The American AssocialidD of Petroleum Geologists Bulletin

V. 60. No. 12 (DecenibiT \^^b>. }• :i42-2166. 17 Figs., ? Tables

GEOLOGIC NOTES

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 fold­ing, fault-transported slices, splinter faults, and de-collement have resulted from fault movement. The wide variety of both brittle and ductile structures com­monly 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.

INTRODUCTION

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, defin­ing 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) sam­pled 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 strati­graphic 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 Forma­tion. The stratigraphic displacement decreases both to the north and south over an exposed trace of 6 mi (10 km), but the stratigraphic displace­ment 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 ex­plained best if this part of the fault surface is en­visioned 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

2142

Geologic Notes 2143

Divio*

LEGEND Oho- Hillwash Alluvium TOu- Undifferentiated Tv - Volconics Kc - Colorado Group Kk - Kootenai Formotion Jm - Morrison Formotion l̂ d - Oinwoody Formation Pp - Ptiosphorio Formotion

Anticline

—e—' Syncline

Overturned Anticline

— y — • Overturned Synclin*

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. 1—A, 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 subsur­face 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 duc­tile structures abound at the site of the Sandy Hollow step (Figs. 4, 6).

2144 Geologic Notes

•? ^W- A\ -^

€ ^H

C'ot* SfciA : ' " ! " • " ': % - , ! '.

•j*f=-

• « . » _ .

No Ver t ica l Exaggeration

FIG. 2—Aerial 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

DjnKleberg

COLORADO GROUP

KOOTENAI FM.

gastropod Is.

MORRISON FM,

DINWOODY FM.

PHOSPMORIA FM.

baiol chart

QUADRANT QUART2ITE

AMSDEN FM

l«50'S=:^:=

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

LIMESTONE

f ^ — I SILTSIONE

| ^ = V ^ LIM* SiLISTONE

SILTY LIMESTONE

DOLOMtTE

CHERT

COAL

FIG. 3—Generalized 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 addi­tion, 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 lime­stone was torn from the west flank of the underly­

ing Sandy Hollow anticUne and was stretched across the fault-truncated edges of the lower Col­orado Group strata of the overridden block on the platform of the fault's step (Fig. 4). Pieces of hanging-wall basal Kootenai conglomerate, bro­ken 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. 4—Aerial 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 For­mation, 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 gas­tropod limestone were folded strongly and pushed eastward over the lower part of the gas­tropod limestone (Figs. 4, 7). The dominantly up­ward movement of the overriding block at the step-fault riser changed to nearly horizontal movement over the step-fault platform. The over­riding 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. 5—View northward at outcrop of Sandy Hollow thrust and .issociated minor faults along Burma Road (for location see Fig. 2)

'Fault Surface

ndy Hollow Thrust

Thrust Tributary to Sondy Hollow Foult

FIG. 6—Diagrammatic sketch of Sandy Hollow fault plane as it would appear if rocks of upper plate were removed. Sketch is based on tracing from oblique air photo.

2148 Geologic Notes

=•' - j i i f^ -r

m-FIG. 7—View eastward at B (Fig. Ic) of gastropod limestone at top of riser of Sandy Hollow step.

Middle and upper parts of gastropod limestone crumpled and slid over its lower layers as overriding block of Sandy Hollow thrust surmounted top of riser.

ded Dinwoody Formation as that unit sHd across the Sandy Hollow step above the splinter faults (Fig. 4).

This ductile response is in strong contrast to the more brittle splintering effect along the fault zone and to the tear faulting of the overriding block (Figs. 4, 6). Field evidence (Brumbaugh, 1973) from the area of the Sandy Hollow thrust clearly indicates shallow deformation—10,000 ft (3,048 m) or less. In such a tectonic environment, mechanical behavior becomes varied and depen­dent on Uthology. Quartzite and dolomite re­spond in a more brittle fashion, whereas lime­stone behaves in a ductile manner. Thus, the wide variety of structures related to the Sandy Hollow step is more a function of the temperature and pressure than a necessary product of step me­chanics. On the other hand, steps in general tend to be present in thin-skinned environments, so such an association of structures with steps prob­ably is anything but unique.

SEQUENCE OF FAULTING AND FOLDING

The structural relations at the steps of the Sandy Hollow thrust indicate that the fault suc­ceeded much of the folding of Sandy Hollow and Ziegler anticlines. The fault plane at best is folded only sUghtly. The fault was controlled by the folds, for the risers of its steps are located over underlying anticlinal structures, and the plat­forms of its steps across intervening synclines (Fig. 2). A further indication that the fault post­dates much of the folding is shown by the rela­tions of the fault plane to the beds it cuts. At the risers of the steps it dips more steeply than the

beds it cuts; at the platforms of the steps it dips less steeply than the beds it cuts. In addition, its trace cuts at a slight angle across the axes of Zie­gler and Sandy Hollow anticlines (Figs. 2, 6). These relations would be unlikely if the fault oc­curred first and then was folded with the beds.

