Abstract:
The geologic map of the Hogback Mountain quadrangle, scale 1:24,000, was made as part of the Montana Investigations Project to provide new information on the stratigraphy, structure, and geologic history of an area in the geologically complex southern part of the Montana disturbed belt. In the Hogback Mountain area, rocks ranging in age from Middle Proterozoic through Cretaceous are strongly folded within and under thrust plates of equivalent rocks. Continental rocks of successive thrust plates have been telescoped eastward over a buttress of the stable continent. Erosional remnants of Oligocene andesitic basalt lie on highest surfaces eroded across the strongly deformed older rocks; younger erosion has dissected the terrain deeply, producing Late Tertiary and Quaternary deposits of alluvium, colluvium, and local landslide debris in the valleys and canyons.
Different stratigraphic successions are exposed at different structural levels across the quadrangle. In the northeastern part of the quadrangle at the lowest structural level, rocks of the Upper Mississippian Big Snowy Group, including the Kibbey Formation and the undivided Otter and Heath Formations, the overlying Pennsylvanian Amsden and undivided Quadrant and Phosphoria Formations, the Ellis Group, and the Kootenai Formation, are folded and broken by thrust faults. The next higher structural level, the Avalanche Butte thrust plate, exposes strongly folded and, in places, attenuated strata of Cambrian (Flathead Sandstone, Wolsey Shale, Meagher Limestone, and undivided Pilgrim Formation and Park Shale), Devonian (Maywood Formation, Jefferson Formation, and most of the Three Forks Formation), and Mississippian (uppermost part of the Three Forks Formation and Lodgepole and Mission Canyon Limestones) ages. The overlying Hogback Mountain thrust plate contains strongly folded rocks ranging in age from the Middle Proterozoic Greyson Formation to the Upper and Lower Mississippian Mission Canyon Limestone and Cretaceous diorite sills. The highest structural level, the Moors Mountain thrust plate, contains the Middle Proterozoic Greyson and Newland Formations and discontinuous Upper Proterozoic diabase sills.
Rocks are complexly folded and faulted across the quadrangle. At the lowest level in the northeastern part of the quadrangle, Upper Mississippian and younger strata are folded along northwest-trending axes and broken by thrust faults that at outcrop level displace the same rocks. The central core of the quadrangle is formed by the Avalanche Butte thrust plate, which contains recumbently folded and thrust faulted Paleozoic rocks. A succession of four tight recumbent folds within the plate have axial traces that trend northwest and north-northwest, and that are both arched and downfolded along east- and northeast-trending axes. Carbonate rocks of the Mission Canyon and Lodgepole Limestones in the upper part of the Avalanche Butte thrust plate exposed in the canyon of Trout Creek are folded and attenuated in stacked east-directed recumbent folds that developed as a succession of folded duplex thrust slices. The exposed remnant of the next higher structural level, the Hogback Mountain thrust plate, contains northeast- and east-trending folds that are inverted on the upper overturned limb of a younger northwest-trending recumbent fold. The Hogback Mountain thrust fault is itself folded and, in its northernmost exposures, is overturned to dip west beneath the overlying Moors Mountain thrust plate. During post-middle Tertiary deformation, the Hogback Mountain thrust fault moved as a normal fault, down on the east. The structurally highest Moors Mountain thrust plate rests on the Avalanche Butte thrust plate in the southwestern part of the quadrangle and across both the Avalanche Butte and Hogback Mountain thrust plates along the northwest edge of the quadrangle. In the central eastern part of the map area, the edge of a large klippen of the Moors Mountain thrust plate rests on a major inverted syncline of the underlying Hogback Mountain thrust plate. At the base of both the contiguous thrust plate and its klippen, the bounding Moors Mountain thrust fault ramps steeply upward from south to north across a structural culmination in underlying thrust plates.
