Geology and structure of Stansbury Island (Thesis and maps)

Plate2 Structure sections in Stansbury Island, Tooele County, Utah

Stansbury Island, Tooele County, Utah, is 12 miles long, 3 to 4 miles wide, and is the second largest island in Great Salt Lake. It is at present a peninsula because it is connected to the mainland by mud flats and sand bars. Consolidated sedimentary rocks exposed on the island range in age from Late Precambrian to Mississippian. The PreCambrian System is represented by about 2,800 feet of strata belonging to the Big Cottonwood series and consisting of metaquartzite conglomerates, metaquartzites, and phyllitic shale. The mapping and complete identification of individual formations constituting the Cambrian System was difficult due to intense deformation. However, 2,500 feet of the Prospect Mountain Quartzite and 500 feet of the Pioche ""Shale"" were identifiable in the basal part of the System. The Ordovieian System Is represented by 1,675 feet of strata belonging to the Garden City Formation and composed of a lower member of silty and cherty limestone, a middle member of cherty dolomite, and an upper member of silty limestone which contains intraformational conglomerates. The Silurian System consists of 450 feet of Laketown Dolomite; and the Devonian System consists of 975 feet of strata belonging to the Stansbury Formation which is composed of a lower member of interbedded dolomite and quartzite, and an upper member of almost pure quartzite. Strata of the Mississippian System are over 5,530 feet thick and consist of dolomite, limestone, and minor quartz sandstone and chert. Five formations were recognized In the Mississippian System, namely, the Fitchville(?), Madison, Deseret, Humbug, and Great Blue. The Quaternary System consists of gravel, sand, alluvium and colluvium, and lake bed deposits. Quaternary deposits are largely unconsolidated and unconformably overlie Precambrian and Paleozoic strata. Structurally the island can be divided into three units, namely, the northern, central and.southern structural units. The northern structural unit is composed of Upper Precambrian rocks of the Big Cottonwood series; the central structural unit is a zone of intense deformation; and the southern structural unit is composed of a sharp, north- northeast-trending anticline which is slightly overturned to the west. Folds on the island are the result of tectonic forces on a regional scale. The Island lies at the eastern margin of a miogeosynclinal trough which developed during Late Precambrian time to the west of the Wasatch Line. The subsidence of the miogeosynclinal area was interrupted by the Northern Utah, Western Utah, and Sevier uplifts during late Precambrian, Permo-Pennsylvanian, Jurassic, Cretaceous, and Early Tertiary time. Two relatively minor uplifts, the Tooele and Stansbury uplifts, occurred to the south of the Island area during Late(?) Ordovician and Late Devonian time respectively. These uplifts constituted the primary type of deformation in the area. Geomorphologically the island may be considered as a horst block bounded by roughly north-trending normal faults to the east and west. Topographically the island is a north-trending ridge having a sharp and sinuous crest line along which occur several high peaks. Numerous short intermittent streams run down steep slopes and end in alluvial fans along valley floors. Several terraces mark levels of the pleistocene Lake Bonneville. The most prominent of the terraces are the Bonneville, Provo, and Stansbury terraces. The present Great Salt Lake is a remnant of Lake Bonneville.