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Главная » 2013 » Ноябрь » 27 » Geology of Glacier National Park And the Flathead Region, Northwestern Montana (p5)
Geology of Glacier National Park And the Flathead Region, Northwestern Montana (p5)
Siyeh limestone along Garden Wall from Collenia undosa zone in lower part of Missoula group down to Conophyton zone 1 in the Siyeh on Going-to-the-Sun Highway, about 7.7 miles west of Logan Pass

[Measured by Richard Rezak, 1951]

Collenia undosa Walcott, isolated heads. Laminae are pink; matrix is green argillite1
Argillite, alternating green, tan, and purple; finely laminated34
C. undosa, poorly developed heads, similar to those at top of section2
Argillite, alternating green, tan, and purple, finely laminated71
Argillite, alternating green, tan, and red, finely laminated. Contains irregular bands of calcareous, oolitic, white, pink, and green sandstone. Sandstone is commonly crossbedded and in lower 4 ft of unit contains pyrite cubes; averages 6 in. in thickness73
Argillite, blue, calcareous; weathers tan. Contains two thin beds of C. undosa. Biostrome at base seems to be continuous8
Sandstone, pink, calcareous, oolitic; crossbedded; contains large pyrite cubes2
Argillite and limestone, alternating tan and blue-gray. Becomes more calcareous toward base. Has stringers of pink, calcareous, oolitic sandstone in lower part21
Limestone, pink to gray oolitic with some green argillaceous bands19
Top of Siyeh limestone. Dark-gray to black beds with limestone (dense, fine-grained) predominating20
Collenia multiflabella Rezak, large (2-3 ft diameter) colonies, C. multiflabella zone?2
Limestone, blue-gray to black, finely laminated. Contains small bioherms of Cryptozoon occidentale and C. multiflabella Dawson (emend. Rezak)21
Sandstone, black, crossbedded, oolitic; calcareous. Contains poorly preserved, unidentifiable stromatolites5
Limestone, dark-gray and tan, argillaceous. Has fine-grained tan laminae174
Biostrome of C. multiflabella? Poorly preserved6
Limestone, dark-gray, dense, fine-grained. Shows evidence of algal activity55
Limestone, dark-gray, dense, fine-grained. Contains biostromes of Conophyton?68
Limestone, black, massive, laminated with unidentifiable stromatolites20
Limestone, black, thin-bedded, Contains C. multiflabella and Cryptozoon occidentale in upper part. Lower part contains abundant Conophyton, Conophyton zone 150

Siyeh limestone along Going-to-the-Sun Highway near southwestern end of Lake McDonald

[Measured by Stephan Nordeng, 1950, for the present study. Rezak's revised names of stromatolites given in parenthesis]

Argillite, gray-green, grading into gray impure limestone, with lenses of yellow sandy quartzite. Nordeng suggests that this unit may be equivalent to part of the Shepard formation; so next unit below would be top of Siyeh limestone11
Limestone, dense, thick-bedded, laminated with a biostrome a foot thick about 3 ft from top of unit, containing unidentified poorly developed stromatolites7
Limestone, black, roughly crossbedded, sandy with calcite veinlets4
Limestone, black, dense, fine-grained2
Limestone, light-gray to black, laminated, sandy with layers of mud breccia and some zones of poorly developed stromatolites3
Limestone, fine-grained, sandy, with breccia in lower part15
Limestone, light-gray to green, fine-grained sandy4
Limestone, black, dense with sandy lenses3
Sandstone, light-gray to buff5
Limestone, black7
Sandstone, light-gray to buff9
Covered or so changed as a result of a forest fire as to be indeterminate293
Limestone, black, dense, wavy-bedded nodular135
Bioherm containing Collenia versiformis (= Cryptozoon occidentale) in heads 1/2-1 in. wide, up to 8 in. tall, digitate, showing very slight flexure of laminae. Most heads appear to expand upward although some are almost perfectly parallel. At top the stromatolites take the form of Collenia willisii (= C. multiflabella). Bioherm is overlain by a thin quartzite layer3
Limestone, black, with thin wavy bedding130
Stromatolites of Collenia symmetrica Fenton and Fenton type, with internal structure obscure2
Limestone, black, dense7
Stromatolite zone containing very poorly preserved Collenia frequens? and Collenia symmetrica?1
Limestone, black, nodular40
Limestone, black, dolomitic, with well-developed "molar tooth" structure6
Sandstone and mud breccia2
Zone of well-developed Collenia versiformis (= Cryptozoon occidentale)3
Collenia frequens zone (=Conophyton zone 1). Black limestone with some "molar-tooth" structure. Contains good examples of Collenia versiformis (= Cryptozoon occidentale, possibly some Collenia clappii (= C. symmetrica) but more probably Collenia willisii (= C. multiflabella), also small heads of probable Collenia frequens10
Limestone, black, thick-bedded33
Limestone, black, massive, with numerous gray to greenish argillite beds55
Biostrome within and covered by edgewise conglomerate. Apparently contains some Collenia versiformis (= Cryptozoon occidentale and an undetermined form; possibly Collenia symmetrica2
Limestone, black, thin-bedded30
Biostrome, 2-1/2 ft high, 1-1/2—2 ft in diameter consisting of rather closely crowded colonies of Collenia willisii (= C. multiflabella). The enclosing limestone shows wavy laminae and some mud pebbles2-1/2
Biostrome containing Collenia versiformis ( = Cryptozoon occidentale and an unknown form resembling a gymnosolen4
Not exposed. Thickness of part of the formation measured. Beds below are not exposed along the highway and not susceptible of satisfactory measurement--


Siyeh limestone near Red Eagle Creek

[From field notes of C. S. Corhett and C. E. Williams, 1914]

Limestone, thin-bedded; with numerous beds of sandy limestone and some of calcareous sandstone. The sandy beds are 1-8 in. thick and contain pyrite102
Limestone, rather thin-bedded with argillaceous partings and a few beds of sandy limestone466
Limestone, gray, massive in beds up to 2 ft thick100
Limestone, shaly1
Limestone, gray, massive with bedding planes at 1- to 2-ft intervals11
(Above beds measured east of Red Eagle Glacier and stopped at top of unit called "massive member" in the field notes, almost certainly equivalent to the Conophyton zone 1 of the present report.)

Massive member (Conophyton zone 1) with a 1-ft bed of thin-bedded limestone near the middle78
Argillite, dark-gray, shaly and very fine-grained quartzite8
Limestone, gray, massive with gnarly structure and a few beds of thin-bedded limestone24
Argillite, dark-gray, probably calcareous3
Limestone, massive with gnarly structure with numerous beds of thin-bedded limestone and, in the lower part, some of gray sandstone371
Limestone, massive, sandy1
Limestone, massive, with some thin-bedded limestone103
Limestone, thin-bedded and massive; in alternating units up to 3 ft thick, with numerous beds of gray sandstone140
Limestone, massive, containing a few beds of thin-bedded limestone249
Limestone, thin-bedded, impure; with a few beds of dark-gray shale up to 6 in, thick of massive limestone up to a foot thick171
Limestone, white, baked, thin-bedded (Above beds measured between Red Eagle and Little Chief Mountains, and the baked limestone at the base overlies a sill 90 ft thick.)35
Limestone, white, baked; with pyrite crystals22
Limestone, thin-bedded; becoming more massive upward78
Argillite, greenish-gray, with numerous beds of gray sandstone, each in units up to 4 ft thick35
Argillite, pink3
Sandstone, argillitic3
Argillite, pink 4
Sandstone, buff 3
Argillite, gray2
Sandstone, gray2
Argillite, green, with a few thin beds of limestone16
Limestone, thin-bedded11
Sandstone, buff1
Limestone, thin-bedded8
Argillite, greenish-gray2
Shale, dark-gray2
Limestone, thin-bedded17
Argillite, green6
Limestone, thin-bedded19
Limestone, massive1
Shale, gray1
Limestone, argillitic9
Sandstone, gray2
Limestone, thin-bedded2
Sandstone, gray1
Limestone, gray, thin-bedded2
Limestone, argillitic; with some thin beds of gray sandstone19
Sandstone, gray4
Limestone, argillitic5
Argillite, gray and green11
Limestone, massive1
Argillite, green2
Sandstone, gray1
Limestone, massive1
Argillite, gray13
Limestone, massive1
Limestone and sandstone, argillitic11
Argillite, green; and sandstone in units up to 2 ft thick19
Limestone, thin-bedded argillitic10
Argillite, gray1
Argillite and sandstone, gray15
Argillite, gray5
Limestone, thin-bedded1
Argillite, gray, calcareous44
(Above beds measured on Red Eagle Mountain. The probability is that some or all of the gray and buff argillite and sandstone listed are calcareous, although not so designated in the field notes. The base of the section is at the top of beds interpreted as belonging to the transition zone at the top of the Grinnell argillite.)

     Thickness of beds above the Conophyton zone680
     Thickness of beds from top of sill through Conophyton zone1,183
     Thickness of beds below the sill415
     Total thickness of Siyeh limestone near Red Eagle Creek


An approximate idea of the Siyeh limestone on the west side of Mount Cleveland may be gained from the following section adapted from the field notes of E. M. Parks. Thicknesses given in this section are estimates based mainly on barometric readings.

