WILLIAM J. Mcmannis Montana State College, Bozeman, Montana

Home , Big Belt Mountains, Bridger Range, Horseshoe Hills

WILLIAM J. McMANNIS Montana State College, Bozeman, Montana

LaHood Formation—A Coarse Fades

of the Belt Series in Southwestern Montana

Abstract: Precambrian arkose and arkose con- up to mid-Greyson. These correlations suggest glomerate that contrast sharply with the com- relationships between rocks of the Philipsburg monly fine-grained units of the Belt Series crop district and the type Belt that are quite different out in a narrow zone from the Bridger Range on from those of other authors. the east to the Highland Mountains on the west in The LaHood formed in a partly fault-controlled southwestern Montana. The name LaHood Forma- eastward embayment of the Belt geosyncline. The tion is satisfactory for these beds, but the formation detritus was derived from a rugged source area of is clarified and redefined, and a principal reference older Precambrian metamorphic rocks to the south. section is introduced to indicate more complete, It was dumped by torrential streams into the more accessible exposures. The names North differentially subsiding marginal part of the Belt Boulder Group and East Gallatin Group should sea and was redistributed to more stable bottom be abandoned. areas by slumping and turbidity currents. The LaHood represents a long time-stratigraphic Sedimentary characteristics, regional truncation interval. Its intertonguing relationships with typical and local angular unconformities beneath the Flat- fine-grained Belt formations, and data regarding head Quartzite indicate regional northward tilting, pre-Flathead (Middle Cambrian) truncation of broad folding, and faulting during and after deposi- Belt strata, permit correlations that show that tion of Belt sediments but before the Flathead LaHood equivalents range from oldest known Belt Quartzite formed.

CONTENTS Introduction 408 6. Pre-Flathead restored section showing strati- Previous work 408 graphic relationships of LaHood facies, Acknowledgments 408 Highland Mountains to Bridger Range . 427 Coarse facies of the Belt Series (LaHood Formation) 408 7. Pre-Flathead restored section showing strati- Discussion 408 graphic relationships of LaHood facies, Description by area 415 Highland Mountains to type LaHood Stratigraphic relationships of the coarse facies. , 425 section 428 Introduction 425 8. Pre-Flathead restored section showing strati- Intertonguing relationships 425 graphic relationships of LaHood facies, Big Discussion of restored sections 426 Belt Mountains to Corbly Canyon . . . 429 Depositional environment and sequence .... 433 9. Pre-Flathead restored section showing strati- Environment of deposition 433 graphic relationships of LaHood facies, Sequence of deposition 433 Canyon Ferry to Dry Creek 430 Summary and conclusions 434 10. Pre-Flathead restored section showing strati- References cited 435 graphic relationships of LaHood facies, Appendix: Principal reference section of the La- Elkhorn Mountains to Bone Basin . . .431 Hood Formation 436 11. Pre-Flathead restored section showing stratigraphic relationships of LaHood facies, Figure Flint Creek Range to Highland Mountains 432 1. Outline map showing Belt outcrops, gross facies distribution, and location of restored strati- Plate Facing graphic sections, Montana .... 410 1. Outcrops and thin sections of LaHood Forma- 2. Generalized form isopach map of Belt deposits, tion, Montana 418 central Montana embayment . . . 412 2. Algal structure, "molar-tooth" structure, and 3. Belt, Cambrian, and pre-Belt relationships boulders in LaHood Formation, Montana . 419 along Pass fault in the central Bridger Range, Montana 415 Table 4. Correlation chart for LaHood Formation . . 420 1. Average mineral composition of LaHood arkose 5. Geology of a part of the Highland Mountains, by areas in Montana 414 Montana 423

Geological Society of America Bulletin, v. 74, p. 407-436, 11 figs., 2 pis., April 1963 407

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overlying Cambrian beds that he took to sup- INTRODUCTION port the Late Precambrian age of the Belt. His A very coarse clastic sedimentary facies in cross section (1935, PL 8, fig. 4) shows the very the Belt Series of southwestern Montana con- significant regional unconformity in which trasts markedly with generally fine-grained progressively older Belt units are truncated rocks of the Belt to the north and west. from north to south, but he made no mention Local studies of small parts of these rocks of the coarse facies. have been made mainly in connection with Sahinen (1938) discovered Belt arkose nearly areal mapping problems (Alexander, 1955; as far west as 113° W. long., in the Highland Berry, 1943; H. D. Klemme, 1949, Ph.D. Mountains south of Butte, Montana (Fig. 1). thesis, Princeton Univ.; Lowell, 1956; Mc- In the late 1940's and early 1950's new inter- Mannis, 1955; Perry, 1950; Sahinen, 1938; est in the coarse facies was generated by the 1950; Peter Verrall, 1955, Ph.D. thesis, Prince- work of several Princeton University Ph.D. ton Univ.; G. D. Robinson, in press). Recent candidates (Alexander, Klemme, McMannis, but unpublished field work by W. R. Lowell Verrall) and by U. S. Geological Survey and in the Jefferson Island quadrangle and G. D. Indiana University men (Klepper, Robinson, Robinson in the Toston quadrangle has sup- Lowell). plied additional information. This paper at- tempts to synthesize existing data and to clarify ACKNOWLEDGMENTS the regional relationships and paleogeographic I wish to thank the National Science Foun- significance of the coarse facies. dation for generous financial support (Grant No. NSF- G6568) and Montana State College PREVIOUS WORK for administration of the grant and for use of Hayden (1861) mentioned the coarse-grained laboratory and library facilities. Appreciation strata and named them (1884) the East Galla- is also expressed to R. G. Alexander, A. B. tm Group. Van Hise (1892, p. 285) pointed Berg, R. N. Miller, C. P. Ross, U. M. Sahinen, out the significance of the coarse-grained beds and Peter Verrall for published and unpub- when he noted that "... the ancient shore line lished data and germs of ideas. Particular crossed the area from 12 to 15 miles south of thanks are due G. D. Robinson of the U. S. the northern limit of the Three Forks sheet." Geological Survey and W. R. Lowell of Indiana Thus he recognized that the coarse facies repre- University for use of unpublished map data sents a near-shore deposit, and that the source concerning location of Belt outcrops, and for area lay to the south where Belt rocks are discussions of mutual problems. absent. D. B. Andretta and D. L. Keil served as Peale (1893) described the East Gallatin field assistants; A. W. Shelden, S. A. Alsup, Group in the Horseshoe Hills and northern K. L. Hudson, and P. A. Glancy helped with Bridger Range, and mentioned a similar se- laboratory work. C. Tacke typed the manu- quence in Jefferson Canyon, suggesting that script, and Jean Myers drafted the illustrations. they probably are Late Precambrian and that The manuscript was critically read by John de they are overlain by Middle Cambrian beds. la Montagne, R. A. Chadwick, M. J. Edie, and G. P. Merrill (in Peale, 1893, p. 49) added G. D. Robinson. general petrographic characteristics of the coarse arkose. COARSE FACIES OF THE BELT Peale (1896) repeated his description and SERIES (LAHOOD FORMATION) added that nowhere is the Belt found in normal stratigraphic contact with the pre-Belt meta- Discussion morphic rocks. However, his own map of the Coarse-grained arkose and conglomerate, as Three Forks quadrangle shows one small out- well as interbedded finer-grained sedimentary crop in normal position on the north side of rocks of the Belt Series crop out in a narrow Jefferson Canyon near what is now called east-west zone extending from the northern Huller Springs. Berry (1943) also mapped this half of the Bridger Range to, and across, the outcrop and measured a Belt section there. Highland Mountains (Fig. 1). These outcrops Peale suggested that Belt sediments were never occupy a narrow marginal zone along the south deposited to the south. side of an eastward-trending embayment of the Deiss (1935) presented evidence of a marked Belt geosyncline (Fig. 2). Northward, within unconformity between the Belt Series and the a few miles, the coarse debris grades into fine-

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grained carbonate, argillite, siltite, and quartz- gested type area) are complexly folded and ite typical of the Belt Series. (Siltite is here faulted, and a complete section is not available denned as silicified and(or) slightly metamor- for measurement. Also, the name North Boulder phosed siltstone). River has been changed to Boulder River Belt strata are absent from a large part of (U. S. Geological Survey Jefferson Island southwestern Montana (Fig. 2), and Middle quadrangle map, Montana, 1950). Thus the Cambrian Flathead Quartzite rests with pro- name North Boulder Group is unsatisfactory found unconformity upon pre-Belt metamor- and should be abandoned. phic rocks. To the southwest, west, and north In some ways the name LaHood Formation of this positive area the Flathead rests on a is also unsatisfactory, but the unit does contain thick sequence of Belt sedimentary rock, a rocks representative of the coarse facies, and factor that grossly influenced differing struc- the name is retained here. However, since the tural patterns developed during the Laramide type LaHood section represents only the upper orogeny. The transition from the area without 2700 feet of the coarse facies, because of a Belt strata to that underlain by a thick Belt faulted base, it seems desirable to establish a sequence appears to be very abrupt, but perti- more complete principal reference section and nent outcrops are too few to demonstrate its to clarify the limits of the formation. nature. The relationships suggest fault control DEFINITION: It is here proposed that the along this margin of the depositional basin. LaHood Formation include all dominantly Outcrops of the coarse facies cannot be coarse Belt strata along the southern margin traced continuously northward because of in- of the central Montana embayment of the folded and infaulted younger strata, inter- Belt geosyncline. As now known, the area of ruption by the Boulder batholith, or cover of distribution extends, in a strip several miles Cenozoic sediments. To the north, where Belt wide, from the Bridger Range on the east to strata reappear, they are predominantly fine- the Highland Mountains on the west. Arbi- grained clastic rocks and carbonate. trarily, the name applies where Belt strata con- To the east, Belt rocks disappear beneath the sist of less than 75 per cent of fine-grained rocks younger strata of the Crazy Mountains Basin, and greater than 25 per cent of coarse arkosic and to the west they are interrupted by tightly debris. The type locality and the type section folded and extensively faulted Mesozoic and are as described by Alexander (1955, p. 15, 17), Paleozoic rocks in the "disturbed belt." Farther that is, approximately 1 mile east of LaHood west, Belt quartzite and argillite are typical Park, along a southwest-flowing stream in the fine-grained rocks which have not been satis- SEJ4, sec. 12, T. 1 N., R. 3 W., and the SW^, factorily correlated with the better known sec. 7, T. 1 N., R. 2 W. sections to the north and northeast. Mapping in Belt terrain is not sufficiently Alexander (1955, p. 17) proposed the name refined to separate tongues of fine-grained LaHood Formation for this coarse Belt facies formations even though they may be several in the Whitehall area. He designated the type hundred feet thick. Thus in areas such as the section (p. 17) as ". . . along the creek which southern Horseshoe Hills the Belt strata are crosses U. S. Highway 10 S. from the northeast called the LaHood Formation, even though approximately six-tenths of a mile southeast of only about one third of the exposures are coarse LaHood Park." arkosic material, and in the Toston quadrangle Two other names have also been used for the Belt strata equivalent to the LaHood are the coarse facies: East Gallatin Group and mapped as Greyson and Newland formations, North Boulder Group. The former has long because more than 75 per cent of the rock con- since been abandoned and requires no further sists of fine-grained materials typical of those mention. Ross proposed the name North formations farther north. Boulder Group in an abstract (1949, p. PRINCIPAL REFERENCE SECTION: About 6 111,113), commented on it in a review (1956, miles east of Alexander's type LaHood section p. 690), again mentioned it in abstract (1958, a small sliver of pre-Belt metamorphic rock has p. 1792), and discussed it in a U. S. Geological been uplifted on the north side of the Jefferson Survey Open-File Preliminary Report (1959, Canyon fault. Unconformably overlying these p. 25-27, p. 167-178). No formal description metamorphic rocks is a complete section of the has been made, nor has a type section been LaHood Formation, about 4170 feet thick, selected and described. Furthermore, exposures which is in turn overlain unconformably by adjacent to the North Boulder River (sug- Middle Cambrian Flathead Quartzite. This