In the southwest corner of the quadrangle, strata of the Greyson Formation are faulted down on the west along a normal fault that, in the Nelson quadrangle adjacent on the west, is listric down to the west with the Moors Mountain thrust fault. Movement on the normal fault is younger than middle Oligocene.
Purpose:
The geologic map of the Hogback Mountain quadrangle provides new geologic information for public use and for use by governmental agencies for land-use planning, including assessment of natural resources, natural hazards, and recreational potential, and for land-use management. The map provides an interpretation of the complex geology and terrain to recreationalists using the Gates of the Mountains Wilderness. The map provides data for scientific interpretation of the geology of part of the south end of the Montana disturbed belt.
Supplemental_Information:
Map political location: Lewis and Clark and Meagher Counties, Montana
Map scale: 1:24,000
Geology mapped in 1976 and 1977
Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government.
Although this Federal Geographic Data Committee-compliant metadata file is intended to document the data set in nonproprietary form, as well as in ARC/INFO format, this metadata file may include some ARC/INFO-specific terminology.
GEOSPATIAL FILES INCLUDED IN THIS DATA SET:
hogpoly: polygon coverage containing geologic unit contacts, faults, dikes, and labels
hogline: arc coverage containing cross sections, anticlines, synclines
hogpoint: point coverage containing strike and dip attitudes of bedding and foliation, and other point-data map entities
Theme:
Theme_Keyword_Thesaurus: USGS Thesaurus
Theme_Keyword: geologic maps
Theme_Keyword: geologic structure
Theme_Keyword: bedrock geologic units
Theme_Keyword: faulting (geologic)
Theme_Keyword: folding (geologic)
Theme_Keyword: tectonic processes
Theme:
Theme_Keyword_Thesaurus: American Geological Institute (AGI) Glossary of Geology
Theme_Keyword: Montana disturbed belt
Theme_Keyword: Lewis and Clark line
Theme_Keyword: thrust fault
Theme_Keyword: thrust plate
Theme_Keyword: recumbent fold
Theme_Keyword: klippen
Theme_Keyword: strike-slip fault
Theme_Keyword: inverted fold
Theme_Keyword: detachment zone
Theme_Keyword: Moors Mountain thrust fault
Theme_Keyword: Moors Mountain thrust plate
Theme_Keyword: Hogback Mountain thrust fault
Theme_Keyword: Hogback Mountain thrust plate
Theme_Keyword: Avalanche Butte thrust plate
Theme:
Theme_Keyword_Thesaurus: ISO 19115 Topic Categories
Theme_Keyword: geoscientificInformation
Place:
Place_Keyword_Thesaurus:
U.S. Board of Geographic Names (BGN) Geographic Names Information System (GNIS)
Place_Keyword: USA
Place_Keyword: Montana
Place_Keyword: Lewis and Clark County
Place_Keyword: Meagher County
Place_Keyword: Big Belt Mountains
Place_Keyword: Gates of the Mountains Wilderness
Place_Keyword: Hogback Mountain
Place_Keyword: Middleman Mountain
Place_Keyword: Beaver Creek Big Belt Mountains
Place_Keyword: Trout Creek Big Belt Mountains
Place_Keyword: Getty Oil Company No. 3-10 Federal Hogback Mountain drill hole
Place:
Place_Keyword_Thesaurus: Augmented FIPS 10-4 and FIPS 6-4
Place_Keyword: f30049 = Lewis and Clark
Place_Keyword: f30059 = Meagher
None. However, citation of publication reference and acknowledgment of the U.S. Geological Survey would be appreciated in all products that use or are derived from these data. These data should not be used at scales larger than 1:24,000.
Radiometric dating of samples from adjacent areas applicable to this quadrangle was performed by Harald H. Mehnert, U.S. Geological Survey. Technical reviews by Karl K. Kellogg and David A. Lindsey, U.S. Geological Survey, helped improve the quality of the geologic map, illustrations, and text. Sharon A. Smith graciously and painstakingly translated the map data into digital files. Carlos Vulgamott made preliminary digital copy of the map and cross sections.
