are described from the Cedar Valley Formation and correlative strata exclusive of the Appalachian Basin. The Subrensselandia, Tropidoleptus, Mucrospirifer-Pholidostrophia, Atrypid-Gypidulinid, Chonetid-Leptaena, Brevispirifer, Emanuella, and Warrenella Communities are described from the Dundee and Rogers City Formations and correlative strata exclusive of the Appalachian Basin. The evolution of these brachiopod communities is traced from the Emsian through the early Famennian by assigning related communities to Community Groups. The temporal changes in the Community Groups are discussed in terms of the constituent genera and communities, Communities not assigned to Community Groups are also considered. The Leptaena-Schizophoria. Assemblage Zone is defined to include strata correlative with the Union Springs and Cherry Valley Formations of New York. The Fimbrispirifer-Pentagonia Assemblage Zone is defined to include strata correlative with the Centerfield Member of New York. The Rhyssochonetes-Hypothyridina Assemblage Zone is defined to include strata which correlate with the Tully Formation of New York. These assemblage zones are the basis for detailed correlation of the upper Middle Devonian strata of eastern North America. The late Middle Devonian fauna of eastern North America represents a mixture of older Eastern Americas Realm and Old World Realm faunas; many older Eastern Americas Realm stocks died out during the earlier Eifelian. Other Old World Realm genera migrated into the Eastern Americas Realm during transgressions associated with the Leptaena-Schizophoria and Rhyssochonetes-Hypothyridina Assemblage Zones; these genera distinguish these zones from older and younger rocks. The Eastern Americas Realm of the late Middle Devonian is divisible into two subprovinces. The Appohimchi Subprovince occupies the Appalachian and Illinois Basins and the clastic shelf to the east of the Appalachian Basin. The Michigan Basin-Hudson Bay Lowland Subprovince occupies a broad carbonate platform extending from Michigan west to Iowa and north to Hudson Bay. Provinciality between these subprovinces rose from the base of the Cazenovia to a peak at the time of the deposition of the Skaneateles Formation, fell during the Fimbrispirifer-Pentagonia Assemblage Zone, rose during the upper Tioughnioga, and declined sharply during the Taghanic. The Fimbrispirifer-Pentagonia Assemblage Zone is associated with a transgression and change in current patterns which allowed Appohimchi genera to migrate onto the Michigan carbonate platform. Profiles of depositional topography are constructed for several intervals of late Middle Devonian time. Shelf (undathem), slope (clinothem), and basinal (fondathem) deposits are distinguished on these profiles. Chains of communities occur along these profiles. Benthic Assemblages 1 and 2 are on the shelf, Benthic Assemblage 3 ranges from shelf through slope, Benthic Assemblage 4 from slope to basin, and Benthic Assemblage 5 is basinal. Migrations of community chains and shelf margins correspond to transgressions and regressions. Biostratigraphic and lithostratigraphic data allow tracing of seven transgressive-regressive events over much of eastern North America; these cycles can be used to refine correlations based on other stratigraphic techniques.
|
|---|