Paleoclimate and the Origin of Middle Pennsylvanian Cyclothems (Fourth-Order Sequences) of North America
C. Blaine Cecil, Frank T. Dulong, Ronald R. West, N. Terence Edgar, 2002. "Paleoclimate and the Origin of Middle Pennsylvanian Cyclothems (Fourth-Order Sequences) of North America", Sequence Stratigraphic Models for Exploration and Production: Evolving Methodology, Emerging Models and Application Histories, John M. Armentrout, Norman C. Rosen
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Transcontinental correlations of a Middle Pennsylvanian fourth-order sequence provided evidence for the relative importance of allocyclic controls on the formation of Pennsylvanian cyclothems. These correlations (and related studies) further indicated that eustasy was the primary control on accommodation space in most basins, whereas tectonic subsidence provided additional accommodation space in a few basins. Temporal and spatial variations in climate, however, were the primary controls on physical and chemical sedimentology. The climate model developed herein suggested 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 and 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 low pressure rainy belt (doldrums) developed in the equatorial region of Pangea during lowstands. During interglacials, the doldrums belt (low pressure belt) degenerated and was replaced by seasonal swings in the ITCZ, in response to seasonal sensible heating of the atmosphere over both northern and southern hemisphere land masses. As a result, the climate in low latitudes changed from relatively wet conditions during glacial intervals to drier and more seasonal conditions during interglacial periods.
Paleoclimates in the eastern United States during glacial intervals are indicated by the following: (1) intense chemical weathering of paleosols, (2) low fluvial sediment supply, (3) peat formation (now coal) during lowstands in response to a permanent low-pressure rainy belt and wet conditions, (4) deposition of black shale in basin centers during the early stages of transgression in response to low wind speeds and poor wind-driven circulation in epeiric seas prior to significant deterioration of the doldrums belt, and (5) transport, deposition, and preservation of eolian sediments (in basin margins) following a period of weathering of lowstand exposure surfaces (western United States). Paleoclimates during interglacial intervals are indicated by an influx of fluvial sediments as the doldrums belt disappeared (eastern United States), and deposition of marine limestone west of the Appalachian basin 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) have controlled sedimentation in cratonic depositional environments as sea level rose and fell. Although tectonics and eustasy control accommodation space, paleoclimate cycles (coincident with eustasy) control the lithostratigraphy of upper Middle Pennsylvanian cyclothems at any given paleo-latitude in the tropical regions of Pangea. Furthermore, the present study negates “deep water” models for the origin of Middle Pennsylvanian black shale and autocyclic models (delta plain, back barrier, or fluvial depositional environments) for peat formation as precursors to Pennsylvanian commercial coal deposits.