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O GENERAL LOCATION OF STRATI GRAPHIC SECTION _^ LINE OF RESTORED SECTION BLACKTOP HIGHWAY

EHI COARSE ARKOSE DOMINANT

V7/\ SOME INTERBEDDEO ARKOSE FINE-GRAINED BELT STRATA

as- '•'"--

Figure 1. Outline map showing Belt outcrops, gross facies distribution,

section is on the Jefferson Island, Montana, Hood Formation, and the only one revealing 15-minute quadrangle map, along a ridge in the base. Other apparent depositional contacts the W^ sec. 13, T. 1 N., R. 2 W., 14 miles by with pre-Belt metamorphic rocks, near the road west of Three Forks, Montana, 1J^ miles Mayflower mine (sec. 31, T. 1 N., R. 3 W.), north of U. S. Highway 10, and half a mile and in the upper part of Bone Basin (sec. 10, west of Huller Springs (Fig. 1). T. 1 S., R. 4 W.), are obscured by mantle This is the best exposed section of the La- (Fig. 1). The formation at the reference section

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SE; B.6E-

1.3*. H-2W. and location of restored stratigraphic sections (See Figures 6-11), Montana

consists of interbedded coarse arkose and arkose higher in the section are similar, but fragments conglomerate, with some saddle-forming fine- are smaller, rarely more than 6 inches long. grained arkose. The lowermost 650 feet con- The uppermost 150 feet of the section includes tains pebbles, cobbles, and some boulders as some beds of arkosic siltite. The measured much as 18 inches long, mainly of gneiss, but section is appended. including some fragments of metaquartzite DISTINGUISHING CHARACTERISTICS: Where metadiorite, and schist. Conglomerate beds coarse-grained rock dominates the LaHood

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O V s.> CONTOUR INTERVAL - 2500 FT.

ZONE OF INTENSE THRUST FAULTING O

c z

Figure 2. Generalized form isopach map of Belt deposits, central Montana embayment

Formation generally forms dark prominent in the arkose and conglomerate (PL 1, figs. ridges and ledges; where interbedded fine- 1,2). Graded bedding is common but ranges grained clastic rocks and carbonate dominate, from distinct in the silt and finer sand sizes to the sequence weathers to rounded barren talus- indistinct in the coarse sand and gravel sizes. covered hills, broken by arkose ledges. Cross-bedding is absent except for rare occur- The coarse-grained arkose and conglomerate rences in siltite. The coarse-grained clastic weather dark brownish green, in places reddish, rocks are remarkably even-bedded, although and locally, as along the Jefferson River from minor scour features are seen at their bases. Lewis and Clark Caverns to Milligan Canyon, Interbedded fine-grained clastic rocks in some are tinted by pink feldspar and secondary places have a varvelike appearance. Carbonate hematite. On fresh fracture the coarse strata ranges from thin-bedded to thick-bedded and are normally dark gray green. The finer-grained in places is very finely laminated, generally strata are mainly dark gray to black on un- where associated with biostromal algal de- altered surfaces. Weathered and otherwise posits. altered argillite ranges from white to light gray Sedimentary structures typical of shallow- to shades of red. Carbonate commonly weathers water deposition, which characterize much of to medium tan, although in some places it the Belt Series in Montana, are absent from weathers pale gray. Siltite is either moderate the LaHood Formation. Cross-bedding, ripple gray green or dark green and weathers from marks, mud cracks, raindrop imprints, salt gray green to shades of red. casts, worm borings, and other such features Locally, below the contact with Cenozoic have not been found in the coarse facies; how- sediments, or where these sediments have been ever, they are present in thicker tongues of removed recently, the siltite, argillite, and fine-grained rock and in the overlying fine- even the coarse arkose are altered to various grained formations nearby. bright shades of red. This phenomenon is The LaHood contains abundant intraforma- particularly notable along and west of Dry tional slump structures ranging from a few Creek in the southeastern Horseshoe Hills, inches across to 10 feet or more. These are east of Clarkston in the northwestern Horse- common only in interbedded siltite and fine- shoe Hills, and along the lower reaches of grained arkose but occur sparingly in coarse Boulder River east of Whitehall. In these arkose and in carbonate beds. localities the reddish color has been used by Most grains are angular, even in the cobble some workers as a criterion for recognition of and boulder sizes, except locally as in the west Spokane Shale equivalents. Detailed strati- end of Jefferson Canyon (PL 1, figs. 1-4). In graphic observations prove that this is er- most of the coarse beds the pebbles and larger roneous; without exception the reddened rocks fragments consist mainly of quartz, feldspar, or are much older than the Spokane. Further, the gneiss. In a few localities dark quartz-feldspar bright red of the Belt rocks in these places is amphibolite, metaquartzite, metadiorite, and very unlike the dull purplish red tone of the marble fragments are notable. typical Spokane Shale. The red color is a More than 200 thin sections were studied in secondary product of alteration by deep weath- an effort to determine general variations in ering prior to deposition of Cenozoic sediments. grain size, sorting, mineral composition, altera- Remnants of the original dark-gray or gray- tion, and addition of material. Specific petro- green color are found in larger fracture- graphic variations are discussed under the bounded fragments. However, the alteration heading "Description by Area." is so complete in some places, particularly along Many of the thin sections show considerable the Boulder River, that it has formed a fair predepositional and postdepositional alteration pottery and brick clay. The alteration along resulting in destruction of feldspars, especially the Boulder River is more or less stratigraphi- orthoclase and plagioclase. Microcline is gener- cally restricted and can be traced around the ally less altered. Much of the plagioclase is not nose of a large syncline. It appears that the twinned, and the feldspar-staining technique of original material in this interval was particu- Bailey and Stevens (1960) was employed as an larly susceptible to alteration to clay. aid in differentiating the feldspars. Much of the Bedding in the LaHood ranges from thin matrix is commonly altered to chlorite. The laminae in the interbedded argillite, siltite, mineral composition of about 50 selected less- and some carbonate to massive thick bedding altered sections were determined by the Rosiwal

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method. Average mineralogic composition of are very abundant in the Jefferson Canyon the coarse arkose was calculated for the major specimens but elsewhere are sparse to absent. subareas. Secondary carbonate is common, particularly The part counted as matrix includes smaller in the more altered specimens, but the quantity fragments of quartz and feldspar, as well as varies irregularly from place to place. Primary other small detrital mineral grains including (detrital) carbonate grains are uncommon in ferromagnesian minerals, garnet, zircon, sphene, most localities but are notable in Jefferson apatite, and opaque oxides. Mica was counted Canyon. Scattered intraformational pebbles of as part of the matrix even though much of it limestone are common in localities containing appeared to be in detrital grains as large as the interbedded limestone. Other small rock frag- major rock constituents. Sericitic mica derived ments are common, as indicated in the discus- from feldspar, however, was not counted. Much sion of conglomeratic beds, and among these

TABLE 1. AVERAGE MINERAL COMPOSITION OF LAHOOD ARKOSE BY AREAS IN MONTANA No. of Ferro- Second- Ac- sec- Ortho- Micro- Plagio- magnesian Primary ary cessory tions Area Quartz clase cline clase Matrix minerals carbonate carbonate minerals 8 Bridger Range 37.3 1.6 11.5 29.6 16.1 2.9 0.1 0.8 tr 5 Horseshoe Hills 35.5 0.0 9.3 30.6 19.6 1.0 tr 4.0 tr 7 Caverns Area 35.2 11.2 17.4 10.9 23.7 0.6 0.0 1.0 tr 6 Jefferson Canyon 32.0 2.0 6.0 30.0 16.0 10.0 1.0 3.0 tr 5 Mayflower Area 31.8 0.3 6.2 40.3 17.0 3.5 0.1 0.5 0.3 7 Bone Basin 37.3 0.0 11.2 28.9 18.4 tr tr 3.8 0.4 6 Highland Mtns. 45.2 7.6 5.6 3.2 35.0 3.0 0.0 tr 0.4 3 Pioneer Range* 77.0 8.7 4.6 2.7 6.5 tr 0.0 tr tr 4 St. Paul Gulch 31.3 10.5 9.8 25.0 18.4 2.8 0.2 2.0 tr * Included for comparison

