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Quaternary rocks northeast of American Falls Reservoir, Idaho, include more than 140 m of poorly indurated sedimentary rocks and intercalated basalt flows. Previous stratigraphic studies in the region include reconnaissance and detailed mapping and topical studies based mostly on surface exposures. The present study extends the Quaternary stratigraphy into the shallow subsurface by using lithologic data from drillers’ logs of about 240 water wells.

Pre-Quaternary rocks of the area include Miocene bimodal volcanic and volcani-clastic rocks and Pliocene basalt of Buckskin Basin. The surface developed on the pre-Quaternary rocks slopes gently to the west and northwest except locally, as at Ferry Butte, where doming is inferred to be the result of shallow emplacement of a rhyolite intrusive body.

Quaternary rocks of the study area consist of coarse-grained alluvium deposited by the Snake River, thin distal edges of basalt flows from sources to the northwest, fine-grained alluvial fan deposits derived from highlands to the southeast, two fine-grained fluvial and lacustrine deposits (Raft Formation and American Falls Lake Beds), and deposits of the Bonneville Flood.

The Raft Formation, present only in the subsurface in the area, is correlated with the Raft in exposures along the Snake River southwest of American Falls and at the northern end of the Raft River basin on the basis of stratigraphic position and virtually identical thickness and lithology. The minimum age of the Raft Formation in the study area is constrained by an overlying basalt flow dated at 0.2 Ma.

Driller’s logs and surface exposures show that the American Falls Lake Beds are at a higher elevation and are coarser grained northwest of the Snake River than they are southeast of the river. These differences are attributed to slightly different depositional environments caused by the geometry of the lake basin. Dating of a basalt flow that overlies the American Falls Lake Beds north of Springfield will provide valuable information on the history of American Falls Lake.

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