The area of souther Elhorn Mountain, a previously unmapped area, is located north of Malad City, Idaho, in the Bannock Range. The mapped area is within the Basin and Range province. The mapped area measures 5.9 miles in the north-south direction and approximately 8.8 miles in the east-west direction. The oldest stratigraphic unit, in the mapped area, is the Ute Formation of Middle Cambrian age. Other units of Cambrian age, in ascending order, are: Blacksmith Formation, Bloomington Formation, Nounan Formation, and St. Charles Formation. The units of Ordovician age are the Garden City and the Swan Peak Formations. The youngest unit of Paleozoic age, in the mapped area, is the Fish Haven-Laketown Formation of Ordovician-Silurian age. The Paleozoic units are composed predominantly of limestone and dolomite; some unites contain varied amounts of quartzite and shale. The Salt Lake Formation and associated volcanic rocks of Tertiary age are present locally in the mapped area. Lake Bonnevill Group, colluvial deposits, and alluvial deposits, all of Quaternary age, are also present. A thrust fault, folds, and normal faults are present in the mapped area. A bedding-plane thrust fault is present between the Bloomington and Nounan Formations. A broad anticline is suggested by the attitudes of the Paleozoic rocks of southern Elkhorn Mountain. Small-scale folds are exposed locally. The normal faults are of two major trends, northwest and north. These fault sets are contemporaneous and they cut Salt Lake Formation. These sets intersect and form numerous fault blocks. The normal faults cut the thrust fault and the fold. Thus, the thrust fault and the fold are isolated within various fault blocks throughout the area. The marginal normal faults are responsible for the present topography. Remnant blocks of a major landslide are also present. The structural features of the area are the result of two major structural events, the older Laramide orogeny and the more recent Basin and Range faulting. The Laramide orogeny, active from Late Jurassic to early Eocene, was responsible for the thrust faulting and folding. The Basin and Range faulting, active from Oligocene to Holocene, was responsible for the normal faults and the landslide. Marginal normal faults are probably active at the present time. (101 pages)

Northern Elkhorn Mountain was unmapped previous to this investigation. The mapped area is located north of Malad City, Idaho, in the Bannock Range. It is within the Basin and Range Province. The mapped area measures 5.4 mi. in the north-south direction and 8.9 mi. in the east-west direction. The oldest exposed stratigraphic unit, within the mapped area, consists of orthoquartzite and is of Early Cambrian age. Cambrian formations of the mapped area, in ascending order, are as follows: Camelback Mountain Quartzite, Gibson Jack Formation, Elkhead Formation, Bloomington Formation, Nounan Formation, and St. Charles Formation. Units of Ordovician age are the Garden City and Swan Peak Formations. The youngest unit of Paleozoic age, found within the mapped area, is the Fish Haven-Laketown Formation of Ordovician-­Silurian age. Rock types comprising the Paleozoic units are orthoquartzite, limestone, dolostone, and shale. Tertiary units present, within the area, are the Salt Lake Formation and volcanic rocks with the composition of andesite. These units occur only in isolated parts of the mapped area. Colluvial and alluvial deposits of Quaternary age are present in the valley west of Elkhorn Mountain and in the southeastern and northeastern parts of the mapped area. Numerous high-angle normal faults dominate the structure of the area. They trend generally north and northwest. A major high-angle normal fault extends along the western side of Elkhorn Mountain and is responsible for the present topographic relief. Several small asymmetrical anticlines and a low-angle thrust fault are also present. The structural features, within the area, resulted from two major periods of crustal deformation. The first event was the Laramide orogeny. Compressional forces, generated during this event, produced the anticlines and the thrust fault. Movement was eastward. The second event was Basin and Range faulting. It produced the high­-angle normal faults. Basin and Range faultinq has been active from Oligocene to Holocene. The marginal normal fault, west of Elkhorn· Mountain, is probably active at the present time.

