OUTLINE OF MAJOR POINTS
The rocks of the two regions range in age from Precambrian to Recent. Most were deposited in shallow seas. In addition, there are extensive deposits of continental origin of several different kinds. The thickest units belong to the Belt series of Precambrian age. Study of these has contributed to revision of concepts as to the character and stratigraphy of the rocks of the series. The principal contribution, obtained through areal geologic mapping over a broad region, is the evidence of marked lateral changes within short distances. In the regions represented in plates 1 and 2 the exposed part of the Belt series is now regarded as consisting, in ascending order, of the Ravalli group, Piegan group, and Missoula group. Each of the groups comprises subordinate units but subdivision is incomplete at present.
Sufficient new information in regard to the stromatolite content of the rocks has been obtained to produce new concepts as to the conditions during deposition of the Belt strata and to justify the hope that intensive study would yield additional valuable data applicable to problems in ecology and stratigraphy. Rezak (1957) has developed new names for the stromatolites, which make many of the names previously in use obsolete.
Paleozoic rocks are exposed only in areas where intense deformation has taken place, so that many of the details of their stratigraphy were not worked out and the rocks are mapped in systemic rather than formational units. The formations are broadly similar to those known to be present farther south but minor differences exist. The Cambrian system is represented only by scattered exposures, in most of which several of its formational units are not exposed. All the Ordovician and Silurian, part of the Devonian, and all the Permian systems are believed to be unrepresented.
No Triassic rocks were recognized. The Ellis group (Jurassic) is present but in most places is represented by narrow fault slivers. Slightly consolidated sedimentary rocks of Tertiary age line the sides of the major valleys and form erosional remnants in the plains close to the mountain border. They are extensively masked with Quaternary deposits, from which they could be discriminated satisfactorily only by detailed work. Pleistocene and Recent glacial and fluviatile deposits floor most of the mountain valleys and cover broad expanses on the plains.
The only intrusive igneous rocks are sills, dikes, and irregular masses, most of which were originally gabbro or diabase, now much altered. These have been found only in rocks of the Belt series, principally in the Siyeh limestone. They are themselves of Precambrian age and are thought to be genetically related to the Purcell basalt, flows of which are intercalated in the upper part of the Belt series.
BELT SERIES (PRECAMBRIAN)
All of the sedimentary rocks of supposed Precambrian age in northwestern Montana belong by definition (Wilmarth, 1925, p. 108-112) to the Belt series. The series comprises a very thick sequence of beds, largely argillaceous and quartzitic but containing also much carbonate. Shallow-water features, such as ripple marks and mud cracks, are abundant. The rocks are thoroughly consolidated and partly recrystallized but retain many of their original larger features. The units within the series are commonly thousands of feet thick, with few distinctive horizon markers and many points of similarity. Much of the broad region underlain by the Belt series in the Northwestern United States has been studied only in preliminary reconnaissance as yet. In consequence, much diversity in nomenclature and correlation of units in different areas persists. The definitions of stratigraphic units adopted in the present report differ only in details from those of previous workers in the general vicinity of Glacier National Park. This fact, however, would not be clear from reading published reports. Hence, a statement, as precise as available data permit, as to the stratigraphic nomenclature employed is essential before the rocks can be described. Some units are designated by lithologic terms as assignment of formal names would be premature.
In a tentative classification of the Belt series in general (Ross, 1949), the representatives of the series in the region that includes Glacier National Park are divided into the Ravalli group at the base, the Piegan group in the middle, and the Missoula group at the top. These groups are broadly equivalent to the Ravalli, Siyeh, or Wallace, and Missoula groups of Clapp (1932, p. 22) and the Ravalli, Piegan, and Missoula groups of Fenton and Fenton (1937, p. 1875-1903). They do not differ fundamentally from the classification adopted by Campbell and his associates in their field notes or from the summary by Dyson (1949a, p. 4-10) except that these geologists do not use formal group designations. Similarly, in the paper by Clapp and Deiss (1931, p. 673-696) that deals specifically with problems of Belt stratigraphy, the Ravalli and Piegan groups are not mentioned, although no essential difference in interpretation of the stratigraphy is involved. An idea of the relations between the subdivisions of the Belt series in the vicinity of Glacier National Park and in other regions can be had from the correlation table (p. 17).
For convenience in following the discussions below, two additional tables are included. The first of these (p. 18) summarizes the ideas of Fenton and Fenton (1937) in regard to subdivisions of the Belt series in the parts of Glacier National Park that they studied. Fenton and Fenton have done the most detailed stratigraphic work so far accomplished and as a result propose to divide the formations of the Belt series into many members. At present, no basis exists for tracing most of their members on the geologic map of Glacier National Park; hence, the members are not adopted here. They are presumably significant in the particular localities where they were described, and some may prove to be widespread when further mapping is done. The second table (p. 19) gives the map units in the Belt series that are employed in the present report. In a broad way, the grouping here adopted corresponds to that of the Fentons and other recent investigators. Changes in the position of the boundary between the Piegan and Missoula groups are suggested with the aim of keeping lithologically similar beds together so far as possible. No basis other than similarity in lithologic characteristics is as yet available for the grouping of strata of the Belt series, although work done during the present investigation leads to the hope that the stromatolites and the trace element content of the different rocks will aid in this respect when further information has been accumulated.
Correlation chart for the Belt series in northwestern Montana
[Thicknesses are maxima for the different localities. Formation names whose authors are not given are in general use in the senses here employed]
Vicinity of Libby, near the western border of MontanaGlacier National ParkMissoula and vicinity including the Philipsburg quadrangleHelena and vicinity including Belt and Little Belt Mountains
Missoula group of Clapp and Deiss.Top not exposed.
Libby formation, 6,000 ft.