chlorite, clay, and iron-oxide stain are also are some fragments of intraformational siltite. included in the matrix. Grain size varies markedly from place to Table 1 shows the average mineral compo- place. Grossly there is a gradation from coarse sition of the coarse arkose, area by area. Several to fine from south to north. Sorting is uni- significant variations appear. Quartz content versally poor (PI. 1, figs. 3, 4), although the decreases westward from the Bridger Range to graded beds are rudely sorted, and some the Mayflower area, and thence westward it in- carbonates and argillites are uniformly fine- creases to 45 per cent in the Highland Moun- grained. tains. Orthoclase variations are irregular, partly Alteration varies irregularly. No significant at least because the mineral is difficult to patterns were noted except that beds closely recognize because of intense alteration. The underlying Tertiary deposits, now or formerly, amount of microcline also varies irregularly tend to be thoroughly decomposed. and not because of difficulties in recognition, In some specimens carbonate fills fractures for it is little altered. Orthoclase and microcline and interstices between grains. Some of this are abundant in specimens from the Lewis and carbonate appears to have been derived from Clark Caverns area, whereas elsewhere they are decomposition of plagioclase; however, in areas not. Plagioclase, much of it in the albite- where carbonate strata are interbedded it may andesine range, decreases westward from the have been derived penecontemporaneously Horseshoe Hills to the Caverns area, then in- from connate waters. In a few areas arkose and creases toward the Mayflower area, and de- siltite have been silicified by contact metamor- creases westward to minor amounts in the phism. Highland Mountains. Matrix percentages in- DIMENSIONS AND SHAPE OF OUTCROP: The crease westward from the Bridger Range to the wedge-shaped LaHood Formation extends in Caverns area, drop sharply in Jefferson Canyon, an 80-mile-long strip, 2-8 miles wide, from the and then become more abundant toward the Bridger Range on the east to the Highland Highland Mountains. Ferromagnesian minerals Mountains on the west (Fig. 1). The southern

margin is everywhere abrupt and generally ward in this section grains become less coarse, seems to be a fault. The northern margin is an but sporadic boulders and blocks as large as 2^ intricate interdigitating facies boundary with feet occur to within 2000 feet of the top of the several units of the Belt Series. LaHood. Some siltite and fine-grained arkose GEOLOGIC AGE: The LaHood Formation is are interbedded in the upper 1000 feet. Late Precambrian, much younger than the A mile or so north of the Corbly Canyon pre-Belt metamorphic rocks to the south, and section, carbonate beds intertongue with the considerably older than Cambrian. Correlation upper few hundred feet of the Belt sequence. with better known Belt units of nearby areas About 3 miles farther north, in the North is a major objective of this paper and is deferred Cottonwood section, the uppermost 2000 feet to later sections. of the arkose contains interbedded siltite and

LEGEND Qs QUATERNARY BASIN FILL K CRETACEOUS J JURASSIC

V PENNSYLVANIAN M MISSISSIPPIAN D DEVONIAN UC UPPER CAMBRIAN M€ MIDDLE CAMBRIAN pCb BELT p«a PRE-BELT SCALE

Figure 3. Belt, Cambrian, and pre-Belt relationships along Pass fault in the central Bridger Range, Montana

dense impure carbonate beds. This is the thick- Description by Area est known section of the LaHood Formation, BRIDGER RANGE: Northeast of Bozeman, in measuring about 10,250 feet, with the base not the southern half of the Bridger Range, south exposed. of the Pass fault zone, Middle Cambrian Flat- About 4}^ miles farther north, just south of head Quartzite rests unconformably on pre- Felix Canyon, only 2600 feet of LaHood is Belt metamorphic rocks (Fig. 3). The upper- exposed, and the strata consist of interbedded most 10-30 feet of the metamorphic rock was impure carbonate, siltite, and arkose in about deeply weathered prior to Flathead deposition. equal proportions. About 1500 feet below the North of the fault zone Flathead Quartzite top is a "molar-tooth" and algal zone, which rests disconformably on as much as 10,250 feet corresponds to a similar zone exposed in the of LaHood Formation. There the uppermost Horseshoe Hills to the west. Southward this few feet of the Belt is also much decomposed. zone occurs nearer and nearer the base of the Adjacent to the Pass fault the LaHood con- Flathead Quartzite, and it apparently is tains blocks and subrounded boulders of gneiss truncated somewhere north of the North Cot- up to 8 feet across. The formation measured tonwood section. more than 7100 feet thick just south of Corbly The similarity of lithologic type and se- Canyon, adjacent to the fault, and the base is quence in the Felix Canyon section to that of not exposed (Fig. 3). Northeastward and up- the Horseshoe Hills and Toston quadrangle, as

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well as areas farther north, establish these beds with the Chamberlain Shale of the type Belt near the north end of the Bridger Range as sequence farther north. Careful study and Newland Limestone equivalents. These strata partial remeasurement of his sections indicates are truncated southward by the Flathead that much of what was called argillite is really Quartzite and also include greater amounts of impure platy carbonate and calcareous argillite, arkose in that direction. The lower part of the lithologies which are identical to those of the LaHood Formation in the North Cottonwood type Newland. None of his Chamberlain is like section is probably equivalent to older units the type Chamberlain, as Verrall admits (p. of the type Belt, that is, to the Chamberlain 18). Although it is quite likely that older buried Shale and Neihart Quartzite. portions of the LaHood Formation are equiva- The coarse-grained LaHood arkose of the lent to the Chamberlain, the exposed strata Bridger Range appears to be slightly metamor- are Newland equivalents. phosed, as indicated by the chloritic matrix The coarse detritus in the Horseshoe Hills is and partially interlocking grain boundaries, finer-grained than much of that in the LaHood but the sedimentary texture remains. The of the type area and of the Bridger Range to the arkose averages approximately 37.3 per cent east. The arkose contains scattered quartz and quartz, 29.6 per cent plagioclase, 11.5 per cent feldspar pebbles up to 2 inches long, averaging microcline, 1.6 per cent orthoclase, 16.1 per about half an inch. Graded beds are common, cent matrix, 2.9 per cent ferromagnesian ranging from less than 1 foot to more than 15 minerals, and minor calcite (mainly secondary), feet in thickness. Many such beds consist of garnet, sphene, and other accessory minerals. coarse angular grains of quartz and feldspar at In the Bridger Range the contact between the base and grade upward into particles of silt the LaHood and the pre-Belt metamorphic size. Scour contacts at the base are very com- rocks is at the Pass fault (Fig. 3). The extreme mon. Mineral composition of the arkose is coarseness of the LaHood deposits adjacent to similar to that of areas nearer the southern the fault, the rapid gradation to finer material margin of the LaHood Formation (i.e., Bridger upward in section and away from the fault, and Range, Jefferson Canyon) except that the the peculiar relative displacement across the arkose of the Horseshoe Hills averages a few fault zone, which cannot be explained by post- per cent higher in matrix and carbonate.These depositional movement alone, suggest that this facts, and the finer average grain size of the fault was active in Precambrian time and that detritus, suggest a more distant source for it was responsible in part for development of the these materials, or a somewhat less rugged coarse facies. source area. SOUTHERN HORSESHOE HILLS: The southern Carbonate in this area is very similar to the Horseshoe Hills area, a few miles east of Three Newland of the Toston quadrangle and to the Forks, Montana (Fig. 1), contains up to 4700 type Newland on Newland Creek north of feet of Belt strata, overlain on the north by White Sulphur Springs, as well as to carbonate Flathead Quartzite, and covered on the west, in the northern part of the Bridger Range. It is south, and east by Cenozoic sediments. Abun- generally thin-bedded to laminated, very dense, dant impure carbonate, calcareous argillite, dark-gray, impure, siliceous magnesian lime- siltite, and siliceous argillite are interbedded stone, which commonly weathers tan. Two with thin and thick arkose. Arkose is sufficiently chemical analyses reported by Peale (1893, p. abundant so that these outcrops can be mapped 16) average 63.5 per cent CaCOs, 4.1 per cent as LaHood Formation; however, the inter- MgCO3, 2.2 per cent Fe2Os and A12O3, and bedded fine-grained strata are typical of the 29.4 per cent insoluble silica. Most bedding is Newland Limestone of areas to the north. regular, but some is irregular, and some Peter Verrall (1955, Ph.D. thesis, Princeton carbonate occurs as isolated or anastomosing Univ.) measured three sections of the Belt in nodules up to 6 feet across. Biostromal algal this area: one at Nixon Gulch, with an exposed colonies (PL 2, fig. 1) are common in a zone thickness of 1228 feet, another 2.5 miles east characterized by "molar-tooth" structure (PL of there, with a thickness of 2485 feet, and a 2, fig. 2) and finely laminated carbonate beds. third about 6 miles east of Nixon Gulch near These three carbonate rock types show grading Dry Creek, with an exposed thickness of 4720 or intertonguing relationships, suggesting that feet. He thought that argillite was most abun- "molar-tooth" structure may be organic. dant, particularly in the lower part of the sec- Two zones of this type occur in the Horse- tion, and attempted to correlate the lower beds shoe Hills; the upper occurs in a lithologic

sequence almost identical to that of a similar feather edge near Garden Gulch in T. 4 N. The zone in the Bridger Range. This appears to be Greyson, which is more than 6000 feet thick the only reasonable key horizon within these where overlain by the Spokane, is only 2800 sections, and the sequence serves to correlate feet thick at the south edge of the Toston quad- across the 7-mile-wide covered area between rangle at Big Davis Gulch, where it is capped these Belt exposures. Peter Verrall (1955, Ph.D. by Flathead Quartzite. More than 5000 feet of thesis, Princeton Univ., p. 13-18, Fig. 3) gives Belt rocks were removed by pre-Flathead a more comprehensive description and discus- erosion in a north-south distance of about 15 sion of this zone in which he suggests that miles. Peter Verrall (1955, Ph.D. thesis, Prince- variation in distance of the "molar-tooth" and ton Univ.) found an angular unconformity of algal zone below the basal Cambrian indicates about 10° between the Greyson and overlying pre-Flathead warping and subsequent erosion. Flathead in the Big Davis Gulch area. However, his usage of the algae Collenia sym- The Newland Limestone of the Toston quad- metrica as basis for correlation of these beds rangle contains numerous thin interbeds of cal- with the basal part of the Siyeh Limestone of careous arkose and feldspathic arenaceous lime- Glacier National Park is hazardous (C. P. Ross, stone. Similarly, thin arkose is interbedded 1959, U.S. Geological Survey Open File with fine-grained quartzite and siltite of the Prelim. Rept., 192 p.). lowermost 1000 feet of the overlying Greyson South of the Horseshoe Hills, the relation- Shale. In neither of the formations is arkose suf- ships of the LaHood Formation to pre-Belt ficiently abundant to warrant use of the name metamorphic rocks are obscured by Tertiary LaHood Formation. However, the fact that and Quaternary cover. However, projection arkose is interbedded with the Newland and of the Belt margin from Jefferson Canyon to lower Greyson but is absent in the rest of the the Bridger Range suggests that Horseshoe Greyson and overlying Spokane is very sig- Hills exposures lie several miles north of the nificant. This suggests that the detrital source boundary, a conclusion supported by sedi- may have been largely eroded away by the mentary characteristics. Hackett and others time of later stages of Belt deposition, and pos- (1960, p. 52 and PL 2) infer a major east-west sibly that late Greyson and younger seas may fault (Central Park fault) crossing the Gallatin have spread southward over the subdued source Valley beneath a surficial flexure in young area. Cenozoic sediments and suggest that this fault Greyson strata in the northern part of the may coincide with the original margin of the quadrangle, and Newland beds in the Big Belt Belt sequence. Westward, this inferred fault Mountains (Fig. 1), a few miles farther north- boundary probably merges with the Willow east, do not include arkose, which indicates the Creek fault of Robinson (1961, p. 1005), and abruptness of the facies change from LaHood to eastward it probably joins the Pass fault of the usual Belt lithologies. Bridger Range (McMannis, 1955, p. 1420- Arkose in the Toston quadrangle contains 1421, 1425). This fault zone is believed to have more quartz than the typical LaHood arkose, moved during Belt as well as Laramide time. and the feldspars are generally much more de- TOSTON QUADRANGLE: The Toston quad- composed, some grains being entirely changed rangle lies northeast of Three Forks, Montana, to sericitic clay. Carbonate is generally much and includes the southern Big Belt Mountains, more abundant in the Toston arkose, with the northern Horseshoe Hills, lower Sixteen abundant interbedded calcareous feldspathic Mile Creek, and rocks on both sides of the quartzite, and feldspathic arenaceous carbonate. Missouri River (Fig. 1). Here the Precambrian These features are consistent with a position Newland, Greyson, and Spokane formations somewhat farther from the source than that of (part of the type Belt Series, older to younger) exposures to the south (Fig. 1). crop out. The still older Belt units, the Cham- CAVERNS AREA: The Caverns area lies along berlain and Neihart, are not exposed, and the north side of the Jefferson River, west of younger Belt strata have been removed by Three Forks, and east of Jefferson Canyon pre-Flathead erosion. (Fig. 1). Here, the LaHood Formation consists In the Toston quadrangle the Cambrian almost entirely of coarse arkose and arkose con- Flathead Quartzite rests on progressively older glomerate. The sequence is generally described Belt units from north to south. The Spokane in the discussion of the principal reference sec- Shale, which is more than 2000 feet thick in the tion which lies in the western part of this strip northern part of the quadrangle, thins to a of outcrop.

