GEOLOGY OF THE NORTHERN END OF THE BIG HOLE MOUNTAINS, MADISON & TETON COUNTIES, IDAHO

New geologic mapping was completed in the northern Big Hole Mountains, directly south of the easternmost Snake River Plain (ESRP), to define the stratigraphy of Miocene volcanic rocks and characterize their deformational history. Based on textural and compositional differences the previously undifferentiated Kirkham Hollow Volcanics are now divided into 7 distinct rock units. The lower sequence of rocks include: (1) undated, 0-130 m thick, rhyolitic lava flow (2) 5-30 m thick, rhyodacitic ignimbrite (3) 6.61 Ma (Ar-Ar, san), 0-130 m thick dacitic ignimbrite and (4) 6.59 Ma (Ar-Ar, san), 0-450 m thick, dacitic lava flow. Units 1-3 form a well exposed, disconformable sequence in the western half of the map area. Unit 4 forms a volcanic plateau in the northeastern quarter of the map area. The upper sequence of rocks includes the regionally persistent Tuff of Edie School, Tuff of Kilgore, and the Huckleberry Ridge Tuff (HRT). The nearly coeval ages of units 3/4 and the 6.57 Tuff of Edie School provide evidence for a complex ESRP eruption history that conflicts with a simple model of eastward migrating, hotspot-related volcanism.

Northwest striking, west dipping normal faults form half-grabens in the northwestern Big Hole Mountains. Major faults are spaced ~1.5 km and show 100-600 m of slip. Miocene rocks are tilted 10-25° northeast, with a slight decrease in tilt upsection and to the east. Normal faulting is interpreted to have begun about 4.4 Ma and ceased before emplacement of the untilted, 2.06 Ma HRT.

The northern Big Hole Mountains decrease in elevation northwards, ultimately merging with the ESRP. This topographic change is interpreted to reflect half-graben uplift of the Big Hole Mountains as well as subsidence of the ESRP. The HRT is restricted to incised ravines along the northern edge of the range indicating differential uplift/subsidence appears to have initiated prior to 2 Ma.

The Big Hole Mountains have large Holocene landslides made up of Miocene volcanic rock sliding over less competent Cretaceous rocks. Recent landslides of the loess over the HRT could adversely affect the watershed of the area.