Striped Peak formation, 2,000 ft.Main body plus several subdivisions, including the Shepard formation. Purcell basalt near base. Total thickness not measured but may be 10,000-20,000 ft. The lower part of the group is the Miller Peak of Clapp which includes the Kintla, Roosville, Mount Rowe of the Fentons, and the Shepard formation.Sheep Mountain quartzite, Garnet Range quartzite and argillite, McNamara, argillite and quartzite, Hellgate quartzite, Miller Peak argillite of Clapp and Deiss. Include Spokane of Calkins. Total thickness is 18,000 ft.Hellgate quartzite, Miller Peak argillite of Clapp and Deiss (including Marsh of Walcott), 3,500 ft.
Helena limestone, 4,000 ft.
Empire shale, Spokane shale. Greyson shale, 7,400 ft.
Piegan group of the Fentons revised.Wallace formation 17,000 ft.Siyeh limestone of Fenton and Fenton, which has been defined and subdivided on different bases by other geologists 5,000± ft.Newland or Wallace formation 7,800 ft. Base not exposed.Newland limestone 4,500+ ft.
Ravalli group of Calkins, revised.St. Regis formation, 1,260+ ft.
Revett quarzite, 1,200+ ft.
Burke formation, 1,500+ ft.
Maximum for the group 10,000 ft.Grinnell argillite 4,000± ft.
Appekunny argillite 5,000+ ft.
Altyn limestone, 2,000+ ft.
Grinnell argillite of Langton, 7,000 ft.Ravali formation of Calkins
Appekunny argillite, 6,000+ ft.
Chamberlain shale, 1,500 ft. Relations of this unit to rest of group not known.
Prichard formation, 9,700 ft. Bottom not exposed.
Prichard formation, 5,000 ft.
Neihart quartzite of Calkins; correlation with Neihart of the locality not established 1,000+ ft.Neihart quartzite, 700 ft.
Total thickness.44,700 ft.26,000-30,000 ft.45,800 ft.21,600 ft.
Comparison of the two tables (p. 18, 19) shows that the differences are in details and that the principal ones are in the upper part of the series. In the table giving the Fentons' classification, their names for the algae are used, whereas in the second table Rezak's are used. The differences in the thicknesses given result in part from differences in the limits placed on the units and in part from actual variations in thicknesses in different localities. Differences in methods of measurements may also enter. The three formations in the Ravalli group (as that term is here used) are sufficiently distinctive, so that they can be recognized and mapped with confidence. All who have worked in the region are in essential agreement as to the limits of these three. One might query whether the Appekunny and Grinnell argillites would not better be term "formations" because both, and particularly the Appekunny, contain much quartzite.
The Siyeh limestone is a distinctive and easily recognized unit, but different geologists place somewhat different limits on the formation. The Campbell party, Dyson (1949a), and, so far as can be inferred, also Willis (1902, p. 316, 323) included beds of argillaceous and arenaceous composition at the top of the Siyeh. Fenton and Fenton, on the other hand, separate the upper clastic beds and include in the Siyeh only the rocks that are essentially carbonate-bearing—a practice which was followed in the present investigation because the clastic beds are lithologically much like the main body of the Missoula group. Recognition of the kinship of the elastic beds just above the limestone with the main body of the Missoula group has necessitated dropping the group boundary low enough, so that, in the park, the Piegan group becomes so restricted as to include only the Siyeh limestone, as that term is here used. Further discussion of these names and their significance is given in the following description of the Piegan group.
The Belt series in Glacier National Park according to Fenton and Fenton (1937)
GroupFormationMember or zoneCharacterThickness (feet)
Argillite and sandstone4,800.
Miller PeakMount Rowe memberMainly red argillite1,500.
Roosville memberArgillite and argillaceous sandstone; largely greenish550-1,000.
Kintla memberArgillite and argillaceous sandstone; dominantly bright red.860-960.
Dolomite, argillaceous and siliceous, and magnesian limestone; dark gray, green gray and brown,585-1,500.
Strata, argillaceous and arenaceous dominantly red and green, though brown, buff, and gray are also seen. Purcell basalt is interbedded with this unit.180-800.
SiyehGranite Park memberMagnesian limestone, oolite, argillite, and quartzite; gray, greenish-gray, brown. Colonies of Collenia willisii abundant.280-900.
Collenia frequens zoneLimestone, dark-gray, in biostromes consisting mainly of Collenia frequens and Collenia versiformis, with thin beds of limestone and dolomite.100-156.
Goathaunt memberLimestone, dolomite, and subordinate oolite, dolomitic sandstone, and argillite; prevailing dark gray. Collenia willisii abundant.2,000-3,200.
Collenia symmetrica zoneQuartzite, argillite, and argillitic dolomite; weathers green, brownish, or buff; purplish-red argillite in the lower 75 ft. Collenia symmetrica throughout.500-900.
RavalliGrinnellRising BullArgillite, quartzite, and mud breccia; similar physically to Rising Wolf member600-1,100.
Red Gap memberArgillite; dominantly red but incidentally brownish or green; interbedded with pink, white, or greenish-white quartzite; brown sandstone, and sandy argillite,Up to 2,800.
Rising Wolf memberQuartzite, white and pink, interbedded with red argillite200-700.
AppekunnyScenic Point member.Argillite, sandstone, conglomerate, mud breccia; green, purplish, buff, brown, gray, brownish red.200-700.
Appistoki member.Argillite gray, green, olive-brown, and rusty-gray interbedded with greenish, white, and pink quartzite.2,000-2,200.
Singleshot member.Argillite and quartzite interbedded with buff to greenish siliceous dolomite and dolomitic sandstone.300-400.
AltynCarthew memberMagnesian limestone, dolomite, quartzite and intermediate rocks; blue gray, buff, brown, and dark brownish red.700-800.
Hell Roaring memberDolomite and dolomitic limestone; variably siliceous; blue gray and greenish gray; weathers buff. Contains Collenia albertensis.1,200-1,300.
Waterton memberDolomite, dark-gray and reddish, weathers gray, reddish brown, and buff with base not visible.280.
For the purposes of this report, all of the Belt series above the Siyeh limestone as defined above belongs to the Missoula group. This procedure is the only one that permits the setting up of satisfactory map units, but the shift in the position of the boundary between the Piegan and Missoula groups proposed above departs from the ideas expressed by this writer earlier (Ross, 1949). The change is required by the fact that the clastic beds below the Shepard formation and above the characteristic limestone of the Siyeh are lithologically indistinguishable from the main body of the Missoula group.