The mapped area, in southeastern Idaho, includes part of the Bannock Range, on the west, and Cache Valley, on the east. It is centered about 13 miles northwest of Preston, Idaho, and measures 12 miles in the north-south direction and 8.5 miles in the east-west direction. The Bannock Range, in the western part of the mapped area, consists of Precambrian and Cambrian stratigraphic units. The Precambrian units are as follows: (1) lower Precambrian argillite, (2) Precambrian quartzite, and (3) upper Mutual Formation. The Brigham Formation, which overlies the Mutual, is probably of Cambrian age; however, the lower part may be Precambrian. The Langston, Ute, Blacksmith, Bloomington, and Nounan Formations, all of Cambrian age, crop out locally. The Wasatch and Salt Lake Formations bf Tertiary age overlap older rocks near the western margin of the area. The Salt Lake Formation overlaps older rocks in the foothills along the western side of Cache Valley. It is separated from Precambrian argillite, on the west, by a major north-south gravity fault. Arnphibolite plutons intrude the Precambrian argillite at three localities in the Oxford Peak area; basalt flows and volcanic breccia are also present in the Precambrian argillite. The amphibolite represents metamorphosed diabase. Both the metamorphism of the plutons and the presence of extrusive igneous rocks in the Precambrian argillite indicate that the plutons formed during Precambrian time. Two major thrust faults are present in the Bannock Range. The Oxford Peak thrust fault places the Brigham Formation of Cambrian age, as well as Precambrian quartzite and the Mutual Formation of Precambrian age, over Precambrian argillite. The Clifton thrust fault places carbonate formations of Cambrian age and also a lower Paleozoic undifferentiated unit on various older rocks. Near the northwestern corner of the mapped area, however, the Clifton thrust fault underlies the Brigham Formation. Both thrust faults dip westward except where locally folded; the direction of movement was presumably eastward. The thrust faulting is probably related to the Laramide orogeny which occurred during the Cretaceous Period and the early part of the Tertiary Period. A major gravity fault extends along the eastern side of the Bannock Range. It places Salt Lake Formation of Tertiary age, on the east, against Precambrian argillite, on the west. A relatively short gravity fault, also down on the east, offsets the two major thrust faults east of Oxford Peak. The relative collapse of Cache Valley, during the Tertiary Period, produced great relief and, as a consequence, a major landslide formed northeast of Oxford Peak. It transported Precambrian argillite and overlying Brigham Formation down over Precambrian argillite.

Geologic mapping, basin analysis, and tephrochronologic analysis in the Clifton quadrangle of southeast Idaho indicates that the modern Basin-and-Range topography is only a few million years old and that the bulk of Cenozoic extension was accommodated by slip on an older low-angle normal-fault system, the Bannock detachment system. The detachment system was active between ~12 and < 4 Ma and accommodated ~50 % extension. Cross-cutting relationships show that the master detachment fault, the Clifton fault, is the youngest low-angle normal fault of the system, was active at a low angle, and has not been rotated to a low-dip angle through time. Map patterns and relationships indicate that the hanging wall to the detachment system began as a cohesive block that later broke up along listric and planar normal faults that either sole into or are cut by the master detachment fault. The Miocene-Pliocene Salt Lake Formation, a syntectonic, basin-fill deposit of the Bannock detachment system, was deposited during three sub-episodes of extension on the detachment system. Depositional systems within the Salt Lake Formation evolved from saline/alkaline lakes to fresh water lakes and streams to braided streams in response to the changing structural configuration of rift basins in the hanging wall of the detachment system. After breakup of the hanging wall began, the master detachment fault excised part of the hanging wall and cut hanging-wall deposits and structures. The structural geometry of the Bannock detachment system strongly resembles that of detachments documented in metamorphic core complexes. Therefore, we interpret the Bannock detachment system as a proto-metamorphic core complex, akin to the Sevier Desert detachment fault. The Bannock detachment system also collapsed the Cache-Pocatello culmination of the dormant Sevier fold-and-thrust belt, much like the Sevier Desert detachment collapsed the Sevier culmination. Structures of the Bannock detachment system are overprinted by a second episode of extension accommodated by E- and NE-trending normal faults that may be related to subsidence along the Yellowstone hotspot track and a third episode of extension accommodated by high-angle, Basin-and-Range normal faults. This last episode of extension began no earlier than 4-5 Ma and continues today.