ABSTRACT

A widespread paleosol in the Galesburg Formation (Kansas City Group, Missourian) in the northern Midcontinent Upper Pennsylvanian cyclothem (MUPC) outcrop belt exhibits prominent features, including repeating sets of synformal slickensides and mukkara or "chimney" structures, which characterize it as a paleo-Vertisol. The Galesburg paleosol has pervasive low-chroma grayish to greenish coloration, an organic-matter profile, and geochemical characteristics with regard to S, Fe₂O₃, MnO, SiO₂, and Al₂O₃ that indicate wetter soil conditions than those indicated by equally widespread high-chroma paleosols higher in the local stratigraphic succession (particularly a high-chroma paleo-Vertisol in the upper Lawrence Formation). The predominance of low-chroma (i.e., wetter) paleosols throughout the Kansas City Group in the northern Forest City Basin (southeastern Nebraska and southwestern Iowa), as exemplified by the Galesburg paleosol, compared to the greater frequency of high-chroma (i.e., drier) paleosols in overlying Pennsylvanian (especially Virgilian) strata, implicates decreasing rainfall in the interior of North America during the late Missourian. The variability in rainfall through this Missourian-Virgilian shift need not have exceeded the range of rainfall (400-1400 mm/a) associated with modern soil analogs in subtropical to warm-temperate North America. Thus, there is an indication of climate change on a longer-term time scale ( 10⁵-10⁶ a) than the intra- or inter-eustatic-cycle climate change proposed by other authors for late Paleozoic cyclothems in the North American interior.

Regional differences in the nature of transgressive marine strata directly above the Galesburg further suggest: (1) that important information relevant to reconstruction of the sea-level curve is probably missing from the interval between regressive paleosols and subsequent regressive limestones in MUPCs, and (2) that there were probably appreciable differences in the overall mechanism of deposition of MUPC transgressive shales and black "core" shales within any particular interval of MUPC transgressive deposition.