| Description |
The Gilson Mountains and vicinity, Utah comprise approximately 450 square miles in central Utah, A gravity survey was made to cover the entire area, and the geologic mapping was carried out in the Gilson Mountains, the Black Mountains, the southern East Tintic Mountains, and their adjacent valleys which cover about 154 square miles. In the studied area, 24 consolidated sedimentary formations are present, ranging from Late Precambrian to Tertiary in age. The total thickness of the strati- graphic sequence exceeds 35K000 feet. The precambrian rocks are in fault contact with the Paleozoic rocks in Leamington Canyon, Paleozoic formations, chiefly car bonate rocks and sandstones, form the main bulk of the Gilson Mountains, the Black Mountains, and the southern East Tintic Mountains. The major structural features of the Gilson Mountains are three faults: the Gilson thrust, the Champlin thrust, and the Leamington Canyon fault. These faults possibly were formed successively during Cretaceous- Early Tertiary time when the entire eastern Great Basin was subjected to severe crustal disturbance. The Gilson thrust crops out eastward along the northern foothills of the Gilson Mountains and swings southward at the northeastern part of the Gilson Mountains. The Oquirrh Formation is exposed extensively around and outside of the bulge of the thrust, and forms the lower plate. Faunal correlation apparently indicates that the Oquirrh Formation underlies continuously the upper plate of the Gilson thrust, which is made up of older Paleozoic rocks.The Champlin thrust, which crops out on the northern slopes and eastern part of the Gilson Mountains, is marked by formational and bedding discordances. The northeasterly- trending drag folds, vertical and overturned beds, which are closely associated with these thrusts, suggest that these thrust plates moved southeastward. The nature of the Leamington Canyon fault is uncertain! however, its nearly linear course and the high-angle dip suggest a possible strike-slip fault. The gravity data reveal a gravity low (Sage Valley low) in the southeastern corner of the surveyed area, which is caused by the low-density Cretaceous (?) conglomerate and Tertiary volcanic rocks. A gravity high (Lynndyl high), centered near the mouth of the Broad Can yon, is interpreted as caused by the high-density Precambrian rocks that were brought up near the surface by thrusts. Gravity and magnetic data do not show any anomaly in association with the proposed "North Gilson fault" and "South Gilson fault" (Costain, 1960). The major structures in the Gilson Mountains possibly were formed during Cretaceous-Early Tertiary time. The Gilson and Champlin thrustings probably occurred in the same tectonic episode as the Canyon Range thrusting. Apparently, they were derived from different parts of the Sevier uplift on the west, and moved eastward as gravity-gliding masses on the eastern slope of the Sevier uplift. The Leamington Canyon faulting later brought the Canyon Range klippe and the Gilson and Champlin thrust plates into juxtaposition. Owing to the unfossiliferous nature of the Cretaceous (?) conglomerate in the northern Canyon Range, a precise dating of the structures in the Gilson Mountains and in the Canyon Range can not be obtained. However, the common occurrence of Cretaceous-Early Tertiary thrusts and orogenic conglomerates in the nearby areas suggests that the structures and the conglomerate in the studied, area are probably the same age. |
