Pennsylvanian Paleogeography of the Western United States

Two strongly contrasting sedimentary and tectonic settings greatly influenced the Pennsylvanian depositional patterns in what is now western North America. The eastern part of this large region was the western Paleozoic miogeocline of the North American craton; the western part is comprised of a number of accreted terranes, some of which include oceanic deposits of Pennsylvanian age. The Pennsylvanian Period saw the continuation of the collision of Gondwana with Euramerica and the resulting deformation of the Hercynian-AppalachianOuachita belt into a orogenic highland. The Nort h America craton was deformed because it and South America and Africa did not fit together well and stresses from this collision were unequally applied. Futhermore, clockwise rotation resulted in the collision progressing from east to west. Many preexisting structures on the craton helpe d transmit the adjustments that were required by the irregular margins of these colliding cratons, particularly faultbounded uplifts and basins, which were reactivated as transpressional and transtensional features, and numerous linearly persistent fault zones. Strong and rapid tectonic activity, thick deposition in local sedimentary basins, and associated persisten t uplifts are characteristic features of the Pennsylvanian of the western craton of North America in contrast to the relative tectonic quiesence during most of the early and middle Paleozoic. Pennsylvanian deposition on the craton locally included: thick continental red beds (3,000 m or more); evaporites; eolian sands; platform and shelfal carbonates, sandstones, and siltstones; slope and basinal debris beds; and euxinic black shales. The wide range of types and thicknesses of these deposits reflects the great differences in the location, size, basin configuration, sediment sources for the various basins, and the duration and extent of adjacent uplifts. At times during the Pennsylvanian, depositional facies in many basins changed abruptly as new, large scale movements activated additional fault systems and relaxed others. In these basins, depositional cyclicity, which is a well-developed feature on stable parts of -the craton, commonly is masked by a strong tectonic overprint. As North America continued westward, its western margin became torn into uplifts and basins as it pushed against ocean floor deposits of eastern Panthalassa.