In Glacier National Park the Ravalli group consists, in ascending order, of the Altyn limestone, Appekunny argillite, and Grinnell argillite. The Altyn is not exposed in the Flathead region, and its base has nowhere been recognized. This formation does not appear to be exposed in any other part of Montana. It is commonly regarded as part of the Ravalli group—a convenient procedure which is here adopted provisionally. However, one should bear in mind that no evidence exists as to the stratigraphic relations of the Altyn limestone of Glacier National Park to the basal components of the Ravalli group in other parts of Montana or to the underlying Prichard formation (Calkins, 1909, p. 33-42). This is one of many unsolved problems in the regional correlation of the Belt series.
The Altyn limestone forms a narrow band immediately above the Lewis overthrust along the front of the Lewis Range extending about as far south as latitude 48°20' and northward past the Canadian border. Here it immediately overlies the Lewis overthrust and much of it has been cut out by that fault. The formation is also exposed in outlying blocks such as those in Divide and Chief Mountains and on either side of Waterton Lake. The Altyn should occur near the western base of the Swan Range but has not been found there. It is either slightly below the depths so far reached by erosion or is masked by detrital material. Figure 2 shows the general character of the formation in Appekunny Mountain near the former settlement of Altyn, the type locality
Fig2 Altyn limestone in the lower slopes of Appekunny Mountain, northeast of Many Glacier Hotel, Glacier National Park. The view is at the type locality of the formation, close to the site of the former settlement of Altyn. The Lewis overthrust is at the base of the cliffs, and the smooth slopes below are underlain by shale of Cretaceous age, which yields few outcrops. Photograph by Baily Willis (1902, fig. 2, pl. 47).
Subdivisions of the Belt series (upper Precambrian) in Glacier National Park and the Flathead region as used in the present report
GroupFormation or similar subdivisionNotesThickness
MissoulaGrayish-green argilliteTop unit in several localities, but mapped only on Chair Mountain, Commonly absent.Several hundred ft.
Main bodyThe principal map unit of the Missoula group, Includes all beds throughout the group not otherwise designated, within the body subordinate units have been distinguished locally and others will be when further work is done. Consists mainly of red-purple and green argillite, in part calcareous. Includes limestone of varying purity, subordinate quartzite, and some conglomerate. Includes the Kintla argillite and parts of the Shepard formation of previous workers, also the "Spokane" of the Fentons. Contains stromatolite zones, one of which has been mapped as the Conophyton cone 2.Over 5,000 ft. where not deeply eroded. In Flathead region may be as much as 20,000 ft.
Pale-pink quartziteMapped only near Union Peak, but small masses of similar relatively pure quartzite are present in several places in upper part of group.Few hundred ft.
Limestone lensesIntercalated in the main body. Only the larger and more definite masses are mapped. Similar to the Siyeh limestone lithologically but include a larger proportion of argillaceous beds. Stromatolites present.From a few hundred to over 2,000 ft.
Shepard formationQuartzite, calcareous quartzite, and dolomite with subordinate argillite. Includes the yellow-weathering beds rich in carbonate that overlie the Purcell basalt. One or more stromatolite zones.400+ ft.
Purcell basaltDark-greenish and purplish lava, much altered but originally basaltic. In the park the principal flows are at or somewhat above the top of the Siyeh limestone and below the Shepard formation.Up to 200+ ft.
Greenish calcareous argilliteDiscontinuous basal unit grading into Siyeh limestone. Not mapped separately in northern part of the park, partly because basal beds there are more diversified lithologically.Up to several hundred ft.
PieganSiyeh limestone (Will be broken down into several units of formational rank eventually).Limestone, partly magnesian and locally argillaceous, locally oolitic, "Molar tooth" markings are common. Dark bluish gray on fresh fracture and yellowish brown on weathered surfaces. Contains several stromatolite zones, one of which, the Conophyton zone 3, is mapped wherever recognized. Argillaceous beds at the top are here excluded and regarded as part of Missoula group.1,800-5,000 ft. May be greater locally.
RavalliGrinnell argillite with a member consisting of grayish-blue-green calcareous argillite locally distinguished.The grayish-blue-green calcareous argillite is a discontinuous gradational zone at the top of the unit, mapped only in the Flathead region. The main part of the formation contains red-purple, red, and green siliceous argillite, locally calcareous, with some light-colored quartzite.1,000-4,000+ ft. Probably near 3,000 ft. in most places.
Appekunny argilliteDark-gray and greenish siliceous argillite, locally calcareous with quartzite prominent locally. Subordinate reddish beds in places.2,000-5,000+ ft.
Altyn limestone (Assigned to the Ravalli group provisionally. May prove to be pre-Ravalli.)Dark, somewhat impure magnesian limestone and dolomite that weathers a distinctive grayish orange. Contains stromatolite zones.2,000± ft with the base not exposed.
In general, this formation consists of a very light-gray1 magnesian limestone that weathers grayish orange, rendering it conspicuous on distant cliffs. The Fentons (1937, p. 1881-1885) say the thickness is 2,180-2,480 feet, and Dyson (1949, p. 7) gives the average as about 2,300 feet. Willis (1902) reported an upper member of argillaceous ferrugeneous limestone 600± feet thick, and a lower member of massive somewhat siliceous limestone with concretions, 800± feet thick. His figures appear to apply only to the middle one of three members described by the Fentons.
1Rock-color chart (Goddard and others, 1948) used no a guide to color names throughout the present report, except where data on color are taken from the records of other geologists.
The Fentons divide their Altyn formation into three members called, in ascending order, the Waterton, Hell Roaring, and Carthew. The Waterton member of the Fentons corresponds essentially to the Waterton formation of Daly (1912, p. 50-56), which he described as underlying the Altyn formation. This unit, to which the Fentons assign an estimated thickness of 280 feet, with the base not visible, is exposed in Waterton Lakes Park in Canada but is not known to crop out in the United States. Whether the Waterton is to be regarded as a member of the Altyn or as an underlying formation, its absence in Glacier National Park reduces the exposed thickness of the Altyn there to about 2,000 feet.
