RECONSTRUCTING MIOCENE PALEODRAINAGE NETWORKS IN THE COLUMBIA RIVER BASALT GROUP, CLEARWATER EMBAYMENT, IDAHO AND WASHINGTON

Sedimentary units interstratified in the Miocene Columbia River Basalt Group (CRBG) flows preserve a record of the depositional and erosional history during and between emplacement of the flows in the Clearwater Embayment of west-central Idaho and southeastern Washington. These sedimentary units are known informally as the sedimentary interbeds, Sweetwater Creek beds or Latah formation within the embayment. Utilizing the existing, well-defined chronostratigraphic and chemostratigraphic correlations of CRBG flows permitted chronostratigraphic correlations of sedimentary interbeds. Combining the interbed chronostratigraphy together with the facies associations and spatial distribution of the interbeds permitted reconstructions of seven paleodrainage networks in the Clearwater Embayment: one reconstruction between the Imnaha and Grande Ronde Basalts; three reconstructions in the Grande Ronde Basalt; one reconstruction between the Wanapum and Saddle Mountain Basalts; and two reconstructions in the Saddle Mountain Basalt. Paleodrainage reconstructions in the Grande Ronde Basalt are fluvial systems and lacustrine systems created by damming the fluvial systems. Paleodrainage reconstructions in the Imnaha, Wanapum, and Saddle Mountain Basalts are only fluvial systems. Distribution of the paleodrainage networks appear to be primarily fault-controlled similar to the modern drainage networks. The CRBG flows do appear to have short-term influence on the paleodrainage networks by damming and/or displacing the paleodrainage networks especially during emplacement of the large volume flows of the Grande Ronde Basalt.