Siyeh limestone on Mount Cleveland

[From field notes of E. M. Parks, 1914]

Limestone with some argillite near base and a stromatolite zone, 30 ft thick, starting 20 ft above the sill565
Limestone, bluish-gray to dark-blue rather thick-bedded800
Upper part of transition zone, here so calcareous that it may be grouped with the limestone. Consists of thin beds of green argillite and impure limestone. The latter comprises 20 percent of the whole in the lower part and 50 percent in the upper part325
   Total, omitting sill


Another composite section of the Siyeh limestone is given below. Most of this was measured by Corbett and Williams on the ridge between the North Fork of Belly River and Mokowanis River but the upper part was measured by Stebinger and Bennett.

Siyeh limestone near Belly River

[From field notes of Eugene Stebinger and H. R. Bennett, 1914]

Limestone, magnesian, baked in lower part17
Limestone, baked15
Limestone, thin-bedded, slabby; with siliceous markings, occasional beds of blue-gray shale up to a foot thick and some beds of magnesian limestone. Includes a zone up to 2 ft thick containing Collenia183
Limestone containing Collenia10
Limestone, buff thin-bedded, shaly9
Limestone containing Collenia2
Limestone, buff, thin-bedded; with siliceous markings and a few beds of gray shale up to a foot thick148
Limestone, thin-bedded, shaly; and calcareous shale, mostly blue-gray, weathering buff with some beds of massive limestone, some of which contain Collenia150
Limestone, bluish-gray, compact, with stromatolites throughout. Probably the Collenia frequens zone. (Conophyton zone 1 of Rezak) (Above beds were measured by Stebinger and Bennett. Those listed below were measured by Corbett and Williams at a different place. They carried their section to the top of a peak where the rocks which in the present report are referred to as the Conophyton zone 1 are absent. By comparison with exposures in nearby peaks, Corbett infers that the top of his section is immediately below this zone.)67
Limestone, massive; with some bedding planes visible. Collenia in upper 5 ft35
Sill; probably same sill as listed above, but at a lower horizon65
Limestone, predominantly massive; with some thin-bedded limestone. Several beds, especially in lower part, contain Collenia. Upper 10 ft is baked198
Limestone, dark-gray, massive, sandy, crossbedded3
Limestone, dark-gray, massive, with Collenia, Many of the Collenia are shallow; domed forms up to 6 ft in diameter15
Limestone, thin-bedded17
Limestone, dark-gray, rather thin-bedded, sandy3
Limestone, light-gray, massive, sandy2
Limestone, alternating thin-bedded and massive. The thin-bedded material predominates and is in part argillaceous. Some of the massive beds (commonly less than 3 ft thick) contain Collenia646
Quartzite, dark-gray1
Limestone, argillitic9
Argillite, gray, calcareous18
Quartzite, gray, crossbedded1
Argillite, greenish-gray and gray calcareous with thin limestone and quartzite beds33
Limestone, alternating massive and thin-bedded; the former containing Collenia25
Argillite, greenish-gray4
Limestone thin-bedded, argillaceous in upper part29
Alternating beds of calcareous argillite and argillaceous limestone with numerous beds of comparatively pure limestone29
Limestone, rather massive, sandy; with Collenia15
Argillite, calcareous; with several thin quartzite beds and a little argillitic limestone30
Quartzite, white and gray, massive13
Limestone, argillitic8
Limestone, gray, rather thin-bedded2
Quartzite, gray, crossbedded; in beds a foot thick, in part separated by argillaceous layers11
Limestone, argillitic; with a few thin quartzite beds15
Quartzite, gray, massive1
Limestone, gray, argillaceous; with numerous quartzite beds nearly a foot thick31
Quartzite, gray, massive1
Limestone, massive1
Argillite, green, with thin quartzite beds9
Argillite, calcareous, with thin quartzite beds10
Limestone, massive1
Quartzite, light-gray1
Argillite, gray, calcareous5
Limestone, massive; with domes of Collenia up to 2 ft in diameter6
Argillite, gray, calcareous8
Limestone, thin-bedded, argillaceous; with a few quartzite beds up to 6 in, thick22
   Thickness, exclusive of sills



Correlations and Divisions

All the Belt series above the components already described belongs to the Missoula group. The assemblage, where not eroded, is very thick, and its mappable divisions differ markedly within short distances. In a broad way the Missoula group consists of red and green fine-grained clastic rocks with some intercalated limestone, mostly impure. Among the clastic beds all gradations between argillite and quartzite exist. Ripple marks, mud cracks, and other evidence of shallow-water deposition are plentiful. The limestone, especially the relatively pure beds, has close lithologic similarity to the Siyeh limestone described above.

