Although there has been considerable study of Pleistocene and post-Pleistocene soils as stratigraphic markers and environmental indicators, much less attention has been given to older paleosols. In nonmarine sedimentary beds, periods of nondeposition or erosion can be characterized as intermittent, areally extensive, and of variable duration. Under favorable conditions of weathering, soils may develop on exposed rocks and subsequently be preserved by deposition and burial.

The Brule Formation (Oligocene) in northwest Nebraska consists predominantly of fluvial and eolian rocks. Based on paleontologic evaluations, several stratigraphic levels have been suggested as being soils or soil complexes. Of these, two stratigraphic zones were studied as possible paleosols. One of these, the lower ash bed of the Whitney Member, Brule Formation, has physical characteristics similar to those of a soil formed in a semiarid climate.

The lower ash bed is indicated to be a paleosol on the basis of trends in organic matter, particle-size distribution, and calcium carbonate content. The vertical profile shows a zone of higher organic matter in the upper part, a zone of high clay content below, and high calcium carbonate content in the subsoil. The White marker bed of the Whitney Member of the Brule Formation was also studied. The amounts of organic matter, clay, and calcium carbonate do not indicate a simple paleosol origin for the White marker bed.

Comparison of the lower ash paleosol to modern soils provides a basis for additional interpretations. Depth and thickness of zones of accumulation suggest that the paleosol was developed under a grassland cover and are characteristic of Chestnut-type soil. The climatic conditions that typically produce these characteristics include an average annual precipitation of 30 to 45 cm.

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