The Fentons describe the Hell Roaring member as "dolomite and dolomitic limestone, variably siliceous; blue-gray to greenish-gray, weathering to gray, buff or cream; beds 2-24 inches thick. Many beds show laminae of limestone and dolomite and apparently primary dolomite nodules, associated with biostromes of Collenia albertensis Fenton and Fenton (= C. frequens of Rezak). Thickness estimated at 1,200 to 1,300 feet." They remark that, where it can be found, Collenia albertensis (= C. frequens of Rezak) is an index fossil. At Appekunny Mountain they found edgewise mud breccia and lenses of dolomitic sandstone or conglomerate in the upper part of the Hell Roaring member. They also report a zone consisting of beds crowded with Collenia columnaris Fenton and Fenton (= C. frequens of Rezak) at this locality and speak of carbonaceous films described as Beltina and Morania antiqua above their Collenia columnaris zone (= C. frequens zone of Rezak). They say that near Roes Creek, north of St. Mary Lake, the uppermost part of the Altyn that is exposed consists of buff-weathering sandy dolomite containing semirounded and angular pebbles, 2-15 millimeters in diameter. Probably the beds at this locality are in much the same stratigraphic position as those near Swiftcurrent Falls and Appekunny Mountain.
The Fentons describe their Carthew member as "magnesian limestones, dolomites, quartzites, and intermediate rocks which grade upward into the basal Appekunny. Colors range from blue gray through buff to brown and dark brownish red; bedding is thin to thick. Red beds, especially, show thin laminae. Thicknesses estimated at 700 to 900 feet." They have recognized this member only in Canada, where its type locality is, and in the northern part of Glacier National Park. They remark that near Many Glacier Hotel the basal beds of the Appekunny argillite rest on the Hell Roaring member of the Altyn. Farther south they think their Carthew member has been cut out by faulting.
During the present investigation the Altyn was studied east of Many Glacier Hotel and on the lower slopes of Appekunny Mountain nearby. The beds in these localities apparently belong to the Hell Roaring member of the Fentons, The specimen of Altyn limestone whose analysis is listed on page 55 came from near the hotel. The analysis shows the rock to be an impure dolomite. Three analyses listed by Daly (1912, p. 58-61), while differing in details, agree in showing the rock to be dolomitic, which appears to be true of most of the formation. Hence, Altyn dolomite probably would be a more precise name than Altyn limestone, but the latter is so well known that a change at this time would have little value. The analysed specimen and one from close to Swiftcurrent Falls are similar under the microscope. The main body of the rock consists of a mosaic of carbonate grains ranging in diameter from a few hundredths of a millimeter up to about 0.2 millimeter (fig. 6, A and B). A few thin layers of fine-grained carbonate parallel the bedding of the rock. So many of the grains have crystal faces that it is probable the whole aggregate has been recrystallized.
Scattered through the carbonate ground mass, and locally concentrated in layers up to about 10 millimeters thick, are larger pieces of several different kinds. Among the more abundant of these are rounded bodies that themselves consist of carbonate aggregates, mostly 0.5-2.0 millimeters in maximum dimension. Some of the more irregular of the aggregates may be pebbles, but some are undoubtedly oolites. One, illustrated in figure 6A is a cross section of an apparently cylinderical body, 0.8 millimeter in diameter, that is so clean cut as to appear very different from anything else in the section. In a personal letter dated April 2, 1952, J. Harlan Johnson states that in his opinion this body is organic although he is not sure just what it represents. He adds: "It appears to be a piece of spine and, if found in lower Paleozoic limestone, I would not hesitate to say it belonged to a trilobite or a chitonous brachiopod."
In addition to the carbonate aggregates, there are rounded to subangular clastic grains up to several millimeters in maximum dimension. These consist largely of quartz but include much feldspar, mostly alkali plagioclase with some microcline. Much of the feldspar is strikingly fresh, but some is sericitized. A few of the grains have rims of clear added material. Some grains contain both quartz and feldspar, and these seem to be bits of a rather coarse-grained granite. Other grains are very fine aggregates of quartz, some of which include rounded masses that look like silicified oolites.
The presence of witherite in small bodies formed by replacement along bedding planes in the Altyn limestone below Swiftcurrent Falls has been reported (Fuller, 1924). This must be an exceptional occurrence as the mineral has not been reported by other observers, and analyses of the rock show no especial abundance of barium.
Both Nordeng and Rezak visited the locality on the slopes of Appekunny Mountain where the Fentons found their C. columnaris zone. Numerous specimens of stromatolites are present. Nordeng in his field notes commented on the diversity of shapes present, which he interpreted as resulting from variation in conditions during growth. Rezak, with benefit of longer study of the stromatolites, including Nordeng's work, regards the apparent diversity in form as resulting mainly from differences in the angles at which joint surfaces cut the algal heads. Both Nordeng and Rezak report that the Collenia columnaris zone of the Fentons is not a persistent zone. Rezak has searched for the zone in exposures of the Altyn limestone other than those on Appekunny Mountain, and reports the zone to be well developed on Divide Mountain although at other localities it is either poorly developed or absent. In this connection it may be significant that neither of the two sections measured by Stebinger and Bennett and tabulated below contain any beds in which stromatolites are reported.
Two sections of parts of the Altyn limestone were measured by Stebinger and Bennett in 1914 and are taken from their field notes. The first of these is in the lower part of the slope at the east end of Red Eagle Mountain, south of St. Mary Lake. The other is in the mountain north of Crossley Lake near the confluence of Mokowanis and Belly Rivers. There may be some duplication in the lower part of this section because of minor overthrusts and thicknesses of units exposed in cliffs are estimated. The descriptions, including color designations, are adapted from those in Stebinger's notebook. Presumably all the rocks described as limestone in the field notes are magnesian.
