Abstract

Eocene alluvial rocks in the northern Bighorn Basin can be subdivided into three major kinds: fine-grained deposits on which cumulative paleosols developed, lithologically heterogeneous packages associated with immature paleosols, and major sheet sandstone bodies. The heterogeneous packages, which alternate vertically with the cumulative paleosols, are dominated by fine-grained deposits and also contain ribbon sandstone bodies and thin (< 1 m) sheet sandstone bodies. Most ribbon sandstones are less than 3 m thick and are more common in the middle and upper parts of the packages. Cut-and-fill deposits with fine-grained fills are locally important. Because the lithologic packages are readily apparent in the field because of the immature or simple paleosols developed on the fine-grained facies, they are referred to as "simple paleosol packages". The major sheet sandstones were deposited by meandering trunk rivers and locally overlie and truncate the other deposits. Contrary to classical models of meandering streams, only a portion of the fine-grained rocks in the study area were deposited by overbank flooding of a trunk channel. On the basis of their stratigraphic position and few ribbon sandstones, the upper parts of some simple paleosol packages are interpreted as ancient levee deposits associated with a trunk channel. Farther from the trunk channel, overbank floods deposited alluvium on which the cumulative paleosols formed. The greater parts of the simple paleosol packages, including their fine-grained deposits, were deposited rapidly during channel avulsion. Ribbon sandstone bodies represent ancient splay channels on the avulsion belt. Simple paleosol packages in the Willwood Formation support the contention of Smith et al. (1989) that, in many meandering river systems, a significant portion of the fine-grained deposits were deposited as the trunk channel avulsed. Deposits that resemble the simple paleosol packages may provide a means of recognizing avulsion episodes in other alluvial sequences.

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