Abstract: Mapping and Analysis of Extension-Related Folds in the Miocene-Pliocene Salt Lake Formation, Northern Cache Valley, Idaho

EVANS, JEFFREY C.

Extension-related folds have been documented in virtually every rift zone worldwide, many of them prolific petroleum provinces, yet the mechanisms by which they form and their influences on rift basin evolution are incompletely understood. Some types of extension-related folds are important structures in extensional settings, as they are indicators of three-dimensional strain, and folding can influence syntectonic sediment dispersal within the basin.

Multiple episodes of normal faulting affected the Cache Valley in southeast Idaho and northern Utah during the late Cenozoic. The tuffaceous Miocene-Pliocene Salt Lake Formation was probably deposited syn-tectonically with most of the extensional events. A preliminary study of existing geologic maps has identified at least 60 folds within the Salt Lake Formation of the greater Cache Valley region, many of which are large and well exposed. We interpret the folds as being extension-related because there is an obvious association between the folds and normal faults and because there are no known contractional events of this age.

The study area contains many folds in the Salt Lake Formation that form a complex arrangement, as well as low-angle normal faults (previously mapped as a younger-over-older thrusts) and numerous high angle normal faults. The mechanism that formed these folds has not yet been determined, but their spatial association with the adjacent normal faults is apparent. My working hypothesis is that the complex arrangement is due to an interaction of more than one folding episode: an initial folding related to the low-angle detachment system, and a later episode related to high-angle normal faulting.

AAPG Search and Discovery Article #90940©1997 AAPG Foundation Grants-in-Aid