ABSTRACT

Paleosols in the upper Lawrence Formation and in the Snyderville Shale Member (Virgilian) extend over an estimated area of over 14,000 km² in Nebraska, Iowa, Missouri, and Kansas. These paleosols, both within cyclothems, are: (1) critical indicators of nearly basin wide emergence (peak regression of the Midcontinent seaway) and geomorphic stability, and (2) useful stratigraphic markers. Both paleosols appear to be analogous to modern Vertisols: they have nested, synformal-antiformal sets ( 4-10 m wide) of very large, intersecting slickensides. Also, some profiles of the upper Lawrence paleosol also have filled cracks extending to depths of 100+ cm. There are, however, significant differences between the paleosols: high-chrom coloration, strong preserved soil structure, and small iron oxide nodules (upper Lawrence) versus dominantly low-chroma coloration, weak to very weak preserved soil structure, and pyrite segregations (Snyderville). After a period of widespread subaerial exposure, each paleosol was drowned and slightly eroded by a marine transgression.

The upper Lawrence shows a single paleosol across the study area. The paleosol is thickest and best-developed shelfward (northward) and shows topohydrosequence variation at two basinward localities in northeastern Kansas. Stratigraphic trends suggest an appreciable time differential in the south-to-north migration of the Toronto transgression. The Snyderville shows two welded paleosols in southeastern Nebraska and a single one elsewhere in the study area. Elsewhere in the Snyderville, there is local evidence for lowstand incision of streams and small lows that underwent little or no subaerial exposure. Snyderville paleogeography and pedogenesis, however, were markedly different from upper Lawrence paleogeography and pedogenesis. Contrasts between the two paleosols are likely to be rel ted to intercycle changes in geomorphic conditions (driven by patterns of sedimentation, eustasy, and tectonics) and climate.