CONTROLS ON VERTICAL COLOR CHANGES IN ALLUVIAL PALEOSOLS OF THE MORRISON FORMATION, COYOTE BASIN, WYOMING

The Morrison Formation in the Coyote Basin, Wyoming can be subdivided into three distinct intervals each ~10m thick. Two intervals consist of moderately developed red paleosols; the third is a greenish gray heterolithic unit that is bounded by the 2 red paleosol units. The red paleosols, which are developed on siltstones, have hues of 5R and 10R and redoximorphic features including redox depletions and concentrations associated with root channels. They are moderately developed paleosols based on the presence of root traces, abundant pedogenic carbonate nodules, large slickensides, and bioturbation features. Heterolithic units consist of ribbon and thin (< 1 m thick) sheet sandstones surrounded by greenish gray (5GY) finer-grained deposits. The finer-grained deposits have few root traces and small slickensides, indicating that the deposits underwent pedogenesis; however, development was weak compared to the red paleosols.

Red soil colors generally result from hematite, which forms under relatively well-drained conditions. The redoximorphic features of the red paleosols indicate seasonal surface water gleying of the paleosols. Green paleosol colors indicate reducing conditions that resulted from a poorly drained setting. Vertical changes in paleosol color; such as observed in the Coyote Basin, are commonly ascribed to changes in climate. Those in the Coyote Basin, however, are attributed to a process intrinsic to the fluvial system – channel avulsion – because the paleosol colors appear to be associated with two different modes of deposition. The heterolithic nature and weak paleosols of the green interval suggests it formed as avulsion deposits. Because the red intervals lack sandstones, show stronger pedogenesis, and are interbedded with the heterolithic deposits, they are interpreted as overbank deposits. The better-drained conditions of the red paleosols are ascribed to deposition of the parent material on the floodplain at some distance from the active channel. The avulsion deposits were probably more poorly drained because they formed closer to the channel as splay systems prograded onto low areas of the floodplain as the channel underwent avulsion. This study highlights the importance of analyzing the depositional framework when interpreting vertical changes in paleosol successions.