Lower Eocene alluvial paleosols: Pedogenic development, stratigraphic relationships, and paleosol/landscape associations

Two kinds of cumulative floodplain paleosols, red and grey paleosols, formed on overbank deposits of the Willwood Formation in the Sand Coulee area of the Bighorn Basin, Wyoming. Although both kinds experienced down-profile iron movement, due to seasonal saturation of a clay-rich A horizon, the red paleosols were better drained and their B horizons were rubified. Various redoximorphic features indicate that the grey paleosols were poorly drained. The red paleosols show progressive increases in the degree of profile development away from an associated channel sandstone, a paleosol/landscape relationship termed pedofacies by Bown and Kraus (1987). Although the grey paleosols show relatively systematic changes in hydromorphy, consistent pedofacies changes were not recognized. Furthermore, no lateral relationship between the red and grey paleosols was observed. These features suggest that, because of the retarding effects of poor drainage on soil weathering, poorly drained soils are not amenable to pedofacies modelling and that the landscape associations of well drained and poorly drained soils may be difficult to document without unusually extensive exposures. Results of this study also show that the pedofacies model is limited by sediment accumulation rates and the kind of fine-grained facies on which paleosols developed. Although readily observable in Sand Coulee, pedofacies are difficult to recognize in the Elk Creek area, where accumulation rates were approximately half as rapid as in Sand Coulee. The attainment of steady-state conditions is believed to have obscured pedofacies in the Elk Creek area. Finally, pedofacies are only developed on true overbank deposits, which, in the Willwood Formation and probably many other ancient alluvial sequences, make up only a fraction of the fine-grained deposits. Only immature soils formed on the remainder of the fine-grained facies because they were deposited very rapidly, as a result of channel avulsion.