Geology of the Terrace and Hogup Mountains, Box Elder County, Utah

The Terrace and Hogup Mountains are in west central Box Elder County, Utah. They are in the middle of the mapped area which comprises a 300 square mile region between Kelton to the north and the Lakeside and Newfoundland Mountains to the south. No rocks older than upper Pennsylvanian are exposed in the mapped area. Strata which aggregate approximately 22,200 feet are assigned to the following systems and formations: Pennsylvanian -- Oquirrh Formation, Virgilian Series (3000+); Permian -- Oquirrh Formation, Wolfcampian Series (6718+), Diamond Creek Sandstone (2852), Loray? Formation (3420), Park City Formation, Grandeur Member (1838), Phosphoria Formation, Meade Peak Phosphatic (404). Rex Chert Member (1157), Gerster Formation (903); Triassic -- Dinwoody Formation (1647). and Thaynes Formation (329+). Pliocene strata of the Salt Lake Group and Pleistocene strata of the Lake Bonneville Group are present in the Terrace Mountains area but neither of these groups has been differentiated. Triassic strata are paraconformable above Permian strata. Angular unconformities are recognized at the base of the Pliocene and the Pleistocene sediments. Igneous rocks of Pliocene age comprise diabasic and basaltic dikes, basalt flows and a welded tuff deposit. The Terrace Mountains are structurally similar to neighboring ranges except for the apparent absence of high-angle reverse and large scale thrust faults. The Big Pass graben divides the Terrace Mountains into eastern and western structural blocks, each char¬ acterized by long, high-angle, north-south trending boundary faults and minor east-west normal faults. A system of arcuate rotational faults is present in the western block. Folds are broad and of large scale except for those at the north end of the eastern Terrace Mountains, in the Hogup Mountains, and at the south end of Crocodile Mountain. The structure of outlying areas is similar to that of the two principal blocks, but its interpretation is difficult due to the cover of Quaternary lacustrine sediments. The existence of concealed border faults along the eastern side of the area is postulated on physiographic evidence. Desiccation fractures and depression structures are present in unconsolidated sediments. The geomorphic development of the area includes outstanding shore-line features left by Pleistocene Lake Bonneville. The elevations of three major terraces in the Hogup Mountains are: Bonneville - 5248 feet, Provo - 4833 feet, and Stansbury - 4518 feet. Shore lines are as much as 50 feet higher in the southern Terrace Mountains. Non-metalic deposits of economic significance, including bentonitic clay, vitric tuff, gravel and phosphatic shale are present in the area. During late Pennsylvanian and early Permian time the mapped area lay within the northwestern part of the Oquirrh Basin. Environmental conditions changed very little throughout this time. During Leonardian time the Diamond Creek Sandstone and the Loray? Formation were deposited in shallow marine water. Marine con¬ ditions prevailed during Guadalupian time when strata of the Park City, Phosphoria, and Gerster Formations were deposited. A paraconformable relationship is indicated between strata of Permian and Triassic age. The Dinwoody and Thaynes Formations were deposited in shallow water in a subsiding basin. Folds belonging to two groups were formed in late Mesozoic or early Cenozoic time. Folding was followed by block faulting and tilting of the blocks to the northwest. Topography ancestral to that of the present developed during late Cenozoic time, previous to deposition of strata of the Salt Lake Group. Faulting of Pliocene and Pleistocene age is recognized.