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East and west of the reference section the JEFFERSON CANYON: The Jefferson Canyon only significant variation in character is the area lies just west and northwest of the Caverns presence, particularly in the upper few hundred structural block and is separated from it by feet, of reddish arkose, tinted by abundant pink major faults. Arkose, which makes up almost feldspar and secondary hematite. Few beds of all the LaHood Formation in this area, is folded this kind occur elsewhere in the LaHood. and faulted so that it is difficult to obtain a re- Where the pink feldspathic zone is present liable stratigraphic section. Alexander (1955) the overlying Flathead Quartzite is poorly de- studied exposures of the type LaHood Forma- veloped or absent, and the lower few tens of tion in sec. 7, T. 1 N., R. 2 W. but was able to feet of the Cambrian are feldspathic. Where the measure only the uppermost 2700 feet of the upper part of the LaHood is the usual gray coarse sequence because of faulting. green, Flathead Quartzite is well developed and Coarse conglomerate, probably equivalent to generally nonfeldspathic. These relationships part of the lower third of Alexander's section, suggest that the pink and reddish color may is beautifully exposed along the east wall of Jef- have developed by more intense weathering on ferson Canyon (Fig. 1, sec. 18, T. 1 N., R. 2 W. small islands of LaHood that remained above and sec. 13, T. 1 N., R. 3 W.). The coarse frag- sea during deposition of the Flathead sands in ments in this locality have a crude imbrication adjacent areas. Subsequent overlap by Cam- (PI. 1, fig. 2). Most of the flatter cobbles and brian seas produced the locally arenaceous and boulders are oriented parallel to bedding, but feldspathic deposits of the younger Wolsey those that are at an angle to bedding slope pre- Shale. Greater than normal alteration of feld- dominantly southward, indicating northward- spars in these red LaHood arkoses, as seen in moving currents during deposition. thin section, supposrt this interpretation. A pebble count was undertaken here be- Petrographic study of the arkose indicates cause of the unusual variety of rock types that quartz averages about 35 per cent and that among the coarse fragments. The fragments orthoclase is mostly altered to clay but av- counted were those more than 2 inches long, in erages about 11 per cent of the rock. Microcline 11 beds, within a 700-foot stratigraphic inter- is much fresher and makes up about 17 per val. Average percentages of rock types repre- cent. Plagioclase is intensely decomposed and sented are: amphibolite 29.1 per cent, marble sericitized and ranges from a few per cent to as 24.9 per cent, schist 17.3 per cent, gneiss 14.6 much as 42 per cent of the rock, averaging per cent, metaquartzite 6.3 per cent, pegmatite about 11 per cent. Ferromagnesian minerals, 4.4 per cent, and metadiorite 3.4 per cent. The other than biotite, are rare. The micas are in- dominant rock type among the coarse frag- cluded as matrix, which averages 24 per cent ments in the areas to the east and west is gneiss. and ranges from 14 to 30 per cent. The matrix If the percentages indicate the rock-type abun- is mainly very fine quartz and feldspar, clay dance in the source area, the source for the Jef- minerals, chlorite, and secondary iron oxides. ferson Canyon conglomerate was distinctly dif- Secondary carbonate is not common, except in ferent than that for deposits elsewhere in the two specimens that contain about 5 per cent. LaHood Formation. In general this suggests To the east, Belt strata are covered by Ceno- that abundant marble and amphibolite-bearing zoic sediments, and to the west they are sep- metamorphic rocks formed this local source, arated from related coarse deposits by major whereas elsewhere the source was dominantly faults. The beds dip gently to the north where gneissic terrain. they generally are unconformably overlain by Very large angular to subangular boulders of Flathead Quartzite, and on the south they are metamorphic rock are common in the vicinity faulted against Upper Cretaceous Elkhorn of the type section. Across the highway from Mountains Volcanics, except where the small LaHood Park (SE^ NWJi, sec. 12, T. 1 N., sliver of pre-Belt metamorphic rocks intervenes R. 3 W.) several 5-foot boulders of marble are at the reference section. exposed (PI. 2, fig. 3), and about one quarter of About 5 miles south of these outcrops Flat- a mile east of Jefferson Canyon in the center of head Quartzite rests directly on pre-Belt meta- the NW%, sec. 18, T. 1 N., R. 2 W., is a 12- morphic rocks. Thus, at least 4170 feet of La- foot boulder of fibrous amphibole (PI. 2, fig. 4). Hood Formation wedges out in that distance, The great size of these fragments and their gen- and judging from relationships in other areas erally angular shape probably preclude a source this termination may be extremely abrupt at more than a mile away. the Jefferson Canyon fault. The matrix of the Jefferson Canyon con-

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/74/4/407/3432083/i0016-7606-74-4-407.pdf by guest on 26 September 2021 Figure 2. LaHood conglomerate in Jefferson Canyon. Note partly rounded, crudely imbricated cobbles. North toward left Figure 1. LaHood conglomerate near LaHood Park, Montana. Very coarse, poorly sorted mass of conglomerate on left; conglomerate and arkose beds on right

Figure 3. LaHood arkose from Bridger Range, Figure 4. LaHood arkose from Jefferson Can- North Cottonwood section. Note rounding of yon. Light grains quartz; darker grains quartz, some grains. Light grains quartz, dark grains garnet, untwinned feldspar, or ferromagnesian mainly quartz and untwinned feldspar at minerals at or near extinction. Crossed nicols extinction. Crossed nicols

OUTCROPS AND THIN SECTIONS OF LAHOOD FORMATION, MONTANA

McMANNIS, PLATE 1 Geological Society of America Bulletin, volume 74

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Figure 1. Algal structure from LaHood For- Figure 2. "Molar-tooth" structure from La- to mation 2.5 miles east of Nixon Gulch in Horse- Hood Formation 2.5 miles east of Nixon Gulch c shoe Hills in H------ae

Figure 3. Partly rounded, 5-foot marble boulder at LaHood Park, SEJ4 NWJ4 sec. 12, T. 1 N., R. 3 W., Montana Figure 4. Boulder of fibrous amphibole, more than 12 feet long in C NWJ4 sec. 18, T. 1 N., R. 2 W., Montana ALGAL STRUCTURE, "MOLAR-TOOTH" STRUCTURE, AND BOULDERS IN LAHOOD FORMATION, MONTANA

glomerate is 29 per cent plagioclase, 24 per cent weathers to various shades of red in places, a ferromagnesian minerals, 24 per cent quartz, 2 phenomenon attributed by Alexander (1955, p. per cent orthoclase, 6 per cent microcline, 4 per iv, Appendix) to local action of mineralizing cent carbonate, and 11 per cent matrix. Other solutions. This red altered zone is remarkably thin sections of arkose in the type LaHood sec- similar to one of the same type in the Prichard tion and elsewhere in Jefferson Canyon are Formation of the Highland Mountains area. much the same compositionally except that The dark argillite and siltite of the Boulder ferromagnesian minerals average about 10 per River section are not only similar in most char- cent and matrix is somewhat more abundant acteristics to those of the lower Greyson and the (up to 20 per cent). This composition appar- Prichard, but they also strikingly resemble the ently reflects the relative abundance of car- Chamberlain Shale of the Little Belt Moun- bonate and amphibolite in the source area, as tains. It appears that the Newland Limestone compared to that of arkose in areas nearby to of areas farther northeast thins and inter- the east and west. tongues southward with arkosic strata, and that Throughout most of this area the entire for- westward it thins and intertongues with dark- mation consists of coarse arkosic debris, and the gray argillite of the Prichard Formation. As Flathead Quartzite caps these exposures on the Figure 4 and the stratigraphic cross sections north. However, north of the west end of Jef- show, I believe the Prichard to be equivalent to ferson Canyon the coarse material intertongues lower Greyson, Newland, Chamberlain, and with dark-gray argillite. Alexander (1955) has Neihart. referred this argillite to the Greyson Shale, a About 12-15 miles to the north, in the conclusion with which I partly agree. More southern part of the Elkhorn Mountains, Klep- precisely, these "slaty" dark-gray beds are per and others (1957, p. 5) noted that the typical of the lower part of the Greyson and/or upper 1500 feet of the Greyson Shale is "... of the Prichard Formation. Because of south- alternations of drab mudstone or shale with ward pre-Flathead truncation, it appears likely light-colored sandstone or quartzite. A few thin that the LaHood strata in the vicinity of the beds of limestone occur in the upper part." The type section and Jefferson Canyon are equiva- writer found nothing resembling this 1500 feet lent to Newland and older beds of the type of Greyson in the Boulder River area; however, Belt sequence. the lower few tens of feet of Greyson in the On the south the LaHood beds terminate axial part of the Elkhorn Mountains anticline against Paleozoic strata along a large east-west resemble some dark-gray silt shale of the fault, and about 8 miles to the southeast and 10 Boulder River area, and most of the Greyson miles to the south Flathead Quartzite rests di- beds exposed in the southern Elkhorn Moun- rectly on pre-Belt metamorphic rocks. tains are very similar to strata referred to the BOULDER RIVER: The Boulder River area is Ravalli Group in the Highland Mountains. At separated from the Jefferson Canyon area by at the south end of the Elkhorn Mountains 1200 least one major fault and lies adjacent to the feet of Spokane Shale overlies the Greyson and lower reaches of the Boulder River. The Belt underlies the Flathead Quartzite. It is es- exposures are terminated on the north, west, timated that southward truncation by pre- and south by Cenozoic cover. The LaHood Flathead erosion removed at least 3000 feet of Formation of this area is characterized by an section in the 15 miles between there and the intimate intertonguing of arkose and dark-gray Boulder River section. Klepper and others to olive-gray siltite and argillite. A thin zone (1957, p. 6) estimate that the post-Greyson about 1500 feet below the Flathead Quartzite beds were thinned southward about 600 feet in contains calcareous arkose and thin beds of 11 miles by pre-Flathead erosion within the limestone and arenaceous limestone. This zone Elkhorn Mountains. The rate of truncation ap- is believed to correspond to the Newland pears to increase southward. equivalent of the St. Paul Gulch and lower ST. PAUL GULCH: The St. Paul Gulch area lies Bone Basin sections which will be discussed a few miles northeast of Whitehall, Montana, shortly. Minor amounts of feldspathic quartzite and west of the Boulder River area (Fig. 1). A are interbedded in an interval 1420 to 1750 feet total of 6080 feet of Belt rocks was measured below the Flathead, and the interval 2300 to along St. Paul Gulch on the southwest flank of 3750 feet below the Flathead consists mainly of Bull Mountain. The base is not exposed. The dark-gray siltite, silt shale, and minor arkose. upper 2700 feet, overlain by Flathead Quartz- The latter siltite, shale, and arkose unit ite, corresponds to Alexander's Greyson Shale

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/74/4/407/3432083/i0016-7606-74-4-407.pdf by guest on 26 September 2021 PHILIPS- HIGHLANDS HIGHLANDS BONE (i) ST. PAUL ELKHORN BOULDER TYPE PRINCIPAL SOUTH BRIDGER TOSTON BELT BURG MAYFLOWER LAXOOD REFERENCE HORSESHOE DISTRICT (WEST) (EASTI BASIN GULCH MTNS. RIVER SECTION SECTION HILLS RANGE QUAD. MTNS. MISSOULA\ GROUP Y /MlssouC7 ALL SECTIONS *OVERLAIN BY V MID. CAMBRIAN HELENA ? f HELENA ? FLATHEAD QTZI

LIMESTONE / LIMESTONE /HELENA LS | ( ^.EMPIRE /* EMPIRE »^S^ SHALE J SHALE

RAVALLI RAVALLI / W Z / ^ SPOKANE SPOKANE LU < I K° / SHALE SPOKANE,/ SHALE ir GROUP 1 _i GROUP SH /*• J / GREYSON o: s SHALE K LjJ 2 / vVW^VV^A / 0) £ ^__ SHALE SHALE Q_ ^_x"^REYSOR

LAHOOD PRICHARD LAHOOD LAHOOO^^ -~v^ LAHOOD LIMESTONE LIMESTONE PRICHARD PRICHARD FORMATION LAHOOO / ?1 v ? 77^- |BVV *V* V vA FORMATION FORMATION ^^^?**V*^?^ 1- S FORMATION FORMATION FORMATION FORMATION^ FORMATIO^N V 1 _J 3 •^>~ -fV?!/^? ^ LAHOOD ^— — "" ^~*, y •H. .v^vTv-W?^ LlJ f7~lT~ LAHOOD FORMATION SHALE t-a — p f /' DO wiyj«?i^ ^*~ ^ A-l- FORMATION r ° NEIHART ^\?/U?A^ iv? yi?"/~ OTZT.

BATHOLITH p€o CENOZO.C o€o p€o p€. -« WITH

Figure 4. Correlation chart for LaHood Formation. Lower two rows indicate with what the oldest exposed Belt rocks are in contact and the nature of the contact.

(1955, p. i, Appendix), which he describes as Some of the arkose beds in the upper 500 feet "... silt shale; black to brown to purple, are calcareous, but no carbonate beds were laminated to thin-bedded, very uniform with found. only variation being occasional beds same ma- The arkose averages 31.8 per cent quartz, terial to 3 inches in thickness, weathers to a 6.5 per cent K feldspar, 40.3 per cent plagio- variety of browns and tans and forms very clase, 17.0 per cent matrix, 3.5 per cent ferro- smooth grass-covered slopes, irregular patches magnesian minerals, 0.6 per cent carbonate, and of this unit white and hard due to local altera- 0.3 per cent accessories (mainly garnet). These tion by mineral bearing solutions. . . . 2706 strata are compositionally similar to arkose of feet." Below this is a zone 550 feet thick char- nearby areas but are generally higher in plagio- acterized by abundant thin beds of tan- and clase and are notably lower in ferromagnesian gray-weathering limestone, some of which is minerals than the Jefferson Canyon arkose. nodular, and some showing cone-in-cone struc- The LaHood Formation 2 or 3 miles south of ture. Interbedded gray argillite and calcareous the measured section is much coarser, contain- and noncalcareous arkose are common also. The ing angular and subrounded blocks of meta- limy zone is very similar to Newland equiva- morphic rock up to 6 feet across. The southern lents in the Horseshoe Hills and Bridger Range boundary of the Belt here is a fault which and is here considered to be an uppermost New- places LaHood against Cretaceous Elkhorn land wedge edge. Below the carbonate zone are Mountains Volcanics. Flathead Quartzite rests about 2850 feet of coarse arkose, arkose con- directly on pre-Belt gneiss 4 or 5 miles farther glomerate, and some interbedded dark-gray south, on South Boulder Creek. siltite. Some of the arkose beds in the upper 600 Lowell (1956) has described an unconformity feet are calcareous. At several places in the between the Belt strata and pre-Belt meta- lower part of the section the rocks are greatly morphic rocks in an area about 1 mile west of altered because of proximity to small intrusive the Mayflower mine, and just north of the pre- masses. viously mentioned fault. These relationships in Arkose in this section averages 31.3 per cent sec. 31, T. 1 N., R. 3 W., Montana, are de- quartz, 20.3 per cent K feldspar, 25.0 per cent scribed by Lowell (1956, p. 1717) as follows: plagioclase, 18.4 per cent matrix, 2.8 per cent ferromagnesian minerals, and 2.2 per cent car- "Belt strata unconformably overlie Archean rocks bonate (mainly secondary) with minor amounts and are overlain concordantly by Cambrian formations. Belt and Cambrian beds are faulted against of accessories. These strata are very similar to early Tertiary volcanics. Archean rocks are exposed arkose of the Boulder River area but are as five small inliers in Belt strata. Attitudes in notably lower in ferromagnesian minerals and Archean (N. 20°-65° E., 50°S.E.) and Belt (N. somewhat higher in matrix and quartz content. 50°-65°E., 50°-65°S.E.) rocks are similar. Two Alexander (1955) has mapped the upper 2700 linear exposures, one of marble and one of schist, feet of section in this area as Greyson and the trend N. 30° E. for about half a mile on a ridge remainder as LaHood Formation. The LaHood crest. At least 300 feet of relief exists between ridge here appears to be equivalent to Newland and crest and bordering gulches. Ridge slopes, eroded older parts of the Belt. The lower part of the on Belt rocks, are steep. The east ridge faces the top of the Belt series." Greyson at St. Paul Gulch lacks the arkose "This area apparently was a west-facing shore found in this interval in sections along the line when the local Belt sediments were deposited. Boulder River, only 2 or 3 miles to the east. Cliffs and steep slopes characterized this shore line Alexander's map suggests that these areas are developed on Archean rocks. The following field separated by a major fault. evidence supports this interpretation: (1) Archean MAYFLOWER MINE AREA! The Mayflower rocks form the crests of hills, whose steep slopes mine area lies south of the Jefferson River, a (20°-25°) are underlain by Belt rocks; (2) relief few miles south-southwest of the Boulder River of 300 feet on these hills indicates a very steep con- area, and a few miles south-southeast of the St. tact between Belt and Archean rocks; (3) Belt conglomerate, near the south corner of sees. 29 and Paul Gulch area (Fig. 1). A stratigraphic sec- 30 and west of the linear exposures, contains marble, tion was measured in sees. 16 and 17, T. 1 N., schist, and gneiss boulders up to 6 feet in diameter; R. 3 W. All but the uppermost 30 feet of the north and east cf this area Belt strata are con- 4600 feet exposed consists of coarse arkose, conglomeratic." glomeratic arkose, and minor arkosic siltite. The uppermost 30 feet consists of dark-gray The geographic and structural position of the siltite and thin-bedded feldspathic quartzite. outcrops described above suggest that the La-