Many names and descriptive terms have been applied to components of the Missoula group in Glacier National Park and other areas in northwestern Montana. It is not the purpose of the present report to attempt stratigraphic correlations throughout the broad region in which the group crops out; but as names from distant localities have been applied in previous work in Glacier National Park, some discussion is required. As already indicated, the top of the Piegan group and the base of the Missoula group are placed in the present report at the top of the definitely calcareous beds in the Siyeh limestone and at the base of beds so predominantly argillaceous that their resemblance to the dominant part of the Missoula group is obvious. This places the base of the Missoula group in and near Glacier National Park stratigraphically lower than has been done in any previous publication. The decision to do this arose from the relationships of the Missoula group in the southern part of the Flathead region to beds farther south, and, in turn, from the relationships of those beds to the Missoula group in its type locality near Missoula. So far as mapping within the park is concerned, the procedure provides convenient map units and avoids long-range correlations. It seems the only logical procedure so long as lithologic character remains the basic criterion for stratigraphic subdivision and areal mapping in the rocks of the Belt series. Some uncertainty is introduced in the interpretation of data recorded by geologists who used other limits for the divisions of the Piegan and Missoula groups. The principal difficulty is in the interpretation of the field notes of Campbell's men for areas not checked during the fieldwork of 1949 and 1950 (shown on index map of pl. 1). Any inconsistencies that may have resulted are small in terms of a map of the scale and detail of plate 1.

The argillaceous rocks above the Siyeh limestone of the present report correspond essentially to the Spokane formation of the Fentons (1937, p. 1897-1898) and to the red and green argillite band in the Siyeh of Clapp (1932, pl. 1). Clapp and Deiss (1931, p. 691-693) agree with the Fentons in correlating the argillite "band" with the Spokane formation although they do not apply it quite so specifically to rocks in Glacier National Park. The correlation of this unit in Glacier National Park with the Spokane formation of the Spokane Hills (Walcott, 1899, p. 199-215) and other localities in the general vicinity of Helena (Pardee and Schrader, 1933, p. 11, 125-126) must remain questionable, at least, until much further work is done in the broad region between Glacier National Park and Helena. No map now available brings the Spokane formation from the Helena region into the vicinity of the Flathead region, and further, neither the Spokane, of the Fentons, in the park nor units above it can be traced satisfactorily southward even as far as the Flathead region, Consequently, the name "Spokane" is not here employed, and the beds thus designated by the Fentons are regarded as an unnamed part of the Missoula group.

The argillaceous beds immediately above the Siyeh limestone in the northern part of the park are overlain by flows of the Purcell basalt, with some intercalated sedimentary rocks. Data summarized by the Fentons (1937, p. 1903) suggest that in Canada similar lava occurs at widely separated horizons in the Belt series, so that correlations in localities distant from Glacier National Park would have to be made with caution. In the park, flows belonging to the Purcell basalt are succeeded upward by the Sheppard formation of the Fentons (1937, p. 1899-1900), now spelled Shepard because of a decision of the U. S. Board on Geographic Names (Wilmarth, 1938, p. 1980). This unit can be safely mapped only where it is underlain by Purcell basalt, but it is there an easily recognized map unit and the only assembly of sedimentary beds constituting a formation within the Missoula group to which a formal stratigraphic name is attached in the present report. The Shepard formation is equivalent to the Sheppard quartzite of Willis (1902, p. 316, 324), but on the whole, it contains much less quartzite than he supposed. The Shepard formation, which is largely dolomitic, is the equivalent of the upper part of the Siyeh of Clapp (1932) as nearly as can be judged by comparing his small-scale map with plate 1 of the present report. If so, Clapp and Deiss (1932, p. 691) suggest correlation with the Helena limestone of the Helena region. The same objections apply to using Helena in Glacier National Park as those given above in regard to the Spokane formation. Willis (1902, p. 316, 324) proposed the name "Kintla argillite" for the part of the Belt series above the Shepard formation in the northern part of Glacier National Park. However, he observed a thickness of only 800 feet and, as he saw no beds above his Kintla argillite, assigned no upper limit. Daly's (1912, p. 81-82) use of the name is essentially the same as Willis'. It is now known that the Missoula group extends for thousands rather than hundreds of feet above the Purcell basalt. As there is no way of separating the Kintla argillite of Willis from similar beds at horizons above those he saw, the name is not useful at present.

