Geology of the Haystack Range, Goshen and Platte Counties, Wyoming

The Haystack Range, located in southeastern Wyoming, is composed mostly of deformed and metamorphosed sedimentary rocks and igneous rocks of Precambrian age9 Evidences of multiple deformation and metamorphism are the following: (1) pre-existing linear structures caused by mineral parallelism have be􁪽n folded with the rocks and (2) a marked discontinuity in fold axes and rock types exists between the schist, granite, and associated igneous rocks of the Haystack Range and the dominantly carbonate and politic met sediments west of the Haystack Range. The most obvious deformation and metamorphism is probably penecontem-poraneous with the emplacement of the granite masses during the Precambrian era. Sedimentary rocks of Devonian (1), Mississippian, Pennsylvanian, Tertiary, and Quaternary age are found in the area., The Guernsey formation of Devonian (1) and Mississippian age overlies the Precambrian complex with angular discordance􁪽 The Hartville formation of Mississippian (1) and Pennsylvanian age disconformable overlies older rocks of the Paleozoic system. The Arikaree formation of Miocene age overlies with angular discordance all older rocks. Alluvium and colluvium of Recent age is found along drainageways and on hillsides. Folds and faults formed during the Laramide orogeny show parallelism with the structural features of rocks of Precambrian age. Laramide structural features follow Precambrian foliation, fold axes, contacts, and faults. Mineral commodities of economic interest include iron ore, copper, muscovite, beryl, limestone, and crushed rock􁪽 With the exception of limestone and copper, which occur in the Guernse1 formation, commodities are restricted to rocks of Precambrian age. Iron ore, limestone, and crushed rock have been produced in quantity in the area. Small amounts of copper ore have been produced and small quantities of beryl and muscovite have been taken from the pegmatites of the joystick Range.