Paleoclimate and the Origin of Middle Pennsylvanian 4th Order Sequences (Cyclothems) USA

C. Blaine Cecil, Frank T. Dulong, Ronald R. West, and N. Terence Edgar: MS 956, USGS, Reston, VA

Abstract

Temporal and spatial changes in paleoclimate were primary controls on variation in sediment supply, both siliciclastic and chemical, in depositional settings across the North American craton during the Pennsylvanian. Tectonic and eustatic processes, as well as climatically induced changes in sediment supply, controlled accommodation space and sequence stratigraphy within depositional settings. Interbasinal correlations of lithologies sensitive to climate, such as coeval paleosols, provide continental scale records of climatic and eustatic conditions.

Pennsylvanian bio- and lithostratigraphy are indicative of climate change at time scales that range from long-term (tens of millions of years) as Pangea formed and North America moved northward through the paleoequator, to intermediate-term hundred thousand year cycles (controlled by orbital forcing), to very short-term events (perhaps analogous to El Niño events).

The long-term climate of eastern basins of the United States was generally wetter than western basins because of the proximity to the humid tropics. Intermediate-term climate cycles (variation in rainfall) were coeval with sea level cycles (fourth-order sequences). In the east, pluvial parts of climate cycles occurred during low-stand events and were recorded by intense chemical weathering (paleosol sequence boundaries), high terrestrial organic productivity, restricted erosion, and sediment starvation. These conditions resulted in highly leached mineral paleosols (ultisols) and coal beds (histosols) of interbasinal extent. Drier parts of climate cycles in the east occurred during high stands of sea level when maximum erosion and siliciclastic transport resulted in coarsening upward sequences.

Pluvial periods in the western basins are indicated by weathering of exposure surfaces (sequence boundaries). Weathering commonly is followed deposition of loessite that is indicative of climatic drying. A subsequent rise in sea level results in black shale, marine carbonates, and evaporites in basin centers while eolian materials or fan-deltas are deposited along basin margins.

Tectonism controlled basin development and glacial eustasy controlled sea level cycles. Climate, however, was the primary control on sediment supply and lithostratigraphy.