Abstract: Aggradation and Channel Avulsion: A study of the Eocene Shire Member of the Wasatch Formation, West-Central Colorado
Channelized sediments, whether subaerially or subaqueously deposited, are increasingly exploited as sources of hydrocarbon reserves. Avulsion is a critical control on the depositional patterns of these sediments and an understanding of the avulsion process is necessary for the construction of accurate models predictive of subsurface geometry.
Workers have proposed that avulsion is a function of the increase in potential energy associated with channel aggradation and that channels avulse upon reaching a critical aggradation height. In addition, they argue that critical aggradation height scales with channel size. The work presented here tests this theory through a study of the geometry of channel belt sand bodies. In the Shire Member of the Wasatch Formation, near Parachute, Colorado, sand bodies were used to determine the aggradation height of channels just prior to avulsion, the incision created as newly avulsed channels establish position on the flood plain, and, for scaling purposes, the channel flow depths.
The data obtained during this study suggest that channel aggradation and incision do, in fact, scale with flow depth and that channels reach a critical ratio of aggradation height to flow depth prior to avulsion. Channels in the Shire Member aggraded to approximately one flow depth above the surrounding flood plain before avulsing and incised to a depth of about 1.5 flow depths during the early stages of flood plain occupation.
AAPG Search and Discovery Article #90931©1998 AAPG Foundation Grants-in-Aid