Altyn limestone on Red Eagle Mountain
[After field notes of Eugene Stebinger and R. R. Bennett]
Limestone, light-gray to whitish-gray, with a slight bluish tinge, weathers to a buff drusy surface; very compact; interbedded with green argillite in beds 2-3 ft thick8
Limestone, as above, in beds 2-3 ft thick with 6 beds of green argillite 1-8 in. thick appearing as dark bands on the buff surface of the limestone16
Limestone, light-gray to whitish-gray, massive; similar to that above. Very compact but with rude bedding planes visible at intervals of 3-4 ft30
Limestone, as above, with 2 beds, 1-1/2 ft thick, of green calcareous argillite6
Limestone, thin-bedded, with alternate beds 6 in. to 3 ft thick of compact buff-weathering limestone like that above and soft limestone that weathers chalky white. Near the middle is a 3-ft bed of limestone grit with siliceous pebbles one-fourth in. in diameter84
Limestone, light-gray to whitish-gray, siliceous, massive, with slight bluish tinge; weathers to a buff, drusy surface. Very compact but rude bedding planes are visible at intervals of 3-4 ft39
Limestone, light-grayish-green, shaly, in beds 1/16-1/2 in. thick2-1/2
Limestone similar to the 39-ft. unit described above except that near the middle is a massive unit of 20 ft. thick without visible bedding107
Limestone, thin-bedded, chalky3
Limestone, cliff-forming; similar to 39-ft. unit above but with some beds only 2 ft thick and 15 ft thick81
Limestone, thin-bedded in layers 1/8-1/2 in, thick, mostly soft and weathering chalky white, but some beds are compact and weather bluff3-1/2
Limestone, massive, light-gray to whitish-gray, siliceous, with slight bluish tinge; weathers to a buff drusy surface. Very compact but rude bedding planes are visible at intervals of 3-4 ft1-7
Altyn limestone near Crossley Lake
[After field notes of Eugene Stebinger and E. R. Bennett]
Dolomite, bluish-gray to light-gray; weathers to a delicate creamy-gray tint. Beds are 1/2-3 ft thick. Some beds appear sandy on weathered surfaces. In the upper 50 ft green argillite beds, 1/2-1 ft thick, are interbedded with the dolomite, forming a transition zone with the Appekunny argillite above190
Limestone (or dolomite), bluish-gray, siliceous, massive; weathers cream, Forms a single cliff face110
Dolomite similar to that below but in beds 1/2-1 ft thick60
Dolomite, bluish-gray to light-gray, fine-grained; weathers to delicate creamy-gray tint. Beds are 1/2-3 ft thick. Thinner beds commonly have a fine hackly, fractured appearance. A few of them look sandy on weathered surfaces1690
Dolomite similar to above unit but not exposed along line of section. Seen in nearby cliff350±
Limestone (or dolomite), red; weathers deep brownish buff125
1Note says that 150 ft of dolomite like that in the 650-ft unit above underlies the red limestone in Chief Mountain, constituting the lowest unit of the Altyn limestone seen by Stebinger up to the middle of September, 1914, and making the total exposed thickness in this part of the park 1,675 ft. an estimate 275 ft greater than that of Willis.
The Altyn limestone is overlain by the Appekunny argillite, which, like the Altyn, was originally defined by Willis (1902, p. 316-322). The Appekunny argillite is widely distributed and well exposed in Glacier National Park. It forms an irregular band in the northeastern part of the park just west of the exposures of Altyn limestone that are associated with the Lewis overthrust. Another band extends from the Canadian border southeastward in the western part of the park until it joins the band in the northeastern part of the park. There are also exposures near Waterton Lake.
Small exposures correlated with the Appekunny were noted along the road in unsurveyed T. 30 N., R. 18 W., on the west flank of the northern part of the Flathead Range, and more extensive outcrops might be found if further geologic work were done in that vicinity. Appekunny argillite is present low on the southwestern slopes of the Swan Range, mostly in unsurveyed parts of Tps. 26 and 27 N., R. 18 W., but here satisfactory outcrops are rare, and much of the formation is masked by dense vegetation, soil, and hillwash. The southern tip of the main body of Appekunny argillite shown on plate 1 (Glacier National Park) also appears on plate 2 (Flathead region).
The formation as a whole consists of dark fine grained sedimentary rock sufficiently uniform in character, so that Willis (1902) gave it the name Appekunny argillite. Sufficient lithologic variations exist, however, both in the Appekunny and in the overlying Grinnell to give support to the Fentons (1937) and Dyson (1949a) in speaking of these units as formations rather than as argillites. This reflects the fact that both in the park and in more distant parts of northwestern Montana the two units possess greater diversity in lithologic character than could have been appreciated during Willis's pioneer studies. The distinction is subtle, and final decision should be based on consideration of a larger region than that covered by the present report. The Appekunny in the region here described originally consisted of beds containing varying proportions of argillaceous and siliceous material with distinctly subordinate quantities of carbonate. No limestone was present, and pure quartz sandstone was rare. All the components are thoroughly consolidated and sufficiently recrystallized, so that names that imply some metamorphism are required for the rocks in their present condition.
The somber, commonly rather massive rocks of the Appekunny argillite can be recognized and delimited throughout northwestern Montana more readily than some of the other units of the Belt series. It is only here and there that local development of schistosity or abnormal colors in some beds introduce some uncertainty. It does not necessarily follow that the top and bottom as mapped are everywhere at the same horizon. That is, beds lithologically akin to the Appekunny may persist over a greater stratigraphic range in one locality than another. Willis (1902, p. 322) recognized this in his remark that detailed stratigraphic study may develop the fact that the Grinnell and Appekunny argillites are really phases of one great formation and that the line of distinction between them is diagonal to the stratification. To a degree, such a suggestion may be applicable to many contacts throughout the Belt series. The data summarized below are contributions toward testing such suggestions, but more information over even wider areas is needed.
