Geological Survey Professional Paper 294—D Stromatolites of the Belt Series in Glacier National Park and Vicinity, Montana

GENERAL

Detailed stratