SIGNIFICANCE OF MULTIPLE DISCONTINUITY SURFACES WITHIN LATEST PENNSYLVANIAN (VIRGILIAN) CYCLOTHEMS OF KANSAS

The stratigraphic interval examined includes three late Virgilian cyclothems in northeastern Kansas. These 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. This interval contains numerous and closely-spaced discontinuity surfaces, but determining their stratigraphic significance poses a challenge.

Sequence boundaries can be confused with more localized channel scour produced by lateral channel migration. Internally complex incised valley fill deposits that downcut through one or more underlying cycles unambiguously mark sequence boundaries. These are locally present within the clastic members of all three cyclothems. However, in the interfluves sequence boundaries are difficult to identify. We interpret the most well-developed paleosol profile within a cyclothem as equivalent to the sequence boundary. These polygenetic paleosols show evidence of forming first under drier or better-drained conditions during lowstand, and subsequently under more saturated conditions that likely reflect rising baselevel associated with overlying flooding surfaces.

Transgressive surfaces and maximum marine flooding surfaces are more difficult to identify than parasequence-bounding flooding surfaces. Some thin marine carbonate units are seen to pinch out over short distances leaving little indication of their previous presence. However, within one cyclothemic sequence, we have identified a subtle layer of bored hiatus concretions only a few centimeters thick that marks the stratigraphic position of a marine carbonate and the maximum flooding surface. Close examination of similar clastic cycles may reveal that such cryptic unconformities are more common than previously recognized, and may provide keys to identifying sequence tract boundaries within cyclothemic sequences.