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The American AssocialidD of Petroleum Geologists Bulletin V. 60. No. 12 (DecenibiT \^^b>. } :i42-2166. 17 Figs., ? Tables
Exposed Step in Laramide Thrust Fault, Southwest Montana'
D. S. BRUMBAUGH^ and H. W. DRESSER3 Flagstaff, Arizona 86001, and Butte, Montana 59701
Abstract .The Sandy Hollow thrust fault of soutfiwest-ern Montana Is a Laramide overthrust that possesses an unusually well-exposed step. In the step zone fold-ing, fault-transported slices, splinter faults, and de-collement have resulted from fault movement. The wide variety of both brittle and ductile structures com-monly should be associated with steps because they are the inevitable result of step-fault mechanics in the restricted temperature/pressure environment in which thrusts form. Field evidence clearly indicates that preexisting folds controlled the development of this step zone in the Sandy Hollow thrust.
Features associated with thrust faults include fensters, klippen, gouge, various kinds of folds, and steps. This report deals with the examination of structures related to an unusually well-exposed step, which is part of a Laramide overthrust fault of the northern Rockies.
From Dillon to Divide, Montana, Laramide fold axes and thrust faults bend eastward, defin-ing a structural salient called the McCarthy Mountain saUent (Brumbaugh, 1973). The salient has a chord length of 30 mi (48 km) and is 10 mi (16 km) wide from east to west (Fig. IB).
This report concerns the exposed geometry of one fault of the McCarthy Mountain sahent, the Sandy Hollow thrust (Figs. IB, C, 2). Gale (1910) was apparently the first to map the Sandy Hollow thrust. Pardee (Richards and Pardee, 1925) used Gale's field notes and geologic map to describe the geology of the area. Peterson et al (1954) sam-pled and measured the phosphatic Permian rocks adjacent to the Sandy Hollow fault. Steuer (1956) mapped the area including the fault at a scale of 1:24,000.
SANDY HOLLOW THRUST AND RELATED STRUCTURES
Apparent stratigraphic displacement varies considerably along the trace of the Sandy Hollow thrust. Thicknesses of stratigraphic units in the area are given in Figure 3. The maximum strati-graphic displacement on the Sandy Hollow thrust is approximately 1,400 ft (427 m). This can be seen along the Burma Road (Fig. 2) where the
Phosphoria Formation is thrust over the hme-stone in the middle part of the Kootenai Forma-tion. The stratigraphic displacement decreases both to the north and south over an exposed trace of 6 mi (10 km), but the stratigraphic displace-ment probably bears little relation to the actual displacement of a thrust fault that cuts folded beds.
The most geometrically interesting exposures of the Sandy Hollow thrust are at the northern end of the fault trace. There, the fault trace swings abruptly from north-northeast to nearly due east (Figs. IC, 2. 4). An obvious explanation for this change in trend would be erosional effects on an irregular low dip surface. However, field evidence does not support this. The fault as it is exposed at the Burma Road (Fig. 5) has a dip of 50. Between the Burma Road and the locale where the change in strike occurs, the trace of the fault across the topography is straight, indicating that the high dip angles are maintained along this part of the fault. The change in fault trend is ex-plained best if this part of the fault surface is en-visioned as a plunging step (F"ig. 6). Proof of the plunging character of the surface can be seen at an exposure of the fault plane in a dry wash east of the change in strike (at the circled A in Figs. IC, 4). This outcrop clearly shows a northerly dip of the fault surface. Two mi (3 km) farther east the trend of the fault trace changes to north-northeast again al the base of Block Mountain (Figs. IC, 2, 4). This is the base of a second step, the Block Mountam step
"Copyright 1976. Tiie American Association of Petroleum Geologists. All rights reserved
IManuseript received, September 29. 1975; accepted, April 19, 1976.
^Department of Geology, Northern Arizona University. 3Department of Geology, Montana College of Mineral
Science and Technology. This study was supported by grants from the Society of Sigma
Xi and Ihe Office of Research and Advanced Studies, Indiana University
Geologic Notes 2143
LEGEND Oho- Hillwash Alluvium TOu- Undifferentiated Tv - Volconics Kc - Colorado Group Kk - Kootenai Formotion Jm - Morrison Formotion l^ d - Oinwoody Formation Pp - Ptiosphorio Formotion
Overturned Anticline y
Thrust Contact of Sondy Hollow Thruet with "T"on Upper Plote
Decollement Contact Arrow shows direction of motion
A H Locotion of Crois Section, Fig. Z
A/2 km SCALE
FIG. 1A, Index map of Montana. B, Sketch map of McCarthy Mountain salient. C, Geologic map of northern part of Sandy Hollow thrust (T4S,R8W, Sees. 22, 23, 26, 27). Circled letters represent critical localities described in text.
