Abstract

Past controversy about the tectonic or eustatic origin of Pennsylvanian cyclothems is resolved by regional basin-subsidence analysis, global paleoclimate, paleogeography, and plate-tectonic evolution. Differences between marine carbonate (Kansas-type) cyclothems of the northern midcontinent and nonmarine clastic (Appalachian-type) cyclothems of the eastern United States are controlled by laterally changing flexural deformation during plate accretion into a supercontinent, coupled with superposed glacial eustasy. Kansas-type cyclothems accumulated on a relatively stable platform affected only moderately by collision tectonics of North American margins; these cycles are eustatic, characterized by a periodicity comparable to Milankovitch orbital parameters. Appalachian-type cyclothems accumulated in response to episodic thrust loading during plate collisions that developed a series of flexurally deformed, wide and shallow, resurgent foreland basins. This resurgent foreland basin evolution during Paleozoic time increased crustal rigidity in the Appalachians, thus yoking the Appalachian and Illinois basins during Pennsylvanian time. With each thrust load, foreland basin deepening favored transgressive cyclothem facies, which changed to a regressive facies because sediment yield increased as denudation rates increased off uplifted orogens. Illinois-type cyclothems are intermediate between the two end-member processes of flexural tectonics and glacial eustasy characterizing Appalachian-type and Kansas-type cyclothems, respectively. Thus, the Pennsylvanian coal-bearing cyclothems of North America owe their origin to the remarkable coincidence of supercontinent development, concomitant glaciation and eustatic sea-level change, and associated episodic thrust loading and foreland basin subsidence of small magnitude on progressively more rigid crust.

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