Abstract

Extensive, fresh exposures of reddish brown mudstones in the Rock Lake Shale Member in a quarry in southeastern Nebraska show pedogenic features (calcite glaebules, blocky spar crystallaria, peds, silt and clay- or micrite-filled pedotubules, slickensides, mottling, and overall horizonation) which indicate protracted subaerial exposure under increasingly arid conditions. The Rock Lake paleosol is bounded above and below by marine cyclothem deposits. Probable events represented by the paleosol are 1) clay and silt illuviation under humid conditions, 2) subsequent carbonate precipitation under progressively drier conditions, and 3) gleying of the A horizon during a marine transgression. The Rock Lake paleosol developed on a flat and laterally extensive, semiarid to arid plain during a low stand of sea level. Examination of the Rock Lake at other localities (well cores and exposures) in nearby counties shows a distinct eastward facies change toward lower topography, earlier marine transgression, and arrested pedogenesis, which is consistent with transect direction from the Nemaha Uplift to the Forest City Basin.

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