REVISED STRATIGRAPHY FOR SEVERAL SADDLE MOUNTAINS AND WANAPUM BASALT UNITS, COLUMBIA RIVER BASALT GROUP

Recent mapping in the Clearwater embayment in Idaho has clarified ambiguous stratigraphic relationships of several Columbia River Basalt Group units, and in one instance has shown a unit previously identified as CRBG to be Oligocene. Clarification of the stratigraphy allows us to better understand the role these small volume flows play in the eastern Columbia Plateau. In Saddle Mountains Basalt, the basalt of Craigmont overlies the basalt of Swamp Creek, which overlies the basalt of Weippe; the basalt of Grangeville has been documented to underlie the basalt of Weippe and overlie the Asotin Member. Stratigraphic position of the basalt of Feary Creek remains unresolved, although its close association with other Saddle Mountains Basalt units, composition similar to the Craigmont and Swamp Creek units, and an age determination of about 11.5-12 Ma strongly suggest it is Saddle Mountains in age. Basalt of Mica Flat, a previously unidentified unit, overlies a thin sediment interbed above the Priest Rapids Member south of Coeur d’Alene. We now include it in Saddle Mountains Basalt, but its position within that formation is uncertain. The basalt of Icicle Flat, previously included in the Saddle Mountains Basalt, now has been placed in the Wanapum Basalt, and is believed to be equivalent to the basalt of Dodge, Eckler Mountain Member, on the basis of paleomagnetic directions, composition, and its stratigraphic position between the Priest Rapids Member and R₂ Grande Ronde Basalt. The extent of most of these units has also been refined and in some instances expanded. Basalt with Asotin Member chemistry has been found filling a narrow erosional channel in R₂ Grande Ronde Basalt on Craig Mountain, the plateau south of the Waha escarpment and east of the Limekiln fault. The basalt of Potlatch, Onaway Member, previously included in either Wanapum Basalt or Saddle Mountains Basalt, has been shown by dating to be Oligocene.