Document Type

Report

Journal/Book Title/Conference

Montana Bureau of Mines Open File report 525

Publisher

Montana Bureau of Mines and Geology

Publication Date

2005

First Page

1

Last Page

28

Abstract

Geologic Map of the Bachelor Mountain 7.5’ Quadrangle Beaverhead County, Montana Geologic Summary The Bachelor Mountain quadrangle is located in the Horse Prairie Creek drainage basin of southwest Montana and contains exposures of Middle Proterozoic metasedimentary rocks and eighteen map units of Eocene to Oligocene basin-fill deposits assigned to the Medicine Lodge beds. Locally, overlying upper Oligocene to lower Miocene(?) Everson Creek beds and late early to early late Miocene Bannack Pass beds are also preserved as basin-fill deposits. The Medicine Lodge beds were deposited in an early basin, the Grant protobasin (Fig. 1a), during slip on the Muddy-Grasshopper detachment fault. This fault is listric, flattens at ~1.5 km depth, has a regional extent, and has 5-10 km of slip. The Muddy-Grasshopper detachment fault lies east of the map area and formed the eastern margin of the Grant protobasin (Fig. 1a). The Grant protobasin began to break up internally shortly after valley-filling 27.5 to 27.8 Ma basalts were deposited at the top of the Medicine Lodge beds. The Everson Creek beds were deposited during this later breakup of the hanging wall of the Muddy-Grasshopper detachment fault (VanDenburg and others, 1998; Janecke and others, 1999; Matoush, 2002) in an event that formed the present-day Medicine Lodge, Horse Prairie, and Grasshopper structural subbasins (Fig. 1b, c). The Barstovian Bannack Pass beds reflect infilling of remnant lows in these structural subbasins after basin-bounding normal faults had stopped slipping (VanDenburg and others, 1998). The quadrangle lies within the Grasshopper structural subbasin, in a major north-trending rift zone of Paleogene age (Janecke, 1994) (Fig. 1c), and contains more synrift Medicine Lodge and Everson Creek beds than post-rift Bannack Pass deposits. The Grant protobasin (Beaverhead basin of Kickham, 2002, and Matoush, 2002) is the original sedimentary basin that formed in the hanging wall of the Muddy-Grasshopper fault (Fig. 1a). Sedimentary rocks of the former Grant protobasin are now preserved in the younger Medicine Lodge, Horse Prairie, and Grasshopper structural subbasins that segmented it. The Grant protobasin once extended across parts of the adjacent mountain ranges (M’Gonigle and Dalrymple, 1993). The Maiden Peak spur, parts of the Bloody Dick Creek divide area, and the Beaverhead Range west of upper Horse Prairie Creek were once covered by sedimentary rocks of this protobasin (M’Gonigle and Dalrymple, 1993, 1996; VanDenburg, 1997; Kickham, 2002; Matoush, 2002). Late Oligocene normal faulting in the hanging wall of the Muddy-Grasshopper detachment fault dropped the Medicine Lodge beds into the Grasshopper, Medicine Lodge, and Horse Prairie structural subbasins (Fig. 1b). These three structural subbasins are essentially east-tilted half grabens but the Grasshopper structural subbasin is more complex because of basin-scale extensional folds, a large east-dipping normal fault on its western margin, and several smaller west-dipping normal faults that duplicate strata on the east limb of the Bachelor Mountain anticline (Fig. 1c). Bachelor Mountain quadrangle lies near the boundaries of these three structural subbasins, in the southwest part of the present-day Grasshopper geographic basin (Fig. 1c). Quaternary alluvial, fluvial, and hillslope deposits cover large portions of the quadrangle and obscure the older rocks. Pediment gravel deposits form the most laterally extensive Quaternary deposits and were laid down by streams draining into Horse Prairie. Multiple levels of pediment gravel show that the regional base level dropped progressively during the Quaternary.

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