CHRONOSTRATIGRAPHY OF THE MIOCENE SEDIMENTARY INTERBEDS IN THE COLUMBIA RIVER BASALT GROUP, NORTHERN CLEARWATER EMBAYMENT, WEST-CENTRAL IDAHO AND SOUTHEASTERN WASHINGTON

Miocene sedimentary interbeds in the Columbia River Basalt Group (CRBG) provide key evidence of paleogeographic and paleoclimatic conditions that existed throughout the Columbia Plateau between and during emplacement of basalt flows. Stratigraphic, sedimentologic, paleopedologic, chemical, and paleoecologic investigations of interbeds exposed within the northern Clearwater Embayment of west-central Idaho and southeastern Washington reveal a wide variety of: (1) textural variations and sedimentary facies from horizontally laminated claystone to coarse planar crossbedded sandstone to matrix-supported boulder conglomerates; (2) paleosols from ones having only weakly developed A horizons to those with well developed A, B, and C horizons; and (3) fossilized impressions of deciduous leaves, such as Magnolia, and evergreen leaves, such as Metasequoia, that imply a Miocene climate warmer and more humid than the present day. Spatial distribution of these interbeds, their sedimentary facies, and facies associations suggest deposition in mixed-load fluvial, lacustrine, and alluvial (debris flow) systems. Employing the existing, well-defined chronostratigraphic and chemostratigraphic correlations of the CRBG flows permits the chronostratigraphic correlation of sedimentary interbeds in the northern Clearwater Embayment. Sedimentary interbeds in Saddle Mountain Basalt flows of the middle and upper CRBG dominate the northwestern portion of the Clearwater Embayment, which is also known as the Lewiston Basin. Interbeds within Grande Ronde Basalt flows of the lower CRBG dominate the north-central and northeastern portions of the embayment. The newly defined chronostratigraphic correlation of the interbeds coupled with their suggested depositional systems permits development of paleogeographic reconstructions of the northern embayment between 16.5 and 6 Ma. This interbed chronostratigraphy also illustrates the inadequacies of previous stratigraphic correlations of interbeds based on similarities in sedimentary facies with a lack of chronologic constraints. The limited lateral extent and continuity of individual interbed exposures make it nearly impossible to accurately correlate interbed stratigraphy without chronologic constraints.