SUMMARY

This report has described a Laramide thrust or step fault and the structures produced by step-fault mechanics. Such structures include decoUe-ment, splinter faults, smaller scale folding, and fault transported slices. These step-fault struc­tures should be associated quite commonly with steps everywhere because they are the predictable results of step-fault mechanics in the restricted temperature-pressure range in which thrust faults form, marginal to orogenic belts.

The sequence of faulting and folding for the Sandy Hollow fault is indicated quite clearly from field evidence. The classic model of thrust-fault steps is that of the Pine Mountain fault (Rich, 1934) where folding follows faulting. The case for the Sandy Hollow fault is one in which previously existing folds controlled the devel­opment of step zones in the subsequent thrust fault. Such a control by folds on the development and location of step zones provides an interesting alternative to the classic sequence of development of steps as envisioned by Rich for the Pine Moun­tain fault.

REFERENCES CITED

Brumbaugh, D. S., 1973, Structural analysis of the com­plexly deformed Big Hole River area, Madison, Bea-

Geologic Notes 2149

verhead, and Silver Bow Counties, Montana; PhD thesis, Indiana Univ., 96 p.

Gale, H. S., 1910, Rock phosphate near Melrose, Mon­tana: U.S. Geol. Survey Bull. 470, p. 440-451.

Peterson, J. A., R. F. Gosman, and R. W. Swanson, 1954, Stratigraphic sections of the Phosphoria For­mation in Montana, 1951: U.S. Geol. Survey Circ. 326, 27 p.

Rich, J. L., 1934, Mechanics of low-angle overthrust

faulting as illustrated by Cumberland thrust block, Virginia. Kentucky, and Tennessee: AAPG Bull., v. 18, p. 1584-1596. ''

Richards, R. W., and J. T. Pardee, 1925, The Melrose phosphate field. Montana: U.S. Geol. Survey Bull. 780A, p. 1-32.

Steuer, P., 1956, Geology of the McCarthy Mountain area, Beaverhead and Madison Counties, Montana: Master's thesis, Univ. Utah, 71 p.

Alaska Geological Society History^

REGINALD W. ELKINS>

Anchorage, Alaska 99501

The first meeting of the Alaska Geological So­ciety was held in Anchorage December 3, 1957. Those present and their affiliations were: Waring Bradley, Aledo Oil Co.; Henry T. Herlyn, Phillips Petfoleum Co.; Phil O'Rourke, Phillips Petroleum Co.; Ruth A. M. Schmidt, U.S. Geological Sur­vey; Robert S. Velikanje, Dist. Engineers; Roger M. Waller, U.S. Geological Survey; Armour C. Winslow, Humble Oil & Refg. Co.; William Zae-gel, Dist. Engineers. This group held meetings on a regular basis with Roger Waller, program chair­man, and Ruth Schmidt, secretary.

On October 7, 1958, membership dues were ap­proved and Ruth Schmidt was elected the first president. On November 8, 1960, a membership of 39 approved a statement of purpose and by­laws. The group has grown steadily to the present total of 350 members.

Diversity has been a source of strength for the Alaska Geological Society. Geologists from the academic, mining, petroleum, and engineering fields are represented in its membership.

The society grew as geologists found that meet­ings offered exposure to the many facets of Alas­kan geology and the opportunity to meet other Alaskan geologists. The program chairman some­times was at a loss to think of a subject appealing to all the geologic interests represented but it soon was apparent that a good paper on any branch of geologic investigation captured the at­tention of title entire group.

The society now meets regularly twice a month except during the summer when special meetings are held at the call of the president.

Much of the work of the society is done by standing committees. Noteworthy contributions have been made by the Stratigraphic and Publica­tions Committees which have prepared and pub­

lished structure and stratigraphic studies and guidebooks. The Continuing Education Commit­tee has obtained world-renowned speakers and presented outstanding seminars and special pro­grams for the membership.

The society is not interested solely in itself. Last year the Community Services Committee ini­tiated a volunteer-geologist speaker pool and pro­vided the Anchorage Borough School District with science and geology classes by many lectur­ers on a variety of subjects.

Some significant milestones of society growth are as follows;

Initial meeting—Dec. 3, 1957 First full slate of officers chosen—Oct. 7, 1958 Bylaws and statement of purpose accepted—Nov. 8,

1960 Affiliation with AAPG Pacific Section—May 1961 Guidebook and Road Log from Anchorage to Sutton

published—1963 Guidebook and Road Log from Sutton to Caribou

Creek published—1964 First Distinguished AAPG Lecturer, Robert J. Weim-

er—Mar. 9, 1964 First AGI Visiting International Scientist Lecturer,

Umberto Colombo—May 22, 1964 Society Emblem chosen—Oct. 15, 1964 First Membership Directory published—1965 Second Membership Directory published—1967 Central Cook Inlet Basin Structure Section pub­

lished—1967 First symposium on Alaska tectonics held—Feb.

1968

©Copyright 1976. The American Association of Petroleum Geologists. All rights reserved.

•Manuscript received, March 30, 1976; accepted, April 19, 1976.

^Cities Service Company.