PRELIMINARY REPORT ON SOME PRE-LATE PENNSYLVANIAN SUPRA-BASEMENT WEATHERING PROFILES IN SOUTHEASTERN NEBRASKA

Deep well cores from southeastern Nebraska capture pre-Missourian weathering/regolith profiles underneath Paleozoic sedimentary cover. These profiles formed on igneous basement rocks of different ages (Late Proterozoic-Cambrian) and types. Weathering profiles differ significantly in characteristics across the western margin of the Forest City Basin and onto the Nemaha Uplift (NU), which had prominent relief prior to its burial during the Missourian. Core St-3 (near Steinauer, Pawnee Co.) has a thin (30 cm), clayey, basal soil horizon and at least 10 m of moderately-weathered saprolite grading downward into weakly weathered gabbro (dated to 1.2 Ga). This profile has an erosional upper contact with overlying Pennsylvanian cyclothems. Vermiculite, illite, and chlorite dominate in the saprolite from St-3, and kaolinite is a minor constituent. In nearby core St-2, Ordovician sands overlie weakly-weathered gabbro. Cores from Elk Creek, Johnson Co. (EC), which were drilled in the 1970s and 1980s to assay an REE-bearing carbonatite, show laterally-variable paleosols and/or intensely-weathered saprolites atop that intrusion. Weathering profiles are absent in some of the EC cores, having been eroded prior to or during marine transgression over the NU in the Missourian. In the EC profiles yet sampled, R3 illite-smectite and chlorite dominate. Some EC weathering profiles grade upward into basal Pennsylvanian sediments, unlike the St profiles. This gradation includes transitions into pebbly mudstones (colluvial regolith?) and pervasively slickensided claystones (paleo-Vertisols). Gossan-like specular iron oxide "caps" appear in the upper parts of at least two EC profiles.

Results from this ongoing study suggest that supra-basement weathering profiles in the study area represent a complex history (perhaps exceeding 150 my) of weathering, erosion, and burial controlled primarily by uplift and sea-level change. The dominance of illite, illite-smectite, chlorite, and vermiculite minerals (as opposed to kaolin minerals and Al hydroxides) in deeply and pervasively weathered materials is noteworthy. This dominance may reflect weathering regimes without exact modern analogs, as well as later diagenetic processes (e.g., illitization of smectite).