BASAL PENNSYLVANIAN CLASTIC SEDIMENTS ON FLANK OF NEMAHA UPLIFT, EASTERN NEBRASKA-KANSAS BORDER, USA

Basal Pennsylvanian clastic strata (BPC) near the eastern Nebraska-Kansas border on the eastern flank of the Nemaha Uplift (NU) contain coarse, feldspathic sandstones and conglomerates derived from Proterozoic granitic basement in the NU. BPC are sedimentologically distinct from overlying limestone-rich cyclothems and older, shale-rich, Pennsylvanian clastic sediments in the adjacent Forest City Basin. Three well cores penetrating the BPC (KC81-1 at Nebraska Conservation and Survey Division; Kansas Geological Survey Emery Trust #1 and Heinen #1) capture multiple (perhaps 20-25 in one case), well-defined, fining-upward (FU) packages (0.2-5.0 m thick). These packages show upward transitions from coarse, poorly-sorted, clay-cemented, feldspathic sandstones into shales and/or mudstones with paleosols. The basal contacts of FU packages are usually sharply erosional, and the upper contacts are sometimes marked by filled desiccation cracks up to 40 cm deep. In several cases, the lowermost sandstone within a FU package contains mm-scale mud laminae, which effect a distinct laminite-like appearance. In KC81-1 and Heinen #1, BPC directly overlie slightly weathered igneous basement rock.

Several hundred meters of Ordovician-Mississippian sediments, as well as weathered basement rock, were stripped from the NU to yield BPC sediments, which were then trapped in small fault-block basins at the NU margin. Paleosols and desiccation cracks in FU packages indicate a strong subaerial overprint on sediments, and it is likely that BPC were deposited on alluvial fans or, possibly, fan deltas. Gradational contacts between the BPC and overlying Missourian strata indicate that the deposition of BPC closely preceded local cyclothem deposition as sea level rose over the NU during the Late Pennsylvanian. Because of their context relative to a major uplift with complex faulting, BPC may provide new information about the relative roles of tectonics and eustatic sea-level cycles in local sedimentation, as well as the comparative importance of autocyclic and allocyclic depositional mechanisms.