AVULSION DEPOSITS IN LOWER EOCENE ALLUVIAL ROCKS, BIGHORN BASIN, WYOMING

Eocene alluvial rocks in the northern Bighorn Basin can be subdivided into three major kinds: fine-grained deposits on which cumulative pale osols developed, lithologically heterogeneous packages associated with immature paleosols, and major sheet sandstone bodies. The heterogeneo us packages, which alternate vertically with the cumulative paleosols, are dominated by fine-grained deposits and also contain ribbon sandst one bodies and thin (< 1 m) sheet sandstone bodies. Most ribbon sandst ones are less than 3 m thick and are more common in the middle and upp er parts of the packages. Cut-and fill deposits with fine-grained fill s are locally important. Because the lithologic packages are readily a pparent in the field because of the immature or simple paleosols devel oped on the fine-grained facies, they are referred to as ''simple pale osol packages''. The major sheet sand stones were deposited by meander ing 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 fl ooding of a trunk channel. On the basis of their stratigraphic positio n and few ribbon sandstones, the upper parts of some simple paleosol p ackages are interpreted as ancient levee deposits associated with a tr unk 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 bod ies represent ancient splay channels on the avulsion belt. Simple pale osol packages in the Willwood Formation support the contention of Smit h et al. (1989) that, in many meandering river systems, a significant portion of the fine-grained deposits were deposited as the trunk chann el avulsed. Deposits that resemble the simple paleosol packages may pr ovide a means of recognizing avulsion episodes in other alluvial seque nces.