The Appekunny argillite is rarely well exposed in the Flathead region. It was mapped on plate 2 on the basis that scattered outcrops and the soil between outcrops attested to the presence beneath the Grinnell argillite of argillaceous rock in which purple or red beds were essentially absent. In the few outcrops seen on the southwestern slopes of the Flathead and Swan Ranges, hard, in part somewhat calcareous, dark-greenish-gray or greenish-black argillite predominates. Some beds are bluish. No distinctly reddish beds were noted; but under the microscope, material from the Swan Range is seen to contain enough hematite to have a faintly reddish tint. Argillite from Wolf Creek on the west side of the Swan Range, although of uniform appearance in the hand specimen, has a microscopic texture suggestive of movement before the rock became firmly consolidated. Most of the original sediment consisted of a quartzose and calcareous mud in which the quartz grains probably ranged up to about 0.07 millimeters in maximum dimension and were poorly rounded. Micaceous flakes, in part muscovite, in part chlorite, are now abundant and are interpreted as recrystallized clay and other constituents of the mud. Originally laminae of finer grained material, up to about 1.5 millimeters in width, were present. These laminae are now broken into short slabs scattered irregularly through the rock. Apparently after the fine-grained laminae had acquired some cohesion but before the rock consolidated, sufficient movement took place to break up the laminae. A little pyrite is present.
In Glacier National Park the character of the formation is broadly similar. A few reddish beds were noted, but they are widely separated, and most of them are much less brightly colored than those typical of the overlying Grinnell argillite. The field notes of Campbell's men speak of red beds in a few places, but it appears that everywhere these are decidedly subordinate in amount. The restriction of the Appekunny to dark largely argillaceous rocks seemingly corresponds also to Dyson's usage (1949a, p.7, 11).
In the southern tip of Glacier National Park, most exposures of the Appekunny argillite consist of greenish-gray massive argillite, almost as green as the greenish parts of the overlying Grinnell argillite. This rock consists of a uniform quartz mosaic with a mesh of micaceous flakes, largely chlorite. The quartz grains range up to 0.02 millimeter in diameter, and all have strain shadows and fuzzy borders. A little carbonate and possibly some feldspar are present. Some dark-gray to almost black beds and a few light-colored quartzite beds are present. Throughout the formation, ripple marks are fairly common.
In the localities seen in 1950 (see index map, pl. 1), exposures are more continuous than they are in the Flathead region. Here medium-dark-gray relatively massive beds predominate in the lower part, and the upper part of the formation consists mostly of greenish-gray comparatively thin-bedded material. Both here and to the south, the amount of slaty cleavage varies in proportion to the local deformation. For example, some of the dark-gray argillite in outcrops on the slopes southwest of Mount Thompson might be described as slate. In both the upper and the lower parts of the formation in the park, some beds are so calcareous that their nature is obvious at a glance. The three analyses of samples of Appekunny argillite from Glacier National Park in the table on page 55 all record the presence of small quantities of carbonate. Samples could be selected that would show much larger amounts, but none of the beds in the formation could be termed "limestone."
The thin-bedded parts of the formation are represented by the greenish distinctly layered rock from near Many Glacier Hotel, specimen ID—13950 in the table on page 55. In this rock the laminae are 0.5-1.0 millimeter wide. Some individual grains exceed 0.5 millimeter in maximum dimension, but most are 0.015 millimeter or less in diameter. Most are subangular, but a few grains are rounded, and some have grown together, so that the contacts between them are obliterated. Quartz makes up roughly 60 percent of the rock, feldspar (mostly alkali plagioclase) constitutes about 10 percent and the rest of the rock consists mainly of micaceous minerals, mostly chlorite.
The exposures of Appekunny argillite along the Going-to-the-Sun Highway above Lake McDonald include green and dark-gray argillite. The green argillite here, represented by analysed specimen ID—21550, has graded laminae 0.1-1 millimeter wide (fig. 6c). The largest grains are about 0.1 millimeter wide and are in an intricate mat of micaceous shreds, mostly chlorite. A little plagioclase is present. The dark-gray argillite from the same locality, represented by analysed specimen ID—21650, is more irregularly and indistinctly layered than the green rock. Most of it resembles the coarser parts of specimen ID—13950. The rock contains quite a little feldspar and enough carbonate to be readily seen, and pyrite is conspicuous. The three analyses show an average content of about 67 percent of silica, rather high for an argillite. The formation includes a few beds of relatively pure quartzite.
The Fentons saw the Appekunny argillite in the area from near Two Medicine Lake northward into Waterton Lakes Park. They propose to divide it into three members. They call the lowest of these the Singleshot and describe it as containing argillite and quartzite interbedded with buff to greenish siliceous dolomite and dolomitic sandstone. The fine-grained clastic rocks are gray, gray green, reddish, and black; quartzites are greenish, pink, or white. Mud cracks, ripple marks, and mud breccias are abundant. Locally, a basal coarse-grained pinkish sandstone is reported to rest on the Altyn limestone with slight angular unconformity. The thickness of the member is said to be 300-400 feet. As the Appekunny was nowhere found resting on the Altyn in localities mapped in 1949 and 1950 (pl. 1), it may be that the Singleshot member is absent or poorly exposed in the places where relatively detailed study of the Appekunny argillite was made during the present investigation.
The second and much the thickest member of the Appekunny is termed "the Appistoki member" by the Fentons, They say it contains "gray, green, olive-brown, and rusty-gray argillite in thin minor but thick major beds, interbedded with thickly stratified, greenish, white, or pink quartzite." Flat-pebble breccias, mud cracks, and ripple marks are abundant; rain and sleet prints are present in some layers. The thickness in the Lewis Range is given as 2,000-2,200 feet, which corresponds to the total thickness of the entire formation as originally estimated by Willis.
The uppermost member is called the Scenic Point member by the Fentons and is described as containing argillite, sandstone, and "gravelly conglomerate" and as being green, purplish, buff, brown, and dull brownish red at the type locality. North and west from Scenic Point, which overlooks Two Medicine Lake, the member is reported to grade into thickly bedded coarsely mud-cracked argillite, which gives way to thick quartzite and subordinate gray and iron-stained argillite beds. Mud breccias, mud cracks, and ripple marks are abundant. The thickness of the member is given as 200-700 feet.
