Paleosols spanning the Cretaceous-Paleogene transition, eastern Montana and western North Dakota
Paleosols occur in exposures of the latest Cretaceous Hell Creek and Paleocene Tullock (= Ludlow) Formations in Montana and western North Dakota. These units are composed mainly of interbedded siltstones and sandstones of meandering fluvial origin. The paleosols indicate that changes in ancient soil development occurred concomitantly with the better-known faunal transition. In order of decreasing abundance, silty soils, sandy soils, organic soils, and volcanic soils are preserved. Pedogenic features present in these soils include roots, microscopic segregations of amorphous material, birefringence fabric, and soil structures. The features suggest that throughout the Cretaceous-Paleogene transition pedogenic processes in the region produced immature profiles, an observation consistent with the unstable, fluvial setting in which the ancient soils formed. Gleization was a dominant process in this setting, and podzolization modified some sandy soils. The association of the features enables recognition of O, A/E, Btg, Bhs, Bg, BC, Cg, and C horizons. During middle Hell Creek time, soils formed in a poorly drained setting that was only stable enough to permit incipient pedogenesis. The bulk of the pedogenesis occurred in levee and flood-plain deposits; soils also occurred on point-bar and crevasse-splay deposits. In topographically depressed regions, organic accumulations formed with minimal soil development. Matted plant debris is the product of this environment. Gley features and segregations of iron oxides around voids suggest fluctuation of the water table. By latest Hell Creek time, the mean level of the water table rose, and in the lowest Tullock (Ludlow) exposures, extensive ponded deposits are preserved. Vegetation accumulated at a rate sufficient for coal formation. The amount of fluctuation apparently was reduced, and pedogenesis was further inhibited, as indicated by the virtual absence of illuviated clays in the sediments.