T54 Thresholds and Pathways of River Avulsion

Sunday, 9 October 2011
1:30 PM - 5:30 PM
Room 211CD

CHARACTERIZING RELATIONSHIPS BETWEEN CHANNEL, FLOODPLAIN, AND AVULSION DEPOSITS IN THE WASATCH FORMATION (PALEOCENE/EOCENE, COLORADO)

Paper number: T54-15
Program: Technical Sessions
Time: 4:50 PM – 5:05 PM

Authors

Abstract

Avulsion is the process by which a channel is abandoned and re-established onto the adjacent floodplain. Two styles of avulsion have been observed: progradational avulsion, during which abundant crevasse splays and coarse-overbank deposits (collectively, heterolithic “avulsion deposits”) rapidly accumulate adjacent to the channel during avulsion, and incisional avulsion, where new channels erode directly into the floodplain. Conditions that might control the avulsion style include floodplain slope, grain-size variation, vegetation, and water-table levels. In order to understand the different conditions in which these two styles of avulsion occur, we evaluate the relationships between channel, floodplain, and heterolithic avulsion deposits in the Wasatch Formation (Paleocene-Eocene, Colorado). The Wasatch Formation straddles the Paleocene-Eocene boundary and comprises three members: the lower Atwell Gulch Member and upper Shire Member which are both mud dominated, and the middle Molina Member which is sand rich. In each member channel sand-body scale and architecture, floodplain deposits, and avulsion deposits are characterized.

The Atwell Gulch Member contains small, single story channel sandstones that overlie well-developed, clay-rich floodplain paleosols and lacks avulsion deposits. The Molina Member exhibits large, sheet-like channel bodies that are generally preceded by coarsening upward avulsion successions associated with decreased paleosol development. The Shire Member contains large, multi-story channel sandstones sometimes underlain by well-developed floodplain paleosols and other times underlain by avulsion deposits.

The scarcity of avulsion deposits in the Atwell Gulch Member suggests that the primary mechanism for channel relocation was incisional avulsion. In contrast, common avulsion deposits in the Molina Member imply that Molina avulsions were dominantly progradational. Shire Member channel bodies variably overlie avulsion deposits and floodplain deposits, which may indicate a mix of avulsion styles. Shifts in avulsion style through the Wasatch may be linked to changes in floodplain and channel characteristics resulting from climate change at the P-E boundary.