Abstract

Clarkston Mountain, the southern extension of the Malad Range of southern Idaho, consists of highly faulted, moderately east-dipping Middle Cambrian to Silurian strata. The internal structure of the range includes numerous mostly down-to-the-west normal faults, a minor bedding-plane thrust fault, and several anomalous younger-on-older blocks bounded by gently dipping normal faults. Nearly 8,000 feet of lower Paleozoic carbonate and fine-grained clastic strata crop out on Clarkston Mountain. The oldest rocks were previously assigned to the Ute, Blacksmith, and Bloomington Formations, but are herein reinterpreted, respectively, as the Hodges Shale, middle limestone, and Calls Fort Shale Members of the Bloomington Formation. The Bloomington Formation is overlain by the Nounan, St. Charles, Garden City, Swan Peak, Fish Haven, and Laketown Formations. The Miocene to Pliocene Salt Lake Formation and a variety of unconsolidated deposits, including boulder deposits of uncertain Tertiary age and Pleistocene Lake Bonneville deposits, crop out along the flanks of the mountain.

Clarkston Mountain is bounded on the west by the Clarkston Mountain segment of the Wasatch fault zone, which consists of two straight-line parts forming an embayment at Elgrove Canyon. A concealed splay of the Wasatch fault zone is inferred west of the mountain front to account for Salt Lake Formation exposed in this embayment. A 15-foot-high fault scarp in alluvial-fan deposits is preserved at the entrance to Elgrove Canyon, above the Bonneville shoreline; this scarp is probably pre-Bonneville in age. Three-point solutions and well-developed fault breccia on faceted spurs show that the Clarkston Mountain segment of the Wasatch fault zone dips about 45 degrees west. Clarkston Mountain is bounded on the east by the West Cache fault zone and on the south by the Short Divide fault. The Short Divide fault, a major transverse structural feature, is a segment boundary of both the Wasatch and West Cache fault zones.

The east-dipping, lower Paleozoic strata of Clarkston Mountain may be part of the east limb of a large anticline, the axis of which is faulted and now buried under the Malad Valley. These strata were transported eastward during the Sevier orogeny as part of the Cache allochthon, the upper plate of the Paris-Willard thrust. In plan view, a fault map of Clarkston Mountain looks like a shattered pane of glass. This complex faulting may reflect the range’s position in the upper plate of the much younger Bannock detachment.