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Hood Formation is locally very thin, particu- morphic rocks is traced southward the arkosic larly over old topographic highs on the base- sequence thins and the contact rises abruptly ment rocks. The relationships further suggest toward the base of the Flathead Quartzite. that this was a steep, rugged shore line, not Though the precise juncture of metamorphic necessarily related to Precambrian faulting. rocks, arkose, and Flathead is covered, it ap- Although this block of LaHood lies only 3 or pears that the Flathead here overlaps the con- 4 miles south-southwest of the Boulder River tact between the LaHood and pre-Belt meta- area, the thick shale intervals of the latter area morphic rocks. are missing in the Mayflower mine area. This The LaHood debris in upper Bone Basin is illustrates the abrupt intertonguing of the coarse, in the lower part containing scattered coarse- and fine-grained facies, and the rapid boulders of gneiss, schist, pegmatite, and marble southward truncation beneath the Flathead up to 5 feet long. Bone Basin arkose averages Quartzite. It is believed that the LaHood For- 47.3 per cent quartz, 11.2 per cent K feldspar, mation is equivalent to Newland and older Belt 28.9 per cent plagioclase, 11.4 per cent matrix, units in this area. 3.8 per cent carbonate, and traces of ferro- These Belt outcrops are terminated on the magnesian minerals and other accessories. The west by Cenozoic overlap, on the northwest by arkose differs from that of the Mayflower area normal faulting, and on the north by Cenozoic principally in its greater quartz content, and in cover. Cambrian sedimentary rocks overlie the its lesser amount of plagioclase, and in the near- LaHood unconformably on the east. absence of ferromagnesian minerals. BONE BASIN: The Bone Basin area lies a few The preceding relationships, and those of the miles west and southwest of the Mayflower Mayflower mine area, suggest that the shore mine area and 5 miles directly south of White- line along this part of the basin of deposition hall, Montana, along the east side of the Jef- was a steep, rugged coast line, not necessarily ferson River valley (Fig. 1). Flathead Quartz- fault-controlled. Reid (1957, p. 18, 19, 21) pre- ite unconformably overlies the LaHood For- sents convincing evidence of Precambrian mation on the west, and Cenozoic strata cover movements, probably of Belt age on north- the Belt on the north and northeast. west-trending faults within the pre-Belt meta- In the northern part of this area 5750 feet of morphic rocks of the Tobacco Root Mountains LaHood Formation was measured; the base is a few miles south of the Bone Basin and May- covered by Tertiary tuffaceous siltstone. The flower mine localities. These movements may upper 350 feet of this section (Newland equiva- have contributed to uplift of the source for Belt lent) contains interbedded dark-gray arkosic arkose of this and nearby areas. siltite, feldspathic quartzite, and minor dense, HIGHLAND MOUNTAINS: The Highland Moun- thin-bedded, brown-weathering carbonate. A tains area lies 15-20 miles south of Butte, west thick Cretaceous or Tertiary sill occupies the of the Jefferson River, and east of the Big Hole contact between Belt and Cambrian rocks. The River (Fig. 1). The Belt outcrops make up lower part of the section consists mainly of most of the high peaks of the Highland Moun- coarse arkose and conglomeratic arkose, with tains and a west-southwest strip along Camp minor amounts of arkosic siltite. The upper Creek and Soap Gulch west of the mountains part appears to be equivalent to the carbonate proper (Fig. 5). A number of thermally meta- zone in the St. Paul Gulch section on Bull morphosed roof pendants or xenoliths of Belt Mountain, which suggests that about 2900 feet rocks lie in the south end of the Boulder batho- of Belt strata was removed by pre-Flathead lith in ranges 6 and 7 west, to the east and erosion over the intervening 7 or 8 miles. Ac- south of the main area of Belt outcrop. cording to this interpretation, the LaHood here A section measured along the east side of is equivalent to Newland and older units of the R. 8 W. contains a total of about 18,000 feet of Belt Series. Belt strata, the lower 5000 to 6000 feet of which To the south, in the upper end of Bone Basin, is coarse arkosic LaHood Formation. The Flat- in sec. 10, T. 1 S., R. 4 W., pre-Belt meta- head Quartzite truncates the Belt westward so morphic rocks crop out just west of Cretaceous that the Belt section is thinned to about 8000 Elkhorn Mountains Volcanics. The meta- feet thick at the west side of R. 8 W. The upper morphic rocks are overlain on the west by a 10,000 feet is missing in the western section, thin, poorly exposed section of LaHood arkose. indicating an angular unconformity of about The arkose-conglomerate interval is about 900 17° between Belt and overlying Flathead. The feet thick, and as the contact with the meta- Belt strata are deformed into open, north-

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plunging folds along Camp Creek and Soap tional movement probably occurred along the Gulch, and the sequence is terminated on the fault, with the north side thrown up more than south against pre-Belt metamorphic rocks by 15,000 feet. Present geometric relationships are the Camp Creek fault. In the SWJ4 sec. 1, best satisfied by such a sequence of events. T. 2 S., R. 8 W., this fault dips approximately To the west this Belt sequence is probably 60° N.; in other places the trace suggests a terminated against Cenozoic basin fill by a somewhat lower dip. Various units of the Belt covered northwest-trending normal fault. To lie against the metamorphic rocks depending the east and northeast the edge of the Boulder upon their position relative to the north- batholith transects the area of Belt outcrop. plunging folds; lower units appear in anticlinal The LaHood arkose, on the average, is some- zones and higher units in synclinal zones (Fig. what finer-grained than that of areas to the east 5)' and in general ranges from 10 to 20 per cent The Belt section in the eastern part of R. 8 higher in quartz than arkose of the Bone Basin W. consists of: (1) more than 5000 feet of area and farther east. Feldspar is correspond- coarse arkose and arkose conglomerate, the ingly low, ranging from 10 to 15 per cent less, LaHood Formation, overlain by (2) about 7400 and plagioclase is particularly sparse. Mica and feet of dark-gray argillite and minor siltite, matrix are very abundant (Table 1). Grains are which is overlain by (3) about 2500 feet of mainly angular, even the larger fragments, al- gray-green quartzose siltite and gray-green though there are a few rounded cobbles. Coarse quartzite. These in turn are overlain by (4) conglomerate is uncommon, and in it fragments about 800 feet of gray micaceous quartzite and as large as 2 feet long are few. Pebbles, cobbles, interbedded dark-gray argillite, and (5) about and blocks consist mainly of quartz, gneiss, 1100 feet of thin- to thick-bedded carbonate schist, and some metaquartzite. Intraforma- rock (now calc-silicate rock). At the top of the tional fragments of argillite and siltite are com- section is (6) about 1000 feet of gray to pink mon in some beds. In the vicinity of the quartzite, interbedded dark-gray siltite, and in Boulder batholith the arkose becomes so places some conglomerate containing sub- severely metamorphosed that it is difficult to rounded quartzite fragments. distinguish from pre-Belt metamorphic rocks Units (6), (5), (4), (3), and about half of (2) (Fig. 5). are gradually truncated westward so that in the In the upper part of the LaHood Formation, western part of R. 8 W. the Flathead rests on on the eastern edge of R. 9 W., a few thin the middle part of unit (2), the dark-gray feldspathic quartzite beds crop out. These re- argillite sequence (Fig. 5). semble quartzite which occurs on Table Moun- The LaHood Formation along the west tain and East Table Mountain in T. 1 S., R. 7 margin of R. 8 W. is also about 5000 feet thick W. (Fig. 1). Relationships between these last but can be subdivided into three subunits (Fig. two quartzites and Belt strata to the north and 5). The basal 1800 feet consists of coarse arkose west are complicated by intrusions and fault- and arkose conglomerate; these are overlain ing, but the 1000-foot-thick sequence on Table by about 1200 feet of interbedded arkose, Table Mountain appears to lie below the dark siltite, and minor conglomerate. The upper argillite (unit 2), and above the LaHood part of the LaHood consists of coarse arkose, arkose. A few thin beds of carbonate intervene fine arkose conglomerate, interbedded dark- between the quartzite and the arkose. Quartz- gray siltite, and a few feldspathic quartzite ite is absent in this zone in the section measured beds. The middle subunit intertongues and dis- along the east side of R. 8 W. appears eastward into coarse arkose. The dark-gray argillite, and the underlying A few hundred yards south of the fault at the feldspathic quartzite very closely resemble the base of the latter section, Flathead Quartzite Prichard and "Neihart" of the Philipsburg dis- rests directly on pre-Belt metamorphic rock trict about 35 miles to the northwest and are (Fig. 5). Here, as in the Bridger Range, geo- tentatively correlated with these units (Fig. 4). metric relationships suggest that Precambrian It is suggested, however, that the "Neihart" of faulting controlled the margin of the deposi- Calkins and Emmons (1915) is a quartzite unit tional basin, at least early in the depositional within the Prichard and that it does not lie at sequence (LaHood part). It also appears that the base of the Belt sequence. In the Highland the fault was inactive during deposition of Mountains the overlying cross-bedded gray- about 13,000 feet of overlying beds. Prior to green siltite and quartzite units (3) and (4) are deposition of the Flathead Quartzite addi- considered to be the equivalents of Calkins' and

Emmons' Ravalli beds, which they resemble in Princeton Univ.) reported that the coarse considerable detail. The still younger carbonate facies intertongues with Newland and Cham- sequence (unit 5) appears to be more or less berlain in the Horseshoe Hills, but subsequent equivalent to the "Newland" of the Philips- examination reveals only Newland equivalents. burg region, but its metamorphism to calc- Verrall discussed the earlier error of Klemme's silicate rock prevents ready comparison. I be- correlation of red-stained beds with the lieve that the "Newland" of Calkins and Em- Spokane. McMannis (1955) quoted earlier mons (1915) is really a correlative of the workers concerning stratigraphic relationships, Helena Limestone (Fig. 4). The long-range cor- and herein corrects those statements. C. P. Ross relations supporting this contention are too in- (1959, U. S. Geological Survey Open-File volved to have place here, and furthermore sev- Prelim. Rept., p. 168, 170) reported Greyson eral critical areas need investigation before the and Spokane equivalents in the Whitehall contention can be substantiated. The upper- area, which as discussed earlier, is erroneous, and most beds of the Belt sequence in the Highland he perpetuates Klemme's erroneous correla- Mountains (unit 6) are correlated with the tions in the Horseshoe Hills area. G. D. Robin- Missoula Group of the Georgetown thrust area son (Written communication, April 20, 1959) (Poulter, 1956), a part of the old 1-degree provided the mapping which demonstrates that Philipsburg quadrangle. the LaHood facies occurs in lower Greyson and As a result of the preceding tentative correla- Newland rocks in the northern Horseshoe Hills. tions the LaHood Formation of the Highland Intertonguing relationships. The intertongu- Mountains appears to be equivalent to the ing relationships between the coarse arkose and lower part of the Prichard and older (?) strata, finer-grained beds have been described. To and the arkose may represent some of the oldest summarize them: (1) In the Bridger Range Belt strata in Montana. I therefore suggest that arkose intertongues with Newland carbonates, the coarse LaHood deposits were a part of the and older parts of the section are probably initial sediments laid down at the beginning of equivalent to Chamberlain and Neihart. (2) In Belt deposition, and that the greatly elevated the southern part of the Horseshoe Hills the source continued to supply detritus later in coarse facies intertongues with Newland strata. areas to the east than it did in the Highland (3) In the Toston quadrangle minor coarse Mountains. arkose intertongues with the Newland and the PIONEER RANGE: The Pioneer Range area, lowermost 1000 feet of the Greyson. (4) In the west of the "disturbed belt" in R. 11-14 W. Boulder River area the LaHood Formation in- (Fig. 1), contains a thick sequence of dom- tertongues with the lower 1400 feet of the mantly reddish quartzite and argillite. No Greyson, and with older parts of the Belt. (5) coarse LaHood type of arkose occurs there. Re- In the St. Paul Gulch (Bull Mountain) and lationships between the Belt sequence of the Bone Basin areas the coarse material is almost Highland Mountains and that of the Pioneer entirely pre-Greyson, that is, it is equivalent to Range are obscure, and until the Pioneer Newland and older units. (6) In the Highland Range sequence is correlated with better- Mountains the coarse material intertongues known sections to the north and northeast, will with lower Prichard equivalents. Therefore, in remain so. the region as a whole, the LaHood equivalents range from oldest Belt up to mid-Greyson. Stratigrafhic Relationships of the Coarse Fades The type LaHood Formation and the prin- Introduction. Various writers have corre- cipal reference section, which lie midway be- lated the arkose sequence with differing units tween the extremities of the coarse-fades' dis- of the type Belt sequence. H. D. Klemme tribution, probably do not include either the (1949, Ph.D. thesis, Princeton Univ.) found youngest or the oldest of equivalents. The well- red strata containing coarse arkose beds in the established southward truncation of the Belt southeastern and northwestern parts of the sequence undoubtedly has removed at least the Horseshoe Hills and erroneously assumed that Greyson and part of the Newland equivalent. these beds were Spokane Shale equivalents. The thinness of the section and coarseness of Alexander (1955) found that the arkose inter- the grain size suggest that this area may not tongues with Greyson Shale in the Whitehall contain all the older parts of the Belt; that is, area but failed to recognize that it also inter- deposition at this particular place was as rapid, tongues with much older parts of the Belt se- but may not have begun as early as it did in quence. Peter Verrall (1955, Ph.D. thesis, areas nearby. The correlations expressed here

