Paleoclimate Controls on Pennsylvanian Cyclostratigraphy

Cyclic deposition of Pennsylvanian strata in the United States (US) is generally attributed to eustatic changes in sea level driven by repetitive fluctuations of ice volume in the southern hemisphere of Pangea. Although changes in ice volume may account for eustasy, global paleoclimate cycles appear to best explain temporal changes in the lithostratigraphy of Pennsylvanian cycles in low paleolatitudes of Pangea. Low paleolatitude Pennsylvanian paleoclimate cycles are indicated by the following repetitive stratigraphic successions: 1) continental-scale mineral paleosols and low sediment supply all indicate a relatively humid climate during low stands, 2) histosol formation (coal) in equatorial Pangea (eastern and central US) indicate a wet doldrums belt during low stands, 3) tidal to intertidal deposits overlying paleosols are indicative of the onset of sea level rise, 4) subsequent shallow water black shale deposits in basin centers suggest minimal circulation of epeiric seas and very weak surface winds during the early to mid stages of transgression, 5) increases in siliciclastic sediment supply and the onset of prograding deltas in the east and eolian deposition in the western U. S. as sea level rose suggests climate drying, and increasing wind speeds, and 6) maximum fluvial influx in equatorial regions (eastern US), open marine limestones and evaporites (central and western US) suggest maximum seasonality, maximum dryness, increased wind speeds, and maximum wind-driven circulation in epeiric seas during high stands. The temporal changes within Pennsylvanian depositional cycles in North America appear to be best explained by a global paleoclimate model as follows: 1) the low latitudes of Pangea were wettest during glacial low stands because of a relatively stable low-pressure rainy belt (doldrums) and a stable intertropical convergence zone (ITCZ) in response to high pressure over the ice caps and nearly stationary polar fronts that confined the ITCZ to low latitudes, 2) as the southern hemisphere ice melted and sea level rose, the doldrums low pressure belt gave way to seasonal swings of the ITCZ in response to seasonal heating of air masses over both northern and southern hemisphere land masses, 3) high stands were coeval with maximum climate drying in the low paleolatitudes of Pangea because seasonal landmass heating in both hemispheres intensified north-south swings of the ITCZ during interglacials; rainfall was limited to two relatively weak rainy seasons coincident with the passage of the ITCZ.