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Department of Geology, University of Iowa, Iowa City, IA 52242, USA
At least 20 Middle/Upper Pennsylvanian (Westphalian D/Stephanian A, B) major marine cyclothems, each consisting of a thin transgressive limestone, thin offshore dark phosphatic shale, and thick regressive limestone member, extend along outcrop for 600 km from the northern shelf region of Iowa, Missouri and Kansas, to the basinal region of central Oklahoma. Each is correlated lithostratigraphically throughout a gridwork of long cores in the northern region beneath Pleistocene cover, and in good exposures southward. This correlation is confirmed biostratigraphically, along the entire outcrop belt using a combination of conodonts, fusulinids and ammonoids. Most of the cyclothems are separated by thin terrestrial deposits with paleosols, but only rare deltas, for about the northern half of the outcrop distance, which rules out delta shifting as a general control for the vertical alternation of terrestrial and marine deposits across the broad shelf. On the northern shelf, all major cyclothems are traced across the cratonic Forest City basin and over the adjacent Nemaha uplift with little change, which rules out local differential tectonics as a general cause. Presence of Gondwanan glacial deposits at this time, in conjunction with the estimated frequencies of these Pennsylvanian cycles within the Milankovitch band of Earth’s orbital parameters (which control variation in solar heating of the mid-latitudes), indicate that glacial eustasy, rather than distant orogenic movements, was the main control over the cyclothems.
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