Paper No. 5
Presentation Time: 9:20 AM
COLLISIONAL-BASIN ORIGIN FOR JURASSIC MARINE DEPOSITS IN THE BLUE MOUNTAINS PROVINCE, NORTHEASTERN OREGON
Jurassic sedimentary rocks in the Blue Mts Province (BMP) represent a large marine basin for which two hypotheses have been proposed: (1) long-lived forearc basin located between an east-dipping subduction zone in the west and the Olds Ferry arc in the east; or (2) collisional basin formed by crustal loading in a large thrust belt to the east. We seek to distinguish between these models using information about the age and architecture of Jurassic basin-fill, relation to thrust belts in Oregon and western Idaho, terrane affinity of underlying rocks, sediment-dispersal patterns, and new detrital zircon ages. Our analysis restores ~400 km of dextral offset on a large strike-slip fault zone in W Idaho and NW Nevada (Wyld and Wright, 2001) and ~60 degrees of post-Jurassic CW rotation (Housen, this session). All directions are given in restored coordinates. Jurassic strata everywhere overlie a regional angular unconformity. Basal Jurassic deposits young from 196-190 Ma in the Huntington area (where they rest on Olds Ferry terrane arc substrate), to 190-180 Ma in the Izee area (Baker terrane substrate), to 167-164 Ma at Pittsburgh Landing and Coon Hollow (Wallowa terrane arc substrate). In all sections a basal unit of locally-derived fluvial to shallow marine conglomerate and sandstone fines up into distal thin-bedded turbidites and shale. In the Izee and Coon Hollow areas, the marine shale coarsens up into sandy turbidites with west-directed paleocurrents and abundant low-grade metavolcanic, metasedimentary, chert, and argillite lithic fragments (Dickinson and Thayer, 1978; Goldstrand, 1994). While input from an active volcanic source is suggested by 180-Ma detrital zircons, the collisional-basin model is supported by overlap of Jurassic strata onto all older terranes, systematic younging of basal deposits from inboard to outboard position, and progradation of easterly-derived cherty and metamorphic detritus with abundant metavolcanic lithics. We infer that Jurassic subsidence resulted from westward migration of a flexural foredeep basin in response to crustal loading in the west-vergent Salmon River belt and related thrust belt in Nevada. This took place during oblique-sinistral collision of previously amalgamated terranes in the BMP with the Cordilleran arc and back-arc basin, during subduction and arc magmatism along strike to the south in the eastern Klamath Mountains.