GEOLOGIC CROSS-SECTION FROM MOSCOW, IDAHO, TO WEST OF PULLMAN, WASHINGTON

Sixteen members, comprised of an estimated twenty-five to thirty flows, of the Miocene Columbia River Basalt Group and associated interbedded sediments of the Latah Formation have been correlated between the cities of Pullman, Washington, and Moscow, Idaho, along the eastern margin of the Columbia River Basalt Province. The composite stratigraphic sequence examined here exceeds 2700 ft (823 m). Major and minor chemical element determinations were used to place the units into the regional stratigraphic framework comprised, from base upward, of the Grande Ronde, Wanapum, and Saddle Mountains Basalts. More than ninety percent of the sequence belongs to the R₁, N₁, R₂, and N₂ magnetostratigraphic units of the Grande Ronde Basalt.

Most flows were emplaced from the west through a wide gap, centered at Pullman, between basement rocks. These flows either pinched out or became interfingered with sediments as they approached the Moscow area. Descriptions of well chips from flows of the Cold Spring Ridge Member of the Grande Ronde N₁ interval indicate that they were invasive beneath Moscow. A short-lived but major fluvial system developed in the area of Pullman at the N₁–R₂ boundary. A northwest-trending anticline at Pullman began to form in early R₂ and played a role in the distribution of subsequent flows. Termination of the uppermost R₂ flows (Meyer Ridge Member) and the lowermost N₂ flow (Spokane Falls flow of the Sentinel Bluffs Member) produced a basin in the Moscow area allowing for the deposition of a thick sequence of sediments of the Vantage Member of the Latah Formation. The Roza flow of the Wanapum never made it over the anticline at Pullman; however, the subsequent Lolo flow (Wanapum Basalt) was voluminous enough to crest the anticline, flow east beyond Moscow and butt up against the steep eastern margin of the basin, obliterating stream channels in the process. Folding continued as sediments prograded westward over the Lolo flow. These features are important to understanding the subsurface architecture, geologic history, and hydrogeology of the Moscow-Pullman area.