CLIMATIC AND TECTONIC IMPLICATIONS OF PRE-BULL LAKE GLACIAL DEPOSITS, MIDDLE RIDGE, LEMHI VALLEY, EAST-CENTRAL IDAHO

Middle Ridge lies between the Beaverhead and Lemhi Ranges in east-central Idaho and rises 120-180 meters above Gilmore Summit and the headwaters of Birch Creek and the Lemhi River. Bounded by normal faults on the south and west, the ridge is underlain by Pliocene tuffaceous “lake bed” sediments with some interbedded gravels near the top. The gravels are overlain unconformably by Quaternary alluvial and glacial deposits. Large boulders atop Middle Ridge were recognized and thought to be of glacial origin (Buffalo age) in the 1960s. Subsequent reconnaissance workers hypothesized glacial or possible fluvial origins.

Recent field mapping supports the glacial origin of abundant large (up to 4m) Kinnikinic quartzite erratics. These boulders lie up to 150m above the valley floor on the crest and western flank of the ridge. They are abundant in several distinct areas located up to 2.5-3.5 km NE of well-documented Pinedale terminal moraines in the adjacent Lemhi Mountains. CRN age estimates of 350-500ka (uncorrected for boulder erosion) were obtained in the year 2000 on two Middle Ridge quartzite boulders. Such exposure dates clearly indicate pre-Bull Lake deposition.

Understanding the origin and current position of these pre-Bull Lake deposits requires unraveling the geologic history of Middle Ridge starting with: 1) deposition of the Pliocene sequence in a broad intermontane basin between the Beaverhead and Lemhi Ranges; 2) renewed Plio/Pleistocene extensional faulting and mountain uplift with concurrent deposition of coalescing alluvial fans across Middle Ridge; 3) extensive pre-Bull Lake alpine glaciation in the Lemhi Range and piedmont glaciers advancing beyond the mouths of three canyons across the alluvial fan plain; 4) deposition of an ablation moraine leaving large ice-rafted boulders on Middle Ridge; and 5) late Quaternary uplift, slumping, normal faulting, and moderate erosion.

The presence of pre-Bull Lake glacial deposits on Middle Ridge provides new insights regarding the maximum extent of glaciation and the timing of extensional faulting in the area.