Structural Geology of Eastern Part of Richmond and Western Part of Naomi Park Quadrangles, Utah-Idaho

Abstract

The mapped area, which includes the eastern part of the Richmond and the western part of the Naomi peak quadrangles, Utah-Idaho, is located in the central part of northern Utah and southeastern Idaho. It is located along the eastern side of Cache Valley and western side of the Bear River Range. Cache Valley is in the Basin and Range province and the Bear River Range is in the Middle Rocky Mountain province. The mapped area is about 8.6 miles long, in the north-south direction, and 7.8 miles long, in the east-west direction. The Mutual Formation of Precambrian age is the oldest stratigraphic unit exposed in the mapped area. It consists of purple and brown quartzite. The Brigham Formation of Early Cambrian age and the Langston Formation of Middle Cambrian age overlie the Mutual in stratigraphic succession. The Salt Lake Formation of Tertiary age unconformably overlies older rocks; it is also faulted against the Mutual Formation. A major landslide of Precambrian, Cambrian, and Ordovician formations is present in the southern part of the mapped area. It is unconformably overlapped by the Salt Lake Formation. The Lake Bonneville Group of late Pleistocene age is present in Cache Valley and overlaps older rocks along the western side of the Bear River Range. The Precambrian and Cambrian stratigraphic units, except for those of the landslide, dip eastward and form the western flank of the Logan Peak syncline. A small disharmonic asymmetrical anticline, in the Langston and Ute Formations of Cambrian age, indicates eastward movement. Two beddingplane faults locally eliminate the basal Naomi Peak Limestone Member of the Langston Formation. A major normal fault, which is nearly vertical, extends along the base of the Bear River Range. Another normal fault, which is probably nearly vertical, parallels the western edge of the foothills. The folding and bedding-plane thrust faulting involve eastward movement and occurred during the Sevier orogeny. This orogeny began during late Jurassic time and extended into the early part of the Tertiary Period. Basin and Range normal faulting began early in the Tertiary Period. The normal faults produced great relief between Cache Valley and the Bear River Range. The landslide surface is thought to have formed as a west-dipping thrust fault. Reversed movement on this fault, due to removal of support on the valley side, produced the major landslide. (58 pages)