In the southern part of the park and in the Flathead region, no subdivisions of the Missoula group have received formal stratigraphic names. Some names have been used by other workers in areas south of the Flathead region, but none can be used in the Flathead region with confidence because of the marked lateral changes in lithologic character that are a feature of the group. The most complete sets of names are those that have been proposed for the vicinity of Missoula, the type locality of the group, and for areas to the northeast and north. Near Missoula, division of the group into five formations was proposed by Clapp and Deiss (1931, p. 677-683). Clearly these cannot be recognized far from that area, for when Deiss carried his studies into the Saypo, Ovando, and Silvertip quadrangles (pl. 3) short distances away (Deiss, 1943, p. 211-218), he was able to recognize only the Miller Peak argillite, the lowest of the five divisions near Missoula, and proposed three new formations above this basal unit, the Cayuse limestone, Hoadley formation, and Ahorn quartzite in ascending order. While there are significant differences in character and thickness between the divisions of the group in the Silvertip and Missoula areas, Deiss (written communications, 1950) tentatively regards the part of the Miller Peak argillite (the basal formation of the group) present in the Saypo, Ovando, and Silvertip quadrangles as equivalent to the upper 1,000 feet of the Miller Peak argillite in its type locality, which is south of Missoula. He thinks that the Cayuse limestone (1,000 ft) is probably equivalent to the lower part of the Hellgate formation and that the Hoadley formation (4,100 ft) is probably equivalent to the upper half of the Hellgate formation and the whole of the McNamara formation. The Ahorn quartzite (2,100 ft) he regards as equivalent to the lower part of the Garnet Range formation which near Missoula is 7,600 feet thick. As one example of the lithologic changes implied by such correlations, it may be pointed out that Clapp and Deiss say the lower 1,600 feet of the Garnet Range formation is made up of brown and green-gray to gray thin-bedded siliceous micaceous coarse-grained quartzite, with argillitic and coarse-grained quartzitic sandstones near the base. Above this 1,600 foot unit is 600 feet of black-gray to dark-blue-gray sandy micaceous argillite. In contrast, the supposedly equivalent Ahorn quartzite is described as consisting of 1,700 feet of pink thick-bedded quartzite with occasional beds of red sandstone overlain by 400 feet of green and red-gray thin-bedded argillite with a few intercalated thin beds of fine-grained sandstone. Unfortunately no maps showing the distribution of these various divisions of the Missoula group have been published.

Comparison of the data summarized above with descriptions of the rocks in the Flathead and Glacier National Park regions given below shows that there are significant differences between beds in the southern part of the Flathead region and those reported farther south as well as differences from place to place within the two regions covered by the present report. Some of the differences in the character of the uppermost beds of the Missoula group in different localities are explained by Deiss (1935, p. 95-124) on the assumption of marked differences in the amount of removal of the upper part of the group by erosion before deposition of Paleozoic strata began.

Even if this assumption should prove to be correct, there are differences in the character of the beds within distances of a few miles that cannot be accounted for in this manner. It must be remembered that wherever the contact between Precambrian and Cambrian strata in this part of Montana has been observed, the beds are essentially flat and parallel to each other. Deiss (1935, p. 102) calls attention to two localities where angular discordances are reported, but both are far from the areas here considered. Hence, the explanation of differences in character as a result of differences in depth of erosion would have to account for differences in the amount of erosion amounting to thousands of feet within a few miles without any appreciable slopes having been produced. Drastic erosion in post-Missoula time would surely have been recorded in observable discordances at the contact with the beds deposited on the eroded surface. In the absence of known discordances the logical assumption is that there are such marked lateral differences in the components of the Missoula group that formation names applied in areas south of the Flathead region cannot be applied safely to any of the divisions of the group within that region.

For present purposes the whole of the Missoula group except units locally discriminated because of distinctive lithologic features is referred to simply as the main body. When the group is better known, more formal names will be needed, and correlation of some of the map units of the present report with already named formations in other regions may be possible. The main body, as that term is here used, is described first because it comprises much the largest part of the Missoula group and in some places constitutes the whole of that group that is present. High in the sequence in the southern part of the Flathead region, two small but distinctive units are mapped separately but described with the main body. These are greenish-gray argillite and pale pink quartzite. From a regional standpoint they do not appear to have enough significance to warrant separate description. On plate 2 and the southern part of plate 1, a discontinuous basal unit that consists dominantly of green calcareous argillite is mapped separately. Description of this unit follows that of the main body. The unit is probably present also in the northern part of Glacier National Park. If so, it has been included in that area with the main body. The green calcareous argillite, although at the base of the Missoula group where mapped, does not correspond either in character or thickness to the Miller Peak argillite, which Clapp and Deiss (1931, p. 677-683) place at the base of the group. The description of the green calcareous argillite is followed by that of the Purcell basalt, which comprises thin lava flows intercalated in the Missoula group mostly at a single horizon; and this in turn is followed by the description of the Shepard formation. Finally, a description is given of the bodies of carbonate rich rocks, other than the Shepard formation. These form more or less lenticular bodies at several horizons in the Missoula group, and some may eventually be approximately correlatable with such named formations as the Cayuse limestone of Deiss or the Shepard formation of the present report, although not actually coextensive with either. These carbonate-rich bodies bear much resemblance to the Siyeh limestone and are here referred to as limestone in conformity with established custom regarding that formation. Some of these limestone lenses are so nearly at the same relative horizon as the Shepard formation that they must be nearly equivalent thereto. In agreement with this suggestion, a few of them were called Sheppard in the field notes of parties under Campbell. Attempts in 1950 to trace mappable links between the Shepard formation in the northern part of the park and limestone masses farther south were unsuccessful.

