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 Coul ee 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 incr eases in the degree of profile development away from an associated cha nnel sandstone, a paleosol/landscape relationship termed pedofacies by Bown and Kraus (1987). Although the grey paleosols show relatively sy stematic changes in hydromorphy, consistent pedofacies changes were no t 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 drai ned soils are not amenable to pedofacies modelling and that the landsc ape associations of well drained and poorly drained soils may be diffi cult to document without unusually extensive exposures. Results of thi s study also show that the pedofacies model is limited by sediment acc umulation 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 rate s were approximately half as rapid as in Sand Coulee. The attainment o f steady-state conditions is believed to have obscured pedofacies in t he Elk Creek area. Finally, pedofacies are only developed on true over bank deposits, which, in the Willwood Formation and probably many othe r ancient alluvial sequences, make up only a fraction of the fine-grai ned deposits. Only immature soils formed on the remainder of the fine- grained facies because they were deposited very rapidly, as a result o f channel avulsion.