PALEOCLIMATE CONTRASTS BETWEEN THE LATEST PENNSYLVANIAN (VIRGILIAN) CYCLOTHEMS AND EARLIEST PERMIAN (WOLFCAMPIAN) CYCLOTHEMS OF KANSAS

The stratigraphic intervals examined include late Virgilian cyclothems in Wabaunsee County Kansas, belonging to the Wabaunsee Group, and earliest Permian (Wolfcampian) cyclothems in Riley County Kansas, belonging to the Council Grove Group.

The late Virgilian cyclothems are characterized by cross-bedded channel sands, laminated and lenticular-bedded sandy and silty shales, stacked paleosol profiles, thin coal seams, and thin silty marine carbonates. Internally complex incised valley-fill deposits that downcut through one or more underlying cycles are locally present within the clastic members of the cyclothems. Below the very thin coaly seams are saturated “underclays.” Weakly to moderately developed paleosol profiles with abundant pyrite and high organic carbon contents indicate saturated acid-sulfate soils. Also common within the section are paleosol profiles with vertic structures such as pedogenic slickensides and gilgae indicating more seasonal wet/dry monsoonal conditions. At the drier end of the spectrum, are paleosols with carbonate nodules produced during well drained conditions in a sub-humid climate.

By contrast, the clastic intervals of the Wolfcampian cyclothems lack incised valleys, and appear to be devoid of alluvial channel facies. Rather, they are represented by variegated mudstones and siltstones overprinted by stacked paleosols of varying maturity. The stacked paleosols have a drier climatic range than those of the late Virgilian being characterized by monsoonal vertic profiles to semi-arid calcic profiles and salt-influenced natric horizons. Other fine clastic units appear to be deposited in sabkha-like settings with abundant pseudomorphs after gypsum, boxwork structures, and tepee structures. In addition to these nearshore facies, there are structureless and rooted siltstone beds that likely accumulated as eolian silts.

These sedimentologic and paleosol observations indicate a substantial transition from a wetter range of climatic conditions to an increasingly drier range across the Pennsylvanian/Permian boundary.