OMAHA UNDERGROUND: REVISED STRATIGRAPHY AND SIGNIFICANCE OF UPPER PENNSYLVANIAN STRATA BELOW NEBRASKA'S LARGEST CITY

The Lansing (LG), Kansas City, Pleasanton, and Marmaton (MG) groups are thoroughly characterized in Omaha, Nebraska for the first time using new cores. The Excello, Little Osage, Anna, Hushpuckney, Stark, Quivira, and Eudora shale members are continuous and prominent in gamma-ray logs, thus facilitating correlations. Pre-ca 2000 Nebraska Geological Survey reports miscorrelated the “Farley Limestone,” now identified as Argentine Limestone in new cores, as well as the “Argentine” (now Raytown), “Raytown” (now Cement City), “Drum” (now Cherryvale), “Westerville”/“Sarpy” (now Winterset/Dennis), “Winterset” (now Bethany Falls), “Bethany Falls” (now Hertha), and “Hertha” (now Exline) limestones, plus intervening units. Clastic rocks (mudrocks, subordinate sand- mud heteroliths [e.g., Mine Creek Shale], minor sandstones) dominate the MG–LG below the Bethany Falls Limestone Member.

In the MG, the Altamont Limestone is weathered, argillaceous, and partly nodular; its members cannot be identified. The Lenapah Limestone is very heavily weathered (“nodularized”) and amalgamated into paleosols, or effectively absent. The thin (< 0.5 m) Lost Branch Formation consists of an argillaceous and nodular Cooper Creek Limestone and the underlying, thin, Nuyaka Creek Shale Member, which overlies a slightly eroded paleosol.

Evidence for subaerial exposure (pedogenesis, reddening, deep desiccation cracking) abounds in the MG–LG. Paleosol-dominated mudrock units are: Morgan School, Nowata-Memorial (multiple paleosols), Hepler, upper Shale Hill, Elm Branch, Galesburg, Chanute, Liberty Memorial, Lane, and Vilas shales/formations. Paleosols also appear in the Labette, Cherryvale (Fontana Shale Mbr.) and Nelly Bly shales/formations, and in the Bandera Shale. A thin (< 0.3 m) limestone bed near the base of the Galesburg Shale in one core is completely disrupted by ancient pedoturbation in other cores. Many paleosols exhibit large slickensides like those in modern Vertisols. Several paleosols also exhibit an angular (± 30°) contact between the base of the paleosol and the underlying, weathered limestone, indicating that the contact was pedogenically sheared. Similarly, weathered fragments of underlying limestones exist well up-profile (cm to dm above basal contacts) in the lower parts of many paleosols.