Regional Gravity Survey of the Northern Great Salt Lake Desert and Adjacent Areas in Utah, Nevada, and Idaho
From 1957 to 1961 a regional gravity survey was made over the northern part of the Great Salt Lake Desert and adjacent areas in Utah, eastern Nevada, and southeastern Idaho. A total of 1040 stations were taken over an area of about 7000 square miles. The results were compiled as a Bouguer gravity anomaly map with a contour interval of 2 mgal. The Bouguer values ranged from a high of about —120 mgal over the outcrop areas to a low of about —196 mgal over the alluvium-covered graben areas. The gravity high over the Raft River Mountains apparently corresponds with the Raft River Mountains anticline. A belt of gravity contours, with a total relief of 15–20 mgal, extending for 40 miles between the Wildcat Hills and the southern part of the Grouse Creek Mountains and beyond, is interpreted provisionally as caused partly by abrupt thickening and/or downwarping of the rocks of late Paleozoic age in a general northwestward direction, perhaps to form a foredeep (part of the Butte-Deep Creek trough of Steele, 1960) south of the Raft River Mountains; however, this anomaly could also be partly caused by overthrusting. Many Basin and Range faults, grabens, and horsts are indicated by the gravity data. In the northern part of the surveyed area, Junction Valley, Upper Raft River valley, and Curlew Valley are indicated to be grabens. The Newfoundland Range, in the northeastern part of the Great Salt Lake Desert, is a horst flanked by a graben on each side. The northwestern margin of the Great Salt Lake Desert comprises a complex pattern of Basin and Range fault blocks, large and small, that lie along a generally northward-trending belt or zone. The Silver Island-Pigeon fault block, which comprises the Silver Island Mountains, the Little Pigeon Mountains, and Pigeon Mountain, forms an elongate, arcuate horst that is flanked by a belt of grabens on the west (Pilot Valley, Lucin, and Grouse Creek grabens) and east (Wendover, Crater Island, Little Pigeon, and Pigeon grabens). A major fault zone is indicated along the east margin of the Pilot Range. The Pilot-Grouse Creek rift belt, at least 90 miles long, extends northward between Pilot Valley and the Upper Raft River valley and constitutes a major lineament in the earth9s crust along which the graben blocks were displaced downward relative to the adjacent mountain blocks. In the southern part of the rift belt, the grabens are separated by blocks (the Lemay and Lucin horsts) that were probably downfaulted relative to the large mountain blocks, but became lodged at intermediate height; in the northern part, however, the blocks (for example the Junction Valley graben), apparently merely broke from the main crustal unit along this belt of weakness. The indicated thickness of the valley fill of Cenozoic age in some of the graben areas ranges up to about 6000 feet.