CECIL, C. BLAINE, N. TERENCE EDGAR, THOMAS H. AHLBRANDT, JERRY L. CLAYTON, U.S. Geological Survey, Reston, VA and Denver, CO

ABSTRACT: Paleoclimates and the Origin of Carbonaceous Strata in the Pennsylvanian System of the U.S.A.

Long- and short-term paleoclimate change was a primary control on chemical and mechanical weathering, organic productivity sediment transport to both epicontinental and continental margin depocenters and source rock and reservoir formation during the Pennsylvanian in both the eastern and western United States. Pennsylvanian climate change in the United States was primarily governed by continental movement across latitudes, orbital forcing, and orographic and paleoceanographic changes. The climatic control on stratigraphy is, therefore, recorded both temporally and spatially.

Because of its more equatorial position, the eastern U.S. was relatively wet in the Pennsylvanian as compared to the western U.S. In the east, coal beds and coeval chemically weathered upland paleosols (Ultisols) record the more pluvial periods of climate cycles when increased terrestrial organic productivity restricted erosion and, hence, restricted siliciclastic and dissolved load input from fluvial systems. Drier and more seasonal parts of climate cycles resulted in increased siliciclastic flux to terrestrial and marine depocenters. Pennsylvanian source rocks in the eastern U.S. are gas prone due to the climatically induced high input of terrestrial organic matter.

The western U.S. was located in the dry tropics and the long-term dry climate is recorded by sand seas, marine carbonates, and evaporites. Pluvial periods (1) stabilized dune fields through increased terrestrial organic productivity, (2) increased fluvial siliciclastic flux, and (3) affected circulation in epeiric seas where oil-prone, black shale source rocks were deposited.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.