Climate Controls on the Stratigraphy of a Middle Pennsylvanian Cyclothem in North America
C. Blaine Cecil, Frank T. Dulong, Ronald R. West, Robert Stamm, Bruce Wardlaw, N. Terence Edgar, 2003. "Climate Controls on the Stratigraphy of a Middle Pennsylvanian Cyclothem in North America", Climate Controls on Stratigraphy, C. Blaine Cecil, N. Terence Edgar
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Continental-scale correlations of a Middle Pennsylvanian fourth-order sequence have provided evidence for the relative importance of allocyclic controls on the formation of Pennsylvanian cyclothems. These correlations (and related studies) indicate that eustatic changes in sealevel were the primary control on accommodation space in most basins. Tectonic subsidence was a secondary control and provided additional accommodation space in a few basins. Temporal and spatial variations in climate, however, were the primary controls on physical and chemical sedimentology. Interpretations of changes in weathering and fluvial and eolian sediment supply suggest that the climate was wetter during lowstands than during highstands and that the physical and chemical oceanography of epeiric seas responded to changing patterns in atmospheric pressure regimes as sea level rose and fell. In addition, the climate was wetter in the eastern part of the tropical regions of what is now North America relative to the west. Temporal and spatial changes in paleoclimate, therefore, appear to be the primary control on lithostratigraphy.
Previous studies have suggested that repetitive fluctuations in the sizes of continental ice sheets resulted in repetitive eustatic changes in sea level during the Middle Pennsylvanian (e.g., Wanless and Shepard, 1936). Such changes in ice volume and sea level must have been in response to global climate change. The climate model developed herein suggests that repetitive changes in rainfall patterns and surface winds at low latitudes were coincident with the glacial and interglacial intervals. During glacial intervals, a large permanent high pressure cell was associated with a southern-hemisphere ice cap (a nearly stationary polar front). The ice cap minimized annual (summer to winter) thermal variation in the atmosphere (sensible heating) over the southern-hemisphere land mass. As a result, permanent high pressure over the ice cap confined the intertropical convergence zone (ITCZ) to low latitudes, and a permanent low-pressure belt (doldrums) developed in the equatorial region of Pangea. During interglacials, the doldrums belt (low pressure belt) degenerated and was replaced by seasonal swings in the ITCZ in response to seasonal heating of air masses (sensible heating) over both northern-hemisphere and southern-hemisphere land masses. As a result, in low latitudes the climate changed from relatively wet conditions during glacial intervals to drier and more seasonal conditions during interglacial periods. Glacial and interglacial climates are indicated by the following: (1) intense chemical weathering of paleosols, low sediment supply, and peat formation (now coal in the eastern United States) during lowstands in response to a permanent low-pressure rainy belt and wet conditions, (2) deposition of black shale in basin centers during the early stages of transgression in response to low wind speeds and poor wind-driven circulation as the doldrums belt began to deteriorate, (3) transport and deposition of eolian sediments (western United States) in basin margins as sea level continued to rise and the doldrums belt disappeared, and (4) deposition of marine limestone in response to increased wind speeds and wind-driven circulation in epeiric seas coincident with highstands and maximum north–south swings of the ITCZ. All climatic factors (annual rainfall, seasonality of annual rainfall, wind speed, and wind direction) controlled sedimentation in cratonic depositional environments as sea level rose and fell. Although tectonics and eustasy controlled accommodation space, paleoclimate change (coincident with eustatic changes in sea level) controlled the lithostratigraphy of cyclothems at any given paleolatitude in the tropical regions of Pangea. There is no genetic relation between autocyclic delta-plain, back-barrier, or fluvial depositional models and the onset of Pennsylvanian peat formation as a precursor to commercial coal deposits.
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The role of climate as a primary control on stratigraphy is the cornerstone of this volume. The emphasis on climate is in distinct contrast to most previous studies, in which stratigraphic variability has been related to changes in sea level and in tectonic activity. Furthermore, the findings, derived from several years of detailed study of modern and ancient key geologic sections around the world, indicate that traditional depositional models generally do not fully explain the origin of fossil fuels. Although the results of the studies presented in this volume are intended to contribute to the disciplines of sedimentary geology and stratigraphy, the contributors recognize that their results may also contribute to a better understanding of global climate change. The theoretical background of climate control on sediment supply and stratigraphy is presented in the volume. With this background in place, detailed documentation and analysis of climate control on the lithologic variation of a single Middle Pennsylvanian.