Interim Geologic Maps of the Clarkston and Portage Quadrangles, Box Elder and Cache Counties, Utah and Franklin and Oneida Counties, Idaho
Document Type
Report
Journal/Book Title/Conference
Utah Geological Survey Open-File Report 381
Publisher
Utah Geological Survey
Publication Date
4-2001
Abstract
The Clarkston and Portage quadrangles lie astride the boundary between the Basin and Range and Middle Rocky Mountains physiographic provinces. The quadrangles straddle Clarkston Mountain, the southern extension of the Malad Range of southern Idaho, which consists of a complexly faulted sequence of generally east-dipping lower Paleozoic strata. Nearly 8,000 feet (2,440 m) of Middle Cambrian to Silurian 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. Oquirrh Formation strata are exposed in the West Hills and Junction Hills, and the Garden City Formation at Bergeson Hill. We also mapped the Paleocene-Eocene Wasatch(?) Formation, the Miocene-Pliocene Salt Lake Formation, and a variety of unconsolidated sediments, including extensive Pleistocene Lake Bonneville deposits, in the quadrangles.
The Salt Lake Formation consists of a laterally and vertically variable sequence of interbedded, generally white to light-gray, tuffaceous siltstone, sandstone, conglomerate, and limestone, as well as volcanic ash beds, micrite, and porcellanite. We subdivided the formation into six informal subunits. Based on tephrochronologic studies, the Clarkston and Portage quadrangles currently contain both the oldest (10.94 ± 0.03 Ma) and youngest (4.75 ± 0.35 Ma) known Salt Lake Formation beds in the area, making it late-middle Miocene to at least early Pliocene. Salt Lake strata are offset by at least 30 faults that bound at least 25 separate blocks along the south flank of Clarkston Mountain and the Junction Hills; folds with a wide variety of trends also deform these rocks. Chemical fingerprinting of volcanic ashes has proven useful in mapping the Salt Lake Formation in this structurally complex area.
The Wasatch fault zone forms the western margin of Clarkston Mountain and the Junction Hills to the south, and bounds the eastern margin of the Malad Valley, an asymmetric graben filled with up to 5,000 feet (1,500 m) of Cenozoic sediments. Clarkston Mountain is bounded on the east by the West Cache fault zone, which has documented Holocene offset in the Clarkston quadrangle, 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 the upper plate of the Paris-Willard thrust. Clarkston Mountain is cut by numerous mostly down-to-the-west normal faults that may reflect the range's position in the upper plate of the Miocene-Pliocene Bannock detachment. Other structures include a minor bedding-plane fault of possible early Tertiary age, and several klippen bounded by gently dipping faults of probable late Tertiary age, and several klippen bounded by gently dipping faults of probable late Tertiary age. In plan view, a fault map of Clarkston Mountain looks like a shattered pane of glass.
The principal economic resources in the quadrangles include sand and gravel, and ground water. Springs in the quadrangles provide an important source of culinary water for local communities. Because the Wasatch and West Cache fault zones traverse the Clarkston and Portage quadrangles, and because of their proximity to other major normal-fault zones, the Clarkston and Portage quadrangles are faced with significant seismic hazards, including surface rupture, ground shaking, liquefaction, and other seismically induced hazards. Geologic hazards in the quadrangles also include mass movements, flooding, shallow ground water, problem soil and rock, and radon.
Recommended Citation
Biek, R.F., Oaks, R. Q. Jr., Janecke, S. U., Solomon, B. J., and Barry-Swenson, L. M., 2001, Interim geologic maps of the Clarkston and Portage quadrangles, Box Elder and Cache counties, Utah, and Franklin and Oneida counties, Idaho: Utah Geological Survey Open-File Report 381, scale 1:24,000. (superceded by Biek et al., 2003).