ABSTRACT
Analysis of Miocene through Pleistocene siliciclastic and volcaniclastic sequences in the western Snake River Plain of Idaho and Oregon allows detailed paleogeographic reconstruction of sedimentation associated with the development of a rapidly subsiding continental basin. Extensional tectonism was accompanied by voluminous outpourings of basaltic and silicic volcanic material. These in turn were reworked basinward by marginal alluvial fan-braided stream networks into basin-center fluviolacustrine systems. Episodic influxes of both felsic and basaltic tephra are recorded in fossiliferous lacustrine silt and claystones of the Poison Creek and Chalk Hills Formations, radiometrically bracketed between 12.5 to 5 m.y.B.P. Basinward-fining facies indicate deposition in a large lacustrine complex fed by at least 2 major fluvial systems. Complex interfingering of coarse-grained strandline deposits with offshore fine-grained sediments suggests repeated expansion and contraction of the lake system and record the dynamic interplay between basin tectonism and sedimentation. A major unconformity and presence of extensive interbasinal basaltic volcanism (Banbury Basalt) reflects draining of this lacustrine system and renewed extensional tectonism in the basin approximately 4 m.y.B.P. Reestablishment of a major fluviolacustrine complex is represented by deposits of the Pliocene-Pleistocene Glenns Ferry Formation (4-1.8 m.y.B.P.). The fossiliferous mudstone, sandstone, and conglomerate represent alluvial fan, meandering stream, flood-basin, and lake margin systems. Interbedded with these are a series of basaltic tuffs and flows reflecting continuing volcanism along the basin margin and basin center. Continued structural adjustments along high-angle normal faults that bound the basin are indicated by stratigraphic relationships as well as the style of fluvial sedimentation. Periods of rapid basin subsidence are recorded by thin, discontinuous channel sandstone encased by thick over-bank siltstone and claystone. More stable periods allowed development of sheet-like channel sandstone with subordinate amounts of overbank fines. Basin-wide structural adjustments have continued to the present, as indicated by the incisement of these Neogene deposits by the modern Snake River.