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are diagrammed on Figures 6-11, and on the the Canyon Ferry area, east of Helena, south- correlation chart, Figure 4. ward to the Horseshoe Hills. Some of the most Discussion of restored sections. In the re- compelling evidence of LaHood stratigraphic stored sections (Figs. 6-11), measured sections relationships occurs along this line of section. are indicated by broad columns; below these There is little doubt that the arkose is here columns relationships are inferred. These sec- equivalent to lower Greyson, Newland, and tions are located on Figure 1. Figure 6 is a re- older units of the type Belt Series. Southward construction of relationships west to east near truncation of Belt units is also exceptionally the edge of the Belt sequence as viewed from well shown in this area. An interesting reversal the south. Figure 7 is a similar cross section in direction of pre-Flathead truncation is along a line a few miles north of sections in the shown in the northern part of the cross section. west end of Figure 6, where fine-grained inter- The Central Park fault, at the south end of the beds allow closer comparison and correlation. section, is a hypothetical feature suggested by Between the Highland Mountains and Bone peculiarities in Cenozoic relationships south of Basin or St. Paul Gulch, the Boulder batholith the Horseshoe Hills. Evidence presented by and Cenozoic cover make lateral tracing of beds Hackett and others (1960) suggests that such a impossible. Therefore correlations are based fault may exist, but it is not known if the fault upon lithologic similarity and sequence. Verti- was active in Precambrian time. cal exaggeration causes disagreeable effects in Figure 10, a restored section from the Elk- the western part of Figure 6, but the westward horn Mountains anticline to Bone Basin, illus- truncation in the Highland Mountains amount- trates the increasing rate of truncation of Belt ing to about 17° should be taken into account. strata southward beneath Flathead Quartzite, The axial part of the structure producing this as well as the stratigraphic relationships of the truncation is shown as being at the more LaHood arkose to Belt units farther north. The easterly measured section in the Highland LaHood is here equivalent to basal Greyson, Mountains; however it may lie a few miles Newland, and older Belt units. The edge of the farther east. Post-Belt, pre-Flathead warping depositional basin is shown as a steep unfaulted of considerable magnitude is illustrated here, as margin; however, it is possible that sporadic in all other restored sections. The Flathead fault movements nearby helped keep the thickness is exaggerated on all sections. True- coastal slopes steep during deposition of the scale sections included with each restored sec- arkose. Thicknesses of pre-upper Greyson units tion make the visual effect of warping more in the Elkhorn Mountains area are gross realistic, but nonetheless impressive. Figures 7 estimates. and 11 best illustrate my interpretation of the Figure 11 is a restored section illustrating relationships between the isolated Highland relationships between Belt strata in the Ana- Mountains exposures and the closest carefully conda Range and Philipsburg area (George- studied Belt sections and also express my town Lake and Flint Creek Range) and those thoughts on correlation between strata in the in the Highland Mountains. The Anaconda Philipsburg area and those of the type Belt Range and Philipsburg areas lie northwest of Series in the Big and Little Belt Mountains. the area covered by Figure 1. The "Neihart" Figure 8 depicts Belt relationships along a Quartzite of Calkins and Emmons (1915) is restored section from the southeast side of the shown as a unit within the Prichard, but the Big Belt Mountains to the central part of the oldest beds shown (lower Prichard) are not Bridger Range. As in the other sections pre- known to be exposed. The Missoula Group is Flathead truncation is obvious, the rate in- thin east of the Philipsburg and Georgetown creasing markedly to the south. Coarse arkose thrusts, where the line of section passes, whereas intertongues with Newland and older Belt west of those faults these strata alone exceed units, and there is little doubt that the base of 10,000 feet in thickness (Fig. 2; Poulter, 1956). the arkose sequence in the Bridger Range is at The LaHood arkose is shown as the oldest Belt least as old as the Chamberlain, and probably unit in western Montana. Southward trunca- as old as the Neihart Quartzite. The Pass fault tion of Belt formations is also illustrated in this terminates the sequence abruptly on the south restored section. The south margin of the and is believed to have actively maintained a depositional basin is marked by a major fault steep scarplike shore line during deposition of (Camp Creek fault). the coarse beds. Figure 4 shows generalized stratigraphic Figure 9 is a restored section extending from equivalence of the LaHood Formation with

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other units of the Belt Series. The lower two upon field relationships, restored sections, and rows on the correlation chart indicate with stratigraphic correlations. what rocks the oldest exposed Belt strata are in Events preceding Belt deposition in western contact, and the nature of these contacts. Montana are very obscure. Information on this phase of earth history is scanty, little-studied, Depositional Environment and Sequence and poorly understood. Suffice it to say that in Environment of deposition. The almost com- Late Precambrian time the Belt geosynclinal plete absence of cross-bedding, ripple marks, basin began to sink and an eastward-trending mud cracks, salt casts, and raindrop imprints or embayment formed, aided by faulting along other signs of shallow-water deposition suggest part of its southern margin. Early in the strongly that the LaHood arkosic beds and depositional sequence a strongly uplifted area most of the interbedded finer-grained strata to the south began to erode rapidly, and coarse were deposited rapidly without significant re- detritus was transported northward from a working. Possibly they were deposited in mod- rugged barren land into the subsiding marginal erately deep waters, although there is little part of the depositional basin. Evidence sug- evidence of this. Persistence of gray-green and gests that the eastern and western extremities dark-gray colors in unaltered rock suggests lack of the region experienced strong fault move- of oxidizing conditions, and the abundance of ment. As unstable accumulations of coarse disseminated iron sulfide in dark-gray to black detritus built up near shore, slumping and siltite and argillite is indicative of restricted turbidity currents removed this material to circulation. more stable positions farther north on the sea Angularity of all fragments, general coarse- bottom. To the west, north, and presumably ness of grain size, graded bedding, and abun- east, the coarse material became intercalated dant intraformational slump features are evi- with finer-grained sediment, principally dark- dence of transport by turbidity currents and gray to black mud. Toward the end of this slumping in a tectonically unstable situation. early phase of deposition the precipitous coastal Isolated angular fragments up to 2 feet long in slopes were somewhat subdued by erosion, al- a sand matrix also suggest the same mechanisms though great quantities of coarse arkosic debris of transport. Local partial rounding and crude continued to pour into the sea. imbrication of cobbles in strata such as those at Later, during deposition of mid-Prichard and the west end of Jefferson Canyon are suggestive Newland equivalent strata, the western and, to of strong undertow currents on steep bottom a lesser extent, the eastern extremities of the slopes very near shore. source block were less rugged. This is particu- Presence of algal biostromes, "molar-tooth" larly notable in the Highland Mountains area structure, laminated carbonate, and septarian where fine-grained mud and clay were being carbonate nodules in the Horseshoe Hills and deposited in an euxinic environment. Con- other exposures may indicate slow shallow- tinued downwarping in the near-shore area water deposition, but these rock types are in- probably developed a partially closed basin. timately interbedded with graded arkose, a fact Onlap of the source area began. In areas to the somewhat in conflict with extremely shallow- east and northeast conditions favorable for water deposition. Little is known of the ecologic deposition of carbonate were increasingly good, conditions favorable to algal growth in Pre- and the coarse arkosic debris became intimately cambrian time, and correspondingly little is interlayered with limy Newland sediment. known of the origin of carbonate of this Deposition of carbonate was inhibited to the vintage. For these reasons I favor the theory of south by floods of arkosic detritus and to the moderate depths of origin for these interbedded west by increasingly restricted circulation. In fine-grained strata. Those Belt units correlated the early part of this phase of deposition arkosic with the LaHood arkose, i.e., lower Greyson, detritus became intercalated with black silt and Newland, Chamberlain, and Prichard, even in mud, later with arenaceous, silty, and argil- areas many miles from the arkose terrain, do laceous carbonate mud. not show much evidence of agitated shallow- Toward the end of Newland deposition the water deposition. Many upper Greyson, western part of the source was perhaps still Spokane, Ravalli, and Missoula strata show further subdued, but continued rejuvenation such features, however. of the source in areas to the east maintained the Sequence of deposition. The following in- supply of coarse detritus. terpretation of depositional sequence is based During the upper Prichard-lower Greyson

