Description: Members and flows of the Grande Ronde, Wanapum, and Saddle Mountains basalts of the Columbia River Basalt Group were mapped in the Saddle Mountains between Sentinel Gap and the eastern edge of Smyrna Bench. The Grande Ronde Basalt consists of the Schwana (low-MgO) and Sentinel Bluffs (high-MgO) members (informal names). The Wanapum Basalt consists of the aphyric and phyric units of the Frenchman Springs Member, the Roza-Like Member, and the Priest Rapids Member. The Saddle Mountains Basalt consists of the Wahluke, Huntzinger, Pomona, Mattawa, and Elephant Mountain basalts. The Wanapum and Saddle Mountains basalts are unevenly distributed across the Saddle Mountains. The Wanapum Basalt thins from south to north and across a northwest-southeast-trending axis at the west end of Smyrna Bench. The Priest Rapids, Roza-Like, and aphyric Frenchman Springs units are locally missing across this zone. The Saddle Mountains basalt has a more irregular distribution and, within an area between Sentinel Gap and Smyrna Bench, is devoid of the basalt. The Wahluke, Huntzinger, and Mattawa flows are locally present, but the Pomona is restricted to the southern flank west of Smyrna Bench, and the Elephant Mountain Basalt only occurs on the flanks and in three structurally controlled basins on the northwest side. The structure of the Saddle Mountains is dominated by an east-west trend and, to a lesser degree, controlled by a northwest-southeast and northeast-southwest trend. The geomorphological expression of the Saddle Mountains results from the east-west fold set and the Saddle Mountains fault along the north side. The oldest structures follow the northwest-southeast trend. The distribution of the flows, combined with the structural features, indicates a complex geologic history for the Saddel Mountains.
Date: September 1, 1978
Creator: Reidel, S.P.
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