Consideration of the various descriptions summarized above leads to the tentative conclusion that the rocks mapped during the present investigation as belonging to the Appekunny argillite correspond essentially to the Appistoki member of the Fentons. On this basis, most of their Scenic Point member may have been mapped with the Grinnell, and their Singleshot member probably is not exposed in the localities shown on the index maps on plates 1 and 2 as having been mapped in 1949 and 1950. Further, it seems probable that this member was included in the upper part of the Altyn limestone by the geologists under Campbell. As it has a large content of carbonate rocks, such an assignment would be in harmony with the work done in the present investigation.
Willis (1902) credits the Appekunny with a thickness of about 2,000 feet. The Fentons say that in the eastern part of the mountains the thickness is 2,500-5,300 feet, and they cite Clapp (1932, p. 22) as estimating thicknesses as great as 10,000 feet farther west, presumably beyond the limits of Glacier National Park. Clapp gives a minimum thickness in the park of 3,500 feet. Dyson (1949a, p. 7) says the thickness is 3,000 feet or more. In the Flathead region south of the park, the thickness is surely as much as 2,000 feet, and on the southwestern slopes of the Swan Range, it may exceed 5,000 feet. The wide variation in the thickness of the formation suggested by these figures may result from lateral variations in the lithologic character of the rocks, a feature that, in varying degree, is found in all the units in the Belt series. Erdmann (1947, p. 129-130) says that fieldwork incident to his examination of the Bad Rock Canyon dam site on the Flathead River "has demonstrated interfingering of the gray-green Appekunny lithology with the dull purplish-red Grinnell lithology in the 2,400 feet of strata occupying the Grinnell horizon in Bad Rock Canyon," as the Grinnell is mapped by Clapp (1932, pl. 1). Erdmann's conclusion reflects the difficulty in distinguishing the Appekunny argillite from the Grinnell argillite. It is inherent in any attempt at establishing stratigraphic subdivisions in a thick sequence of broadly similar argillaceous rocks such as those that compose these two formations. A similar difficulty led Stebinger to remark in his field notes of August 13, 1914, that the Grinnell-Appekunny contact on the spur between Snyder and Sprague Creeks and Lincoln Creek is "hard to locate or cannot be so located because of the large amount of green argillite in the base of the Grinnell." On the other hand, such subdivisions are needed, if the character of the Belt series is to be understood, and can generally be made with reasonable precision when systematic areal mapping over a sufficiently extensive territory is undertaken. Bad Rock Canyon at the north end of the Swan Range (pl. 3) is outside of the area mapped during the present investigation but has been visited by the writer. The green and purplish rocks so well exposed there are essentially similar to those of the Grinnell argillite in the part of the Swan Range mapped in plate 2.
Alden in his summary of data obtained by Campbell's parties says that the Appekunny argillite near Swiftcurrent Falls is composed mostly of greenish argillite, with some interbedded reddish argillite and lighter-colored quartzite. At the base he reports a transition upwards from buff Altyn limestone to greenish-buff Appekunny argillite. At the top of the formation he notes that there is a transition zone in which beds of maroon argillite are interbedded with green argillite, increasing upwards until the maroon beds characteristic of the Grinnell argillite predominate. In the fieldwork of 1949 and 1950, this transition zone would have been included in the Grinnell.
Nordeng reports in his field notes that the Collenia columnaris zone of the Fentons (= C. frequens zone of Rezak) in the upper part of the Altyn limestone is 50-75 feet below the base of the Appistoki member of the Appekunny argillite near Appekunny Falls. No fossils have been reported from the Appekunny anywhere; so it would appear that the Fentons' Singleshot member is very thin or absent at this locality.
The following is a complete section of the Appekunny argillite measured by Stebinger and Bennett in 1914 at the eastern end of Red Eagle Mountain above the Altyn limestone. (See p. 21.)
[From field notes of Eugene Stebinger and H. R. Bennett, 1914]
Argillite, green, thin-bedded, remarkably uniform; a few gray beds; surfaces weather rusty brown1,412
Quartzite, gray to whitish-gray, coarse35
Argillite, green, thin-bedded; a few quartzitic layers 1/2-3 in. thick; weathers rusty brown on bedding planes200
Quartzite, white, coarse7
Argillite, green; in beds 1/4-6 in, thick; some layers of gray quartzite 1/2-1 in, thick351
Argillite, light-red; and gray to greenish-gray coarse-grained quartzite in close alternation.59
Beds are 1/2-4 in. thick, and argillite and quartzite are about equal in quantity86
Quartzite, white to gray and greenish-gray, coarse-grained155
Argillite, green, dense70
Covered; presumably green argillite155
Argillite, green; in beds 1/4-1/2 in. thick160
Quartzite, light-gray to greenish-gray, coarse grained15
Argillite, green; 1/2-6 in. thick. Interbedded with light-red beds 1/2-2 ft thick in lower 20 ft296
Argillite, light-red; a few green argillite beds 4-6 in. thick33
Argillite, light-red; resembles the Grinnell; in beds 1/2-4 in. thick; has white mica on bedding planes69
Argillite, green; which occasionally has light-purple tint; in beds 1/2-2 ft thick139
Argillite, gray; interbedded with white to greenish white quartzite in beds 2-3 ft thick20
Argillite, green; in beds 1/4 in to 2 ft thick, mostly 1-4 in; occasional beds of banded green argillite with light-purple to reddish tinges214
Quartzite, light- to dark-green13
Argillite, green, dense; mostly in beds 1/2-2 in. thick, but some in massive beds 2-3 ft thick47
Corbett and Williams, also in 1914, measured a section of the Appekunny argillite on the mountain north of Crossley Lake which, while supposedly complete, is much thinner than the section just given. Corbett's field notes state that the measurement started at the highest limestone bed in the transition zone between the Altyn and Appekunny. This transition zone, although excluded from the section as given by Corbett, includes much quartzite and some argillite, so that it might well have been included in the Fentons' Singleshot member of the Appekunny. The section below stops at the horizon designated the base of the Grinnell argillite in Corbett's notes.
