Late Miocene Fanglomerates in Lower Member of Starlight Formation, Northern Portneuf Range, Idaho: ABSTRACT

The lower member of the late Miocene Starlight Formation in the northern Portneuf Range, southeastern Idaho, represents sedimentation influenced by both Basin-and Range faulting and eastern Snake River plain (SRP) volcanism. The lower Starlight contains at least 610 m of fanglomerates and thin, interbedded basalt flows. The formations lower half is interpreted as medial alluvial fan facies and consists of coarse tuffaceous heterolithologic conglomerate lenses and sheets, and interbedded immature tuffaceous pebbly lithic arenites. The fine-grained upper half of the member is interpreted as distal-fan facies. The sandy deposits of the distal facies are cut by numerous gravel-filled channels. On the basis of clast assemblages and preservation of fragile rhyolite tuff grains that could not survive long transport distances, these deposits were likely derived from nearby sources. In addition, paleocurrent data indicate a general westerly transport direction. Individual drainage systems apparently controlled sedimentation, as indicated by different clast assemblages at various locations. The dominant clast type in The Cove, southwest of the Blackfoot River, is Eocene andesitic volcanics. In outcrops 20 km south, Paleozoic and lower Mesozoic sedimentary clasts predominate. Chert pebble conglomerate clasts, found only in southern exposures, may have been derived from distant eastern outcrops of Cretaceous conglomerates.more » Alluvial fan deposits in the lower member of the Starlight Formation developed as a response to early Basin-and-Range uplift. However, they were also influenced by eastern SRP volcanism, as indicated by the presence of tuffaceous sediments and basaltic lava flows. The fan deposits likely originated from a nearby eastern source that is now covered by Tertiary volcanics of the Blackfoot laval field.« less