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| Atokan | |
|---|---|
| Name | Atokan |
| Period | Pennsylvanian |
| Age | Moscovian (Atokan North American stage) |
| Primary lithology | Sandstone, shale, coal |
| Other lithology | Limestone, siltstone |
| Namedfor | Atoka County |
| Namedby | A. H. Purdue |
| Region | North America |
| Country | United States |
| Unitof | Pennsylvanian stratigraphy |
| Subunits | Morrowan, Desmoinesian (correlative units) |
Atokan is a regional Pennsylvanian stage and associated stratigraphic interval originally defined in the Midcontinent and Ouachita foreland regions of North America. It denotes a suite of sedimentary rocks—sandstone, shale, coal, and subordinate limestone—deposited during the Moscovian-equivalent time slice and is central to correlations among the Appalachian Basin, Midcontinent, and Ouachita orogenic forelands. The Atokan interval is significant for its paleontological assemblages, cyclothemic sequences, and economic coal resources.
The Atokan interval occupies a position above Morrowan-equivalent units and below Desmoinesian-equivalent strata within the Pennsylvanian chronostratigraphy. In the Ouachita fold-thrust belt and the Arkoma Basin, Atokan sediments include cyclic fluvial-deltaic sandstone packages, marine-influenced shale, and coal-bearing cyclothems documented in well logs and outcrops correlated with basin-scale transgressive-regressive events recorded in seismic profiles. Key lithostratigraphic features compare to units described in the Anadarko Basin, Fort Worth Basin, and Appalachian Basin and are tied to eustatic oscillations recorded by researchers working on Permian and Carboniferous sequences internationally. Biostratigraphic control frequently uses fusulinacean foraminifers, conodont zonation, and plant macrofossils to refine stage boundaries and correlate with contemporaneous units such as the Bolsovian and Kasimovian in European frameworks.
Fossil assemblages within the Atokan include diverse fusulinids, ammonoids, brachiopods, bryozoans, and foraminifers in marine facies, and prolific plant macrofossils, lycopsids, calamites, and pteridosperm fragments in nonmarine coal measures. Trace fossils and ichnofabrics occur in tidal-flat sandstones correlated to contemporaneous ichnofaunas described from the Pennsylvanian of Eurasia, facilitating paleobiogeographic comparisons with sites studied by paleontologists working on the Carboniferous floras of the Carboniferous Coal Measures and the Moscovian floras of the Russian Platform. Marine microfossils, particularly conodonts, are used alongside fusulinids to tie Atokan horizons to global chronostratigraphic schemes developed by committees and working groups active in Permian-Carboniferous standardization.
Atokan-designated strata are exposed and subsurface across parts of the Midcontinent, Arkoma Basin, and Ouachita foldbelt, with notable occurrences in Oklahoma, Texas, Arkansas, and adjacent areas. Outcrop belts in the Arbuckle Mountains, Ouachita Mountains, and the flanking Anadarko uplift display Atokan cyclothems and coal seams similar to those documented in the Appalachian Plateau and Illinois Basin, enabling basin-to-basin correlation. Subsurface mapping in petroleum and coal exploration wells across the Anadarko and Fort Worth basins has refined the lateral extent and depositional trends of Atokan deposits, linking them to contemporaneous sedimentary provinces such as the Midcontinent Rift and the Western Interior Seaway margins during late Paleozoic plate reconfigurations.
The term was coined in regional stratigraphic studies originating from early 20th-century surveys of the American South-Central United States by geologists associated with state geological surveys and university departments. Pioneering stratigraphers and paleontologists refined Atokan boundaries through biostratigraphy, lithostratigraphy, and coal correlation programs led by investigators working with institutions such as the United States Geological Survey, state agencies, and academic centers. Subsequent work by sedimentologists, basin analysts, and petroleum geologists integrated well-log correlation, core studies, and seismic interpretation, paralleling developments in Carboniferous research by European and Russian colleagues and international stratigraphic commissions.
Atokan coal-bearing cyclothems have been mined historically in regional coalfields and have influenced energy supply, rail transport networks, and regional industrialization patterns examined by economic geologists and mining historians. Sandstone reservoirs and shale seals within the Atokan interval are targets for hydrocarbon exploration in the Anadarko and Fort Worth basins and have been evaluated in studies by petroleum companies and energy agencies. Additionally, Atokan limestones and sandstones serve as local sources for aggregate, dimension stone, and raw materials used in cement and construction industries monitored by state geological surveys and mineral resource assessments.
Subdivision of the Atokan interval employs lithostratigraphic members, cyclothem names, and biostratigraphic zones using fusulinaceans and conodont taxa, facilitating correlation with regional stages such as the Morrowan and Desmoinesian as well as international Moscovian and Bashkirian schemes. Correlative analyses draw on comparisons with stratigraphic frameworks developed for the Illinois Basin, Appalachian Basin, Anadarko Basin, and European Carboniferous sequences, integrating data sets produced by stratigraphic commissions, paleontological monographs, and basin synthesis studies to refine temporal and spatial relationships across late Carboniferous depositional systems.
Category:Pennsylvanian stratigraphy