Appekunny argillite near Crossley Lake
[After field notes of C. S. Corbett and C. R. Williams, 1914]
Argillite, green, lower 2 ft rather sandy, thin-bedded12
Argillite, red, thin-bedded37
Argillite, green and gray, thin-bedded; contains beds up to 4 in. thick. In part covered305
Sandstone, gray; with argillaceous layers4
Argillite and quartzite; interbedded in beds up to 6 in. thick12
Quartzite, light-gray, massive4
Argillite, gray, thin-bedded; a few massive beds up to 5 in. thick and a few thin sandy layers95
Quartzite, dark-gray, massive3
Sandstone, gray, thin-bedded4
Argillite, gray, thin-bedded20
Argillite, green, thin-bedded49
Sandstone, green, thin-bedded3
Argillite, green, thin-bedded3
Sandstone, green, thin-bedded2
Argillite, green; becomes sandy toward top3
Argillite, green, thin-bedded111
Argillite, green, massive; in beds up to 3 ft. thick with some thin-bedded green argillite168
Argillite, light-green, massive15
Quartzite, light-green, massive12
Argillite, light-greenish-gray, quartzitic, thin-bedded; some massive beds up to 2 ft thick45
Quartzite, light-greenish-gray, massive; weathers buff18
Argillite, green, thin-bedded12
Argillite, green, massive2
Quartzite, white, massive14
Argillite, gray, sandy, thin-bedded5
Quartzite, gray, massive1
Argillite, gray, sandy, thin-bedded11
Quartzite, white, massive1
Argillite, gray, thin-bedded4
Quartzite, gray, thin-bedded1
Argillite, gray, thin-bedded12
Quartzite, light-gray, crossbedded8
Argillite, gray, sandy, thin-bedded; some beds of fine grained argillite and 1 bed of white quartzite 8 in. thick36
Quartzite, white, massive11
Argillite, gray, thin-bedded17
Argillite, green, thin-bedded115
Quartzite, white, massive13
Argillite, green, thin-bedded5
Quartzite, light-gray, massive13
Argillite, gray, thin-bedded2
Argillite, green, thin-bedded6
Argillite, green, massive6
Argillite, green, thin-bedded, occasional massive beds up to 2 ft thick2
Argillite, red; some green beds32
Argillite, green; beds up to 5 in. thick72
Argillite, greenish-gray, massive18
Argillite, gray, thin-bedded11
Quartzite, gray, massive; beds 1-3 ft thick, with few thin argillaceous beds21
Argillite, green and gray, thin-bedded16
Argillite, gray, massive4
Argillite, green and gray, thin-bedded122
Argillite, green, massive; 2 beds each 5 ft thick10
Quartzite, buff, massive; beds 2-5 ft thick31
The Grinnell argillite is the uppermost unit of the Ravalli group and the one that is best exposed in the Flathead region. It underlies broad areas in the southwestern parts of the Flathead and Swan Ranges. In these mountains the Grinnell, like the Appekunny, is largely obscured by the extensive cover of vegetation and soil. However, the Grinnell extends high enough to reach the parts of the mountains that have been extensively denuded by glacial action. In these places parts of the formation are well exposed. In Glacier National Park the Grinnell argillite, like the Appekunny which underlies it, is exposed along the southwestern flank of the mountain mass from the Canadian border nearly to latitude 48°30' and thence northward along the eastern mountain flank back to the Canadian border. There are also exposures on both sides of Waterton Valley.
The most distinctive feature of the Grinnell argillite is the purplish and reddish color of many of the beds. Some beds are green and some are white, but most display in varying degree the characteristic color of the formation, which is due to iron in the Grinnell. Like the Appekunny, the Grinnell argillite contains many argillaceous beds. Both formations are fine grained, but the Grinnell is somewhat the coarser. It is also more siliceous and contains less carbonate and more recognizable feldspar than the Appekunny argillite does. Two of the three samples analysed (see p. 55) contain about 70 percent of silica, and none of them contain more than a small fraction of a percent of carbon dioxide. The samples selected for analysis represent the average rock, and the differences in the composition of the two formations would be emphasized if analyses of the more strikingly siliceous and calcareous beds in each were available.
The Grinnell argillite, as can be seen from figure 3, is a fairly uniform even-bedded unit. In some exposures it has a massive appearance but, wherever it is sharply deformed, as in the outcrops pictured in figures 25 and 28, the bedding is emphasized, and individual beds are seen to be thin. The argillaceous beds that compose much of the formation are resistant to weathering, which is indicated by the presence of unweathered joint blocks far from present outcrops. On the slopes northeast of Harrison Lake, for example, such blocks are so abundantly scattered over surfaces underlain by Appekunny argillite that in reconnaissance work it would be easy to imagine that the Grinnell contacts are lower on the slopes than is actually the case. Ripple marked and mud-cracked beds and intraformational conglomerate or breccia are common throughout the Grinnell argillite (figs. 4, 5) but are particularly abundant in the middle part of the formation. Figure 4 shows broken bits of thin dark argillaceous layers enclosed in white quartzite. Figure 5, in addition to ripple marks, shows bulbous forms whose origin is not clear. They are somewhat like the forms that the Fentons (1937, p. 1912-1913) regard as channel fillings.
fig3 Air view northeast from above Gunsight Pass toward the valley of St. Mary River, with Lake Ellen Wilson in the foreground. Shows the general character and topographic expression of the Grinnell argillite, Siyeh limestone and, in the distance, the Missoula group. Metagabbro sills and the Conophyton zone 1 are visible. One of the ridges on the Great Plains that is capped by early glacial deposits can also be seen. Photography by U. S. Army Air Corps. (click on image for a PDF version)
Fig4 Grinnell argillite on Broken Leg Mountain, Nyack quadrangle, Flathead region. Somewhat more distinctly laminated than is common because of the presence of white quartzitic layers containing included fragments of argillite
Fig5 Grinnell argillite on Broken Leg Mountain, Nyack quadrangle, Flathead region. Shows ripple marks and bulbous masses