MIOCENE EVOLUTION OF THE MOSCOW-PULLMAN BASIN, IDAHO AND WASHINGTON

The Moscow-Pullman basin, located on the eastern margin of the Columbia River flood basalt province, consists of a subsurface mosaic of Miocene sediments of the Latah Formation and lava flows of the Columbia River Basalt Group. At least 25 flows emplaced into the basin blocked westward drainages at Pullman, Washington, and caused sedimentation along flow edges and steep mountain fronts to the east near Moscow, Idaho. Approximately 2,700 ft (823 m) of basalt and sediment filled the basin but only uppermost units occur in rare outcrops. Rock chip chemistry from ten wells greater than 700 ft (213 m) in depth was used to form a stratigraphic framework for the Grande Ronde, Wanapum, and Saddle Mountains Basalts and associated Latah sediments. Domestic well reports, outcrops, and regional comparisons were used to determine approximate time lines and develop paleogeographic reconstructions from early Grande Ronde to late Saddle Mountains time. Reconstructions identify distributions of basalt flows and disruptions, obliterations, and reversals of drainages.

The ancient Palouse River, believed to have flowed southwest from Pullman, was overwhelmed numerous times eventually reversing its direction to flow north toward the area of Palouse, Washington, and then northwestward away from the basin. Interbedded sediments are primarily fluvial and representative of a low energy environment; they are clay rich with poorly sorted, coarse-grained channel lag deposits. Overall lack of energy was caused by rapid rises in base levels both regionally and locally with each basalt emplacement. Stream development out of the basin was also slowed by the formation of a small, northwest-trending anticline in the Columbia River Basalt Group near Pullman across the western edge of the basin. Folding began at least as early as the Grande Ronde R₂ interval and continued after emplacement of the last Saddle Mountains flow. At times the anticline played a role in basalt and sediment distribution within the basin. The Lolo flow of the Wanapum Basalt covered the entire basin and the anticline. Clastic sediments prograded away from mountain fronts across the basalt plains in early Saddle Mountains time. Continued anticlinal growth and local basalt flows slowed erosion. Modern stream development is still adjusting to the volcanic and folding events.