Geologic Map of the Hogback Mountain Quadrangle, Lewis and Clark and Meagher Counties, Montana

• Main Author: Mitchell W. Reynolds
• Format: Dataset
• Language: unknown
• Published: USGS Science Data Catalog 2003
• Subjects: geologic maps; geologic structure; bedrock geologic units; faulting (geologic); folding (geologic); tectonic processes; Montana disturbed belt; Lewis and Clark line; thrust fault; thrust plate; recumbent fold; klippen; strike-slip fault; inverted fold; detachment zone; Moors Mountain thrust fault; Moors Mountain thrust plate; Hogback Mountain thrust fault; Hogback Mountain thrust plate; Avalanche Butte thrust plate; geoscientificInformation; USA; Montana; Lewis and Clark County; Meagher County; Big Belt Mountains; Gates of the Mountains Wilderness; Hogback Mountain; Middleman Mountain; Beaver Creek Big Belt Mountains; Trout Creek Big Belt Mountains; Getty Oil Company No. 3-10 Federal Hogback Mountain drill hole; f30049 = Lewis and Clark; f30059 = Meagher; Quadrangle; Buttress; The Quadrangle; Beaver Creek
• Online Access: https://search.dataone.org/view/96178a2b-6681-4e69-9a6a-a84c72572a15

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.