Relationships of paleosol maturity to autocyclic and allocyclic controls on sediment accumulation rates, Willwood Formation (Lower Eocene), Wyoming, U. S. A

A developmental sequence of spodosols, ranging from the minimum levels of soil organization through very mature soils, has been reconstructed from lower Eocene paleosol profiles throughout the Willwood Formation of the Bighorn Basin, Wyoming. In general, the most mature soils formed on the distal parts of flood plains where alluvial topography was essentially featureless and sediment accumulation rates were relatively low. Less mature soils characterize areas of greater relative sediment accumulation on and proximal to alluvial ridges, in other areas proximal to stream channels, and (in some parts of the section) on the distal parts of flood plains that underwent relatively rapid alluviation. Vertical sequences of multistorey Willwood paleosols are bounded above and below by channel sandstones. Between these sands, pedogenic maturity of the multistorey paleosols generally increases upwards, reflecting an overall decrease in sediment accumulation rate as channel courses shifted (avulsed) away from these areas of the ancient flood plain. These small scale cyclic changes in soil maturity are ascribed to autocyclic fluvial mechanism. Paleosols in the northern part of the Bighorn Basin are typically less mature than coeval sequences in the southern part of the basin, reflecting greater sediment accumulation rates, which correspond to a thicker Willwood sectionmore » in the north. This difference is attributed to areally differing rates of basin subsidence. Paleosols are therefore useful in determining stratigraphically controlled changes in relative rates of sediment accumulation in ancient alluvial sequences.« less