Abstract and Figures
Pennsylvanian cyclothems of Midcontinent North America represent a repetitive succession of widespread marine beds, upon which widespread paleosols in the overlying terrestrial beds show that they resulted from extensive marine regressions as well as transgressions over a large land surface. Classic widespread late middle to late Pennsylvanian major cyclothems in this region consist of a thin transgressive limestone and thin offshore dark phosphatic shale (which contains the maximum fl ooding surface), followed upward by a thicker regressive shallowing-upward limestone and a variety of nearshore marine and terrestrial detrital facies as well as paleosols. Cyclothems of intermediate scale lack the black facies in the offshore shale and are less widespread, and those of minor scale lack much differentiation of facies. The greatest number of cyclothems of all scales is along the shelf-basin margin in the Kansas-Oklahoma border region, and the total number decreases as most minor cycles pinch out northward into Iowa, several intermediate cycles disappear eastward into Illinois, and only some of the major cycles extend farther into the shelfward Appalachian Basin. Cycles of all scales can be grouped around each major cyclothem to produce a succession of ~400-k.y.- long groupings, which can be used to calibrate the Pennsylvanian time scale with the few radiometric dates that are available. Conodont-based biostratigraphic correlation of the Midcontinent cyclothems with those of the Illinois and Appalachian Basins and the north Texas shelf and Paradox Basin in Utah has been extended to the cyclothems now recognized on the Russian Platform and in the Donets Basin of Eastern Europe. This biostratigraphic zonation of the Pennsylvanian pantropical zone provides the framework for incorporating the radiometric dates now becoming available from fossiliferous marine successions that contain volcanics into the named units of the Pennsylvanian time scale. This framework ultimately should allow the Gondwana succession with entirely endemic biotas to be correlated more accurately with the pantropical zone. The current lack of apparent correlation of times of major cyclothem formation in the pantropical zone with widespread glacial deposits on Gondwana relates to the fact that only large-scale withdrawals of the sea off of the tropical shelves would correlate with widespread glacial deposits, whereas a succession of major cyclothems that are separated by regressions that only approached the edge of the shelf would appear to be simply a long-term interglacial episode in most of Gondwana.
Interpreted paleogeography of northern (Euramerican) end of Protopangea during late middle to late Pennsylvanian time, showing land (dots), maximum extent of epicontinental seas at highstand (lines), and minimum extent at lowstand (blank), slightly modifi ed from Heckel (2002); reproduced with permission from the Canadian Society of Petroleum Geologists. Slightly earlier marine basins existed from central Britain through Germany into Poland. A.B.—Arkoma-Anadarko Basins; Ap.B.—Appalachian Basin; C.Alps—Carnic Alps; C.B.—Cantabrian Basin; D.B.—Donets Basin; IL—Illinois; KS—Kansas; M.B.—Midland Basin; MOS.—Moscow region; OH—Ohio; S.B.—Sydney Basin; Spbgn.—Spitsbergen; Sv.B.—Sverdrup Basin; UT—Utah.
Correlation of North American and Western European regional stages of Pennsylvanian subsystem with global stages named from Russia (from Heckel and Clayton, 2006). Radiometric dates on global time scale for stage boundaries are estimates from Gradstein et al. (2004), with precise U-Pb zircon dates (marked with asterisks) shown for correlated volcanic strata in southern Urals, from Ramezani et al. (2007)* and V. Davydov (2005, personal commun.)**. Estimated dates on North American time scale are from cyclothem calibration based on these dates, updated from Heckel (2006; see Fig. 5) via correlation with eastern Europe. Dashed lines show boundaries of dominantly terrestrial units [t] in northwestern Europe that cannot be correlated accurately with biostratigraphic framework of pantropical marine succession. Labeled boxes and thick solid lines in right column show more intense glacial episodes based on greater sea-level drop in Midcontinent (from Fig. 3). Vertical dimen sion is not uniform as to time scale. C-P-Carboniferous-Permian; M-K-Moscovian-Kasimovian.
