Geology of parts of the Johnny Gulch quadrangle, Montana

Abstract

An area of about 35 square miles, situated about 30 miles southeast of Helena, Montana, was mapped during the summer of 1952 at a scale of l:24, 000. The area includes a part of the eastern foothills of the Elkhorn Mountains, and is underlain by sedimentary mad volcanic rocks of Cretaceous age that were intruded during late Cretaceous or early Tertiary time by several types of igneous rocks. The oldest rocks in the map area are the nonmarine sandstone, shale, and limestone of the Kootenai formation. These are overlain disconformably by the black shale siltstone, sandstone, and siliceous mudstone of the Colorado group that is subdivided into three map units; a lower black shale unit composed of black shale and silty shale with a basal clean sandstone, all of probable marine origin; a middle siliceous unit composed of sandstone, siltstone, and siliceous mudstone of both marine and nonmarine origin; and an upper black unit composed of black shale of marine origin. Conformably above the Colorado group are crystal lithic turfs of the Slim Sam formation; in places theme grade into and in other places are unconformably overlain by the Elkhorn Mountains volcanics composed of crystal tuff, breccia, flows, and bedded tuff of andesitic composition. The rocks of the Elkhorn Mountains volcanics and older formations have been intruded by diorite porphyry and related rocks and by hornblende quartz monzonite. The diorite porphyry and related rocks include hornblende diorite porphyry, hornblende augite diorite porphyry, augite diorite porphyry, and basalt. Resting with marked unconformity upon older rocks are volcanic sedimentary rocks of early Tertiary age that are locally overlain by thin rhyolite flows. Late Tertiary and Quaternary fans overlie the rhyolite flows. Alluvium, talus, and other mantle are present in small amounts in many parts of the area. The sedimentary rocks of the area mapped form a part of the east flank of a major anticline. A major north-south syncline to the north of map area is believed to have been deflected to the east of the area because of the rigidity of large irregular plutons of diorite porphyry. The location of the plutons may have been controlled by the initiation of the major syncline, by a postulated pre-intrusive fault, or by both. Most of the small-scale structural features are related to the emplacement of the plutons. During emplacement the intruded sediments yielded either by faulting or by folding; the deeper rocks failed by faulting and the shallower rocks failed by folding. The area contains deposits of gold, silver, copper, lead, and zinc, none of which are currently being mined; and a deposit of magnetite which is being mined for use in cement.