Main Body

The greatest part of the Missoula group consists of an assemblage of grayish-red, purplish-red, brownish-red, and grayish-green dominantly argillaceous rocks that differ in details from place to place. Some quartzite, argillaceous quartzite, and argillaceous beds with different proportions of calcium and magnesium carbonates are included. This diverse assemblage, whose division must be deferred until much more detailed studies are accomplished, is provisionally called the main body of the group. The unit is one of the most widespread in the region. It occupies the northeast slopes of the Swan Range, and of the Flathead Range, and extends eastward to the trace of the Lewis overthrust in the middle of the Marias Pass quadrangle. It is conspicuous on the slopes on the northeast side of the Middle Fork of the Flathead below Bear Creek, and probably constitutes the principal rock in the unmapped areas on Hungry Horse and Firefighter Mountains west of Wildcat Mountain. The Apgar Mountains and the area on and south of Desert Mountain are composed of it. Remnants of the same unit extend, in patches isolated by erosion, from Almost-a-Dog Mountain northwestward through the median part of the park past the Canadian border west of Waterton Lake. Some idea of the general appearance of the formation can be gained from figure 3.

The argillite characteristic of this unit is softer and less siliceous than most of the Grinnell argillite. In most places reddish beds predominate, but green ones are rarely entirely absent, and some beds are gray. One mass of green argillite is so conspicuous a feature of Chair Mountain that it was mapped separately. Lithologically identical rocks are present in other parts of the main body but have not been distinguished on the map. Many are in bodies too small to show at the scale of the map. Distinctly purplish rocks, so common in the Grinnell argillite, are almost entirely absent in the Missoula group, a feature which in most localities is sufficient as a basis for discriminating between rocks of the two formations with considerable confidence. However, as noted in the description of the Grinnell argillite, the rocks of that unit, especially where exceptionally reddish, may bear sufficient resemblance to some of those in the Missoula group that confusion is possible where stratigraphic relations are not evident. Although most of the main body is argillaceous, quartzite, of varying degrees of purity, is widely distributed. In places near the top of the group, it is the dominant rock.

One distinction between the beds of the Missoula group and those at lower horizons is that the older rocks seem slightly more metamorphosed than those of Missoula age. In the Missoula group the original shapes of most of the clastic grains are preserved to a greater extent than in the Grinnell argillite. Some are subangular and, locally, even sharply angular, but many are well rounded (fig. 9c). The components of the coarser beds are more perfectly rounded than those of most of the fine-grained layers. Some of the rounded grains are coated with thin films of sericite. Some recrystallization has taken place as some quartz grains are intimately interlocked and some plagioclase grains have clear, added rims. In some of the argillite, thin very fine-grained layers were broken and twisted around before consolidation. The composition of the clastic rocks of the Missoula group is so nearly identical with that of comparable beds in the Grinnell argillite that no distinction can be made on that basis.

Here and there within the dominantly argillaceous main body of the Missoula group, certain beds weather a rusty brown that is similar to parts of the Siyeh limestone and to limestone beds above the Siyeh. Some of these rusty beds are impure limestone in assemblages so intimately interbedded with argillite that they were not mapped separately. Most of them, however, do not contain enough calcium carbonate to be detected by field tests. In a few places argillaceous beds contain small calcareous bodies that appear to be in whole or in part made up of stromatolites (fig. 15). Some are irregular lenses several feet long; others are concretionlike bodies commonly a few inches in diameter but alined within a single bed or assemblage of narrow beds for distances of tens or scores of feet along the strike. Some of these bodies are obviously calcareous, but many appear to be partly or wholly siliceous. The first variety is well displayed on the trail leading to Spruce Lookout, while the latter are common on Patrol Ridge, where together the outcrops probably represent a stratigraphic thickness of some scores of feet at least.

Ripple marks are very common in the argillite of the Missoula group, and mud-cracked surfaces are fairly so (figs. 12-15). The ripple marks tend to be smaller, less accentuated, and somewhat more uniform than those in the Grinnell argillite. In the Grinnell argillite, cross ripples and certain bulbous forms of obscure origin are rather common (fig. 5). Intraformational conglomerate is fairly plentiful but is a less common and striking feature than in the Grinnell.
fig12  Argillite of the Missoula group with sand-filled mud cracks. Upper Twin Creek, Marias Pass quadrangle, Flathead region
Fig13 Cusped ripple marks in argillite of the Missoula group, head of Grouse Creek, Marias Pass quadrangle, Flathead region
fig14 Clay spalls in argillite of the Missoula group, below Mount Bradley, Marias Pass quadrangle, Flathead region
fig15 Laminated argillite of the Missoula group with a small stromatolite, in railroad cut nearly opposite the mouth of Coal Creek, Nyack quadrangle, Flathead region.
Wherever sufficiently detailed observations have been made, variations within short distances in the components of the main body of the Missoula group are manifest. The scattered data available are summarized below.