The cross section of Figure 2 shows a subsur-face step of the Sandy Hollow thrust along the Une from A to H just north of the Big Hole River Valley. This hidden step is required to place the west flank of Ziegler anticline against the west flank of Sandy Hollow anticline.
It is instructive to examine this short stretch of the Sandy Hollow thrust to observe some of the
structural phenomena associated with these steps and jX)ssibly characteristic of steps in general. Field observation of the Sandy Hollow thrust suggests that step zones are zones of difficulty of fault movement with the attendant production of a wide variety of structures. Both brittle and duc-tile structures abound at the site of the Sandy Hollow step (Figs. 4, 6).
2144 Geologic Notes
? ^W- A\ -^
C'ot* SfciA : ' " ! " " ': % - , ! '.
. _ .
No Ver t ica l Exaggeration
FIG. 2Aerial oblique view northward across Big Hole River of Sandy Hollow thrust and associated structures. Cross section shows interpretation of structure along hne from A to H, just north of Big Hole River Valley. Pq = Quadrant Quartzite, Pp = Phosphoria Formation, Trd = Dinwoody Formation, Jm = Morrison Formation, Kk = Kootenai Formation, Kc = Colorado Group, Tg = Tertiary gravel. For location see Figure IB.
Geologic Notes 2145
90 - ^ - ^ ^ ^ J m
>w< f> t?
L E G E N D
[ V V v j POHLLUANtTE
^ j ? l SHALE
K ^ V ] SflNL'STONE
\o ^^ CONGLOME^'A^E
f ^ I SILTSIONE
| ^ = V ^ LIM* SiLISTONE
FIG. 3Generalized stratigraphic column of McCarthy Mountain area.
In turning the corner at the top of the Sandy Hollow riser, the main thrust has splintered into several subsidiary branches (Figs. 4, 6). In addi-tion, large sHces of material have been caught up in and moved along the thrust plane and lodged on the platform of the Sandy Hollow step. A large irregular sheet of upper Kootenai gastropod lime-stone was torn from the west flank of the underly-
ing Sandy Hollow anticUne and was stretched across the fault-truncated edges of the lower Col-orado Group strata of the overridden block on the platform of the fault's step (Fig. 4). Pieces of hanging-wall basal Kootenai conglomerate, bro-ken from the fault-truncated edge of the steep east flank of Ziegler anticline as it moved across the platform, lie discordantly on the gastropod
2146 Geologic Notes
FIG. 4Aerial oblique view northward of Sandy Hollow thrust in area between step in lower middle part of picture and Block Mountain step near right edge. Pp = Phosphoria Formation, Trd = Dinwoody Formation, Jm = Morrison Formation, Kk = Kootenai For-mation, Kkc^l = basal chert pebble conglomerate member of Kootenai Formation, Kkgls = gastropod limestone member of Kootenai Formation at its top, Kc = Colorado Group, Kcss = prominent channel sandstone in lower part of Colorado Group, Tg = Tertiary gravel. Circled A location is same as that on Figure IC. Length of side of photo is approximately 0.86 mi (1.4 km).
limestone and on the Colorado Group shale. A piece of Colorado Group sandstone torn from the upturned edge of a bed in the overridden block below the step platform was carried eastward and lodged against the riser of the Block Mountain step (Fig. 4). Tear faults (Figs. 2, 4) are confined to the overriding block. They represent velocity discontinuities that may result from local binding of the allochthon as it slides over the step.
In addition to these brittle structures, folds were produced in the overridden block at the top of the riser of the Sandy Hollow step (at circled B in Fig. IC). A bedding surface within the gastro-
pod limestone clearly served as a surface of de-collement. The middle and upper parts of the gas-tropod limestone were folded strongly and pushed eastward over the lower part of the gas-tropod limestone (Figs. 4, 7). The dominantly up-ward movement of the overriding block at the step-fault riser changed to nearly horizontal movement over the step-fault platform. The over-riding of the top edge of the step exerted a nearly horizontal stress against the upper part of the step, crumphng the gastropod limestone that here formed this part of the step (Fig. 7). Many fold structures also were produced in the thinly bed-
Geologic Notes 2147
FIG. 5View northward at o