DOI: 10.5066/f73j3b61
title: Geologic map of the Strawberry Butte 7.5' quadrangle, Meagher County, Montana

description: The Strawberry Butte quadrangle, scale 1:24,000, is a legacy map of the U.S. Geological Survey, made as part of the Montana Investigations Project to provide information on the stratigraphy, structure, and geologic history of the area. The quadrangle encompasses an area of about 132 km2 (51 mi2) in the southwest part of the Little Belt Mountains and is about 26 km (17 mi) north of White Sulphur Springs, county seat of Meagher County, Montana. Two contrasting geologic terranes form the area. The northern two-thirds of the quadrangle are underlain by Paleoproterozoic metagranite that contains some inclusions of metadiorite and biotite gneiss. A veneer of Middle Cambrian sedimentary rocks, including the Flathead Sandstone at the base and overlying Wolsey Formation, Meagher Limestone, and Park Shale, is exposed discontinuously across the Paleoproterozoic metamorphic rocks. The southern third of the quadrangle, separated from the metamorphic rocks by the Volcano Valley fault zone, is underlain by Mesoproterozoic strata of the Newland Formation which are deformed in anticlines and synclines generally parallel to the fault zone. Eocene dacite sills and dikes intrude the Newland Formation south of the fault, and Eocene monzonite and dacite are locally present high in the northeast corner of the area. Oligocene andesitic basalt and tuffaceous sedimentary rocks of Oligocene and Miocene age are present in a northwest-trending paleo valley of probable late Eocene age in the northwestern third of the quadrangle and in the southeast corner of the quadrangle. The Volcano Valley fault zone is a complex zone of dominantly high-angle faulting, active during several intervals of geologic time. Prior to deposition of the Flathead Sandstone, the area on north side of the fault zone moved up, so that erosion removed Mesoproterozoic rocks on that side to expose Paleoproterozoic rocks before deposition of the Middle Cambrian strata. Movement along the zone recurred in early Eocene time as the south block moved up and east. Subsequently likely in late Eocene time, the block north of the Volcano Butte fault zone arched upward as normal faults north of that fault accommodated movement along the margin of the arch. A parallel fault zone, the Black Butte Creek fault zone, diverges from, and extends parallel to the Volcano Valley fault zone across the southern part of the quadrangle. The Black Butte Creek fault zone displays both left lateral strike slip movement and normal movement up on the south side. The two major fault zones together form a segment of the northern margin of the Helena structural salient in west-central Montana. Alluvium and terrace gravel of Holocene and Pleistocene age are present along the northwest courses of both Black Butte Creek and Sheep Creek and the southeast course of Newland Creek. Boulder deposits, derived from areas as far as 10-12 km to the east, are present in the southwest part of the area. Debris from excavations in iron gossan mantles slopes on the south block of the Volcano Valley fault zone. The debris and mapped iron-stained bedrock of the Newland Formation have drawn attention to the potential occurrence of economic sulfide mineralization in the area. Subsequent to mapping the quadrangle, an exploration company has identified in the subsurface a limited area of copper- and cobalt-enriched rock in the Newland Formation in the southeast part of the quadrangle and has proposed to mine the deposit. The current geologic quadrangle map defines a surface geologic baseline from which exploration and resource agencies alike can potentially evaluate the areal distribution of possible mineralization as well as the environmental effects of development of a proposed mine.