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stage of deposition, tectonic activity was be- embayment of the Belt geosyncline provided a coming weaker, although minor faulting and locus for deposition of coarse arkose, arkose other differential crustal movements main- conglomerate, and finer-grained interbeds, col- tained production of coarse arkosic debris in lectively called the LaHood Formation, in an areas from Whitehall eastward to the Toston east-west zone extending from the northern quadrangle. Onlap of the source area probably part of the Bridger Range to the Highland occurred in all but the Jefferson Canyon por- Mountains. This detritus was derived from a tion of the coast line. As the end of this deposi- rugged source area of metamorphic rocks lying tional phase drew near, structural activity de- to the south. It was carried and dumped by clined, erosion continued, and the source be- torrential streams into the differentially sub- came more and more subdued. siding marginal part of the Belt sea, where During the remaining depositional cycles of slumping and turbidity currents redistributed the Belt sequence in this region (upper Grey- the material to more stable bottom areas. son through Missoula Group) the source area These deposits began to form in earliest Belt continued to be lowered by erosion; seas and time (Neihart-early Prichard) and, at least in deposition of sediments advanced farther upon the Boulder River, Horseshoe Hills, and Toston the source block. As downwarping became more quadrangle areas, continued to be laid down up uniformly slow, the seas were shallowed by fill- to the time of lower Greyson deposition. Upper ing with sediment. Waves and current action Greyson and younger Belt rocks show no indi- were increasingly effective in redistributing and cation that the source area was notably positive, winnowing the sediments. Broad mud flats and it seems probable that the source was much were common and developed extensive mud subdued and being encroached upon by the sea cracks, raindrop imprints, and other features during later Belt time. characteristic of temporary emergence. Stratigraphic equivalence of the LaHood As deposition neared a standstill, or after it ranges from lower Prichard only on the west, had ceased, renewed tectonic movements be- to a much broader range (Neihart to lower gan. The seas receded (if they were not already Greyson) on the east. It appears that Chamber- gone), Belt sediment was warped into broad lain-Newland time was the period of maximum undulations, and along the old fault zone move- deposition. ments were renewed. There was an over-all Tectonic disturbance after Belt and before northward tilting with development of super- Flathead deposition led to warping and bevel- imposed broad northward-plunging folds. ing of the Belt units, setting the stage for later Accompanying and following this tectonic Paleozoic and Mesozoic events. phase erosion began again in earnest, Belt strata Other conclusions supported by this study were beveled, and the old source area was are: gradually reduced to a nearly flat featureless (1) The names East Gallatin Group and plain. The sediment derived from this phase of North Boulder Group should be abandoned erosion was carried far from this region, per- and the name LaHood Formation accepted for haps to be deposited as the uppermost Pre- these strata. cambrian and lower Cambrian sediments in (2) Geometry and Stratigraphic character- British Columbia, Utah, Nevada, and Cali- istics of the LaHood Formation demonstrate fornia, which have been discussed by numerous tectonic activity of considerable magnitude in authors. These are the nearest known sedi- southwestern Montana during Late Precam- mentary strata of that vintage. brian time. In Middle Cambrian time marine waters ad- (3) Previous correlations of the coarse vanced over a singularly flat western Montana arkose with upper Greyson and Spokane shales surface (Figs. 6-11) and sands of the Flathead are erroneous. were worked and reworked to form a slightly (4) Correlations suggested in this paper transgressive blanket. Local monadnocks per- differ from other regional correlations of the sisted as evidenced by arkosic Flathead or type Belt Series with Belt rocks in other areas. absence of Flathead in some localities. (5) Pre-Flathead beveling indicates that regional northward tilt and broad folding oc- SUMMARY AND CONCLUSIONS curred in latest Precambrian or Early Cambrian A partly fault-controlled eastward-trending time.

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REFERENCES CITED Alexander, R. G., 1955, Geology of the Whitehall area, Montana: Yellowstone-Bighorn Research Project, Contribution 195, 107 p. Bailey, E. H., and Stevens, R. E., 1960, Selective staining of K-feldspar and plagioclase on rock slabs and thin sections: Am. Mineralogist, v. 45, p. 1020—1025 Berry, G. W., 1943, Stratigraphy and structure at Three Forks, Montana: Geol. Soc. America Bull., v. 54, p. 1-29 Calkins, F. C., and Emmons, W. H., 1915, Philipsburg folio, Montana: U. S. Geol. Survey Geol. Atlas No. 196, 24 p. Deiss, C. F., 1935, Cambrian-Algonkian unconformity in western Montana: Geol. Soc. America Bull., v. 46, p. 95-124 Hackett, O. M., and others, 1960, Geology and ground-water resources of the Gallatin Valley, Gallatin County, Montana: U. S. Geol. Survey Water-Supply Paper 1482, 282 p. Hayden, F. V., 1861, Sketch of the geology of the country about the headwaters of the Missouri and Yellowstone rivers: Am. Jour. Sci., Ser. 2, v. 31, p. 229-245 1884, Report of Dr. F. V. Hayden to Director of the U. S. Geological Survey: U. S. Geol. Survey 6th Ann. Rept. 1884-1885, p. 48-53 Klepper, M. R., and others, 1957, Geology of the southern Elkhorn Mountains, Jefferson and Broadwater counties, Montana: U. S. Geol. Survey Prof. Paper 292, 82 p. Lowell, W. R., 1956, Unconformity between Belt series and Archean metamorphic rocks (Abstract): Geol. Soc. America Bull., v. 67, p. 1717 McMannis, W. J., 1955, Geology of the Bridger Range, Montana: Geol. Soc. America Bull., v. 66, p. 1385-1430 Merrill, G. P., 1893, Notes on the petrography of the Paleozoic section in the vicinity of Three Forks, Montana, p. 47-54 in Peale, A. C., The Paleozoic section in the vicinity of Three Forks, Montana: U. S. Geol. Survey Bull. 110, 56 p. Peale, A. C., 1893, The Paleozoic section in the vicinity of Three Forks, Montana: U. S. Geol. Survey Bull. 110, 56 p. — 1896, The Three Forks folio, Montana: U. S. Geol. Survey Geol. Atlas No. 24, 5 p. Perry, E. S., 1950, The Belt series of Montana: Billings Geol. Soc. Guidebook, 1st Ann. Field Conf., p. 40-43 Poulter, G. J., 1956, Geology of the Georgetown thrust area southwest of Philipsburg, Montana: Mont. Bur. Mines and Geology Geol. Inv. Map No. 1 Reid, R. R., 1957, Bedrock geology of the north end of the Tobacco Root Mountains, Madison County, Montana: Mont. Bur. Mines and Geology Mem. 36, 25 p. Robinson, G. D., 1959, The disturbed belt in the Sixteenmile area, Montana: Billings Geol. Soc. Guide- book, 10th Ann. Field Conf., p. 34-40 —•— 1961, Origin and development of the Three Forks Basin, Montana: Geol. Soc. America Bull., v. 72, p. 1003-1014 •—•— in press, Geology of the Three Forks quadrangle, Montana: U. S. Geol. Survey Prof. Paper 370 Ross, C. P., 1949, The Belt problem (Abstract): Washington Acad. Sci. Jour., v. 39, no. 3, p. 111-113 —— 1956, The Belt series in relation to the problems of the base of the Cambrian System, p. 683-699 in Rodgers, J., Editor, El Sistema Cambrico, su paleogeografia y el problema de su base -symposium: 20th Internal. Geol. Cong., Mexico, D. F., Pt. 2, 762 p. • 1958, Belt series (Abstract): Geol. Soc. America Bull., v. 69, p. 1742 Sahinen, U. M., 1938, Geology and ore deposits of the Rochester and adjacent mining districts, Madison County, Montana: Mont. Bur. Mines and Geology Mem. 19, 53 p. •—— 1950, Geology and ore deposits of the Highland Mountains, southwestern Montana: Mont. Bur. Mines and Geology Mem. 32, 63 p. Van Hise, C. R., 1892, Correlation papers, Archean and Algonkian: U. S. Geol. Survey Bull. 86, Ch. VI, Sect. Ill, p. 282-286

MANUSCRIPT RECEIVED BY THE SOCIETY, JANUARY 29, 1962

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APPENDIX: PRINCIPAL REFERENCE SECTION OF THE LAHOOD FORMATION

W. yz sec. 13, T. 1 N., R. 2 W., about 14 miles dium- to coarse-grained, medium-bed- west of Three Forks, Montana, about 1}^ miles ded, less resistant 275 north of U. S. Highway 10, and about half a mile west of Huller Springs on U. S. G. S. lefFerson (8) Arkose and arkosic conglomerate, me- Island, Montana, 15-minute quadrangle map. dium green to gray green, weathering Measurements were made by Jacob staff and reddish brown, coarse-grained, massive, Brunton compass. ledge-forming 480 Unit Feet (7) Arkose and pebbly arkose, medium gray green, medium-bedded, medium- Flathead Quartzite—Contact of LaHood Not to coarse-grained, forms sag 165 with Flathead Quartzite poorly ex- meas- posed, but can be placed within 2 feet ured (6) Similar to unit 5 below, but more resistant, forms low hill 110 LaHood Formation (5) Arkose and conglomeratic arkose, some (16) Arkose and arkose conglomerate, gray fine-grained arkose, gray green, forms green, massive to medium-bedded, peb- low sag 75 bles and cobbles of quartz, quartzite, gneiss, metadiorite, and schist up to 4 (4) Arkose conglomerate, gray green, inches long. Prominent conglomerate weathering reddish brown, massive, zone with pebbles up to 5 inches long, scattered larger boulders of pink gneiss, averaging 1 inch, 215 feet above base of gray gneiss, metadiorite, pegmatite, unit. Beds less conglomeratic between and mica schist up to 18 inches long. 265 and 300 feet above base. Prominent Somewhat more conglomeratic from conglomerate zone between 300 and 125 feet above base to top, ledge- 330 feet above base. From 330 feet forming 180 above base to top, beds are coarse- grained arkose, fine-grained arkose, and (3) Arkose and conglomeratic arkose, gray arkosic siltite. 470 green, massive, fragments up to 2 feet long, sag-forming 175 (15) Arkose, medium- to coarse-grained, pebbly, forms prominent ledges 250 (2) Arkose and arkose conglomerate, gray green, massive, cobbles of quartz, (14) Covered slope below unit 15. Littered quartzite, gneiss, and schist up to 15 with blocks of arkose and Flathead inches long, rounded to subrounded, Quartzite 335 very little lamination or grain orienta- tion, ledge-forming 140 (13) Arkose and conglomeratic arkose, minor fine-grained arkose, gray green, (1) Arkose and arkose conglomerate, gray weathers greenish brown, sag forming 500 green, massive, medium- to very coarse- grained, cobbles and boulders up to 20 (12) Arkose and conglomeratic arkose, gray inches long, some slightly rounded. green, massive, ledge-forming 470 Fragments consist of quartz, quartzite, (11) Arkose, gray green, medium- to fine- gneiss, and schist. Partly covered, con- grained, less resistant 120 tact with the pre-Belt metamorphic rocks placed on basis of float but is well (10) Arkose and conglomeratic arkose, gray exposed 100 yards to the west 175 green, coarse-grained, massive, ledge- forming 250 Total LaHood Formation 4170 (9) Arkose, some pebbly beds, mainly me- Pre-Belt metamorphic rocks

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