On the east side of Cruiser Mountain, Masursky in 1949 found somewhat less than 1,000 feet of ripple-marked and mud-cracked red-purple and yellow-green argillite with a few quartzite beds. Next above he noted two narrow sills, grouped as a single sill on the map because of limitations of scale. Above the sills quartzite beds become increasingly numerous and thicker until at the top the rock is almost exclusively quartzite, which is in part purplish red, in part almost white. The color differences are not diffuse, but occur in speckles, shreds, and bands. The thickness of beds of the Missoula group above the sills is roughly equal to that of the exposed beds below. Unfortunately, available data as to the lateral extent of the quartzitic rocks on Cruiser Mountain do not permit representing them on the map. Most of the Precambrian rocks above the sill that is mapped are quartzitic. Near the crest of Cruiser Mountain, the beds of the Missoula group are overlain by nearly white rather coarse-grained quartzite regarded as belonging to the Flathead quartzite of Cambrian age. The lithologic difference is sufficiently well marked so that Masursky felt that the top of the Precambrian rocks could be fixed with confidence, but he found no evidence of discordance or unconformity at the contact.

On Chair Mountain, about 5 miles northwest, the uppermost beds of the Belt series are distinctly different from those just described. At this locality, exposures on the lower slopes on the Dolly Varden Creek side are poor but consist mainly of purplish-red ripple-marked argillite. On the ridge crest, light-red to pink crossbedded quartzite is overlain by several hundred feet of green argillite, which, in turn, is overlain by Flathead quartzite. The green argillite is increasingly gritty upward. This green argillite is distinguished on plate 2 but was not found in any locality other than Chair Mountain. In most places the horizon at which it might be expected has been removed by recent erosion or has been faulted out.

On Argosy Mountain most of the beds of the Missoula group are quartzitic. Most are reddish, but some are pink to nearly white, and some purplish red and green argillite is present. The beds on this mountain are so much deformed that it would be difficult to determine the proportions of the different kinds of rocks present. The lower slopes on and near Union Peak are underlain largely by red and green, locally gray, thin-bedded argillite. Near the ridge tops some light-red and nearly colorless quartzite is exposed. On and southwest of Union Peak, pinkish quartzite is so conspicuous that it is mapped on plate 2. There is much white, reddish, and greenish quartzite on Lodgepole Ridge, but like that on Argosy Mountain, it is so intermingled with argillite that it could be distinguished only on a large scale map.

Rather pale-reddish or pinkish coarse-grained quartzite crops out along the South Fork of the Flathead River close to the southern boundary of plate 2 and attracts attention in roadcuts and cliffs along the stream for some distance south of that boundary. Presumably this quartzite is near the top of the Missoula group, but the base of the block of Cambrian rocks east of the river is not exposed; consequently, the character of the rocks at the contact is not determinable.

The localities described above are in the upper part of the Missoula group. Erdman (1944, p. 56-59) in connection with his study of the Hungry Horse dam site measured the lower part of the group near the dam site. The descriptions in the section below are somewhat abbreviated from those in his report. He also gives a very detailed section of the unit near the top of the generalized section in which carbonate rocks are abundant.

Generalized section of lower part of Missoula group in secs. 4, 5, 8, 9, T. 29 N., R. 18 W.

[Adapted from section by C. E. Erdman (1944, p. 56-57)]

Top concealed.
Argillite, dull, red and maroon, irregularly bedded, platy; some variegated dull green rock, and some thin beds of gray quartzite; thin red shaly films on some bedding surfaces.1,500±
Argillite, green, dull; irregular beds 1/4-2 in. thick; some layers of gray-green sandstone825
Dolomite (top concealed); mainly lentils and irregular masses of limestone and dolomite intermingled in such a way as to suggest "molar tooth" structure; some greenish argillite and occasional thin layers of pure gray limestone or sandy green limestone. Resembles Siyeh limestone500
Argillite, green to gray-green; thin layers of quartzitic sandstone290
Argillite, red to maroon, dull; few layers of dull green rock and some reddish quartzitic sandstone; has ripple marks and mud cracks240
Argillite, gray-green; mud cracks and ripple marks; some layers of brown quartzitic sandstone1,200
Argillite, red and maroon, dull, with ripple marks, mud cracks and mud lumps; few layers of green and buff sandy argillite165
Argillite, green, dull, with a few limestone and sandstone beds; mud cracks, clay galls and blebs parallel to bedding170
Argillite, red and maroon, dull; red beds appear to lens out to northwest70
Argillite, gray-green, massive; some sandstone or quartzite and limestone layers160
Transition zone, alternating beds about 1 in, thick of gray limestone, greenish-gray argillite, gray quartzite, and buff-weathering dolomite. The whole is sprinkled with sand in grains and lenses, some of which contain mud lumps. Proportion of carbonate rocks decreases upward, and near top of zone some layers are reddish260
Top of Siyeh limestone.


In the part of Glacier National Park south of exposures of the Purcell basalt, the following section of the lower part of the Missoula group was measured in July 1914 by C. S. Corbett and E. S. Williams. The top of this section is at the base of a limestone body thought by Corbett and Williams to be long to the Shepard formation. In the absence of the lava, this cannot be regarded as a definite correlation.

Lower part of the Missoula group on the upper slopes of Almost-a-Dog Mountain

[Measured by C. S. Corbett and E. S. Williams, July 30, 1914]

Argillite, green29
Argillite, red, micaceous; some beds as much as 6 in, thick17
Argillite, green and gray, interbedded; a few red beds. Beds range up to a foot in thickness385
Argillite, pinkish-gray; in beds up to 5 in. thick with sandy layers up to 3 in, thick18
Argillite, red micaceous45
Argillite, greenish-gray, rather coarse-grained14
Argillite, red; in beds up to 6 in. thick32
Argillite greenish-gray; in beds up to a foot thick52
Argillite, red6
Argillite, gray; with numerous beds of green argillite in it. Beds range up to 8 in, thick but are finely laminated. A few beds of gray sandy limestone in the lower part294
Argillite, red; in beds up to 6 in. in thickness26
Argillite, green and gray, interbedded. Gray beds are a little coarser grained than the green ones and range up to 5 in, in thickness25
Argillite, red; with a few thin gray layers56
Argillite, pink, thin-bedded4
Argillite pinkish-gray; weathers buff3
Argillite, green, thin-bedded25
Argillite, gray; with thin sandy layers; weathers pink8
Argillite, red; with some gray beds7
Argillite, gray, sandy, slightly calcareous3
Argillite, red, thin-bedded; with some gray beds15
Limestone, gray, sandy1
Argillite, red; with thin green beds and a few sandy beds up to 4 in, thick24
Argillite, gray and pink, calcareous; with calcareous red sandstone at base12


The Fentons (1937, p. 1897-1898) report the following section of the lowest part of the main body of the Missoula group in the basin above Hole-in-the Wall Falls in the northern part of the park. The sedimentary beds in the section represent what they called the Spokane formation, and others have referred to as the "red band in the Siyeh" or similar expressions. If the greenish calcareous argillite distinguished at the base of the group in the Flathead region had been mapped in the northern part of the park, the lower 75 feet of the section and possibly some of the beds above that would have been included in it.

Partial section of the Missoula group in Hole-in-the-Wall Basin

[After C. L. and M. A. Fenton, 1957. p. 1897-1898]

Diabasic lava belonging to the Purcell basalt, dark-greenish, massive; flow structures pronounced; pipes, pillows, and inclusions of sedimentary material abundant basally180-200
Argillite, green, finely banded, hard and much fractured. Upper part has been caught up and included in the lava flow above3-5
Diabasic lava, dark-greenish; pillows and flow structures less pronounced than in the flow above. Thickness increases eastward6-20
Argillite, dark-green, finely banded, with several sandy layers. Large ripple marks in upper beds. Near the top is a metamorphosed red layer 6-8 in. thick; near the bottom is a bed containing altered colonies of Collenia undosa16
Argillite, with several beds of quartzite, both dark and red; some layers of brownish-red conglomerate. Crossbedding, ripple marks and mud cracks throughout.75
Argillite, red, buff, and buff-red in lower parts; grades upward into greenish, and terminates in red argillites interbedded with quartzite. Ripple marks, mud cracks, and crossbedding common in the quartzite270
Argillite, greenish to buff; thick beds 2-3 ft thick; other, less abundant, beds are thinner15
Limestone, magnesian, gray to buff, thinly bedded, shaly; grades upward into yellow to reddish and finely mud-cracked argillite60
   Total of sedimentary rocks

Top of Siyeh limestone.

In August 1914 E. M. Parks noted that 700 feet of alternating reddish and greenish argillite beds underlies the Purcell basalt on the west side of Mount Cleveland. A more detailed section of the beds in the same locality, obtained by Eugene Stebinger and H. R. Bennett in September 1914 is given below. Stratigraphically, this corresponds to the section in Hole-in-the-Wall Basin just given, but there are many differences in detail. Similar differences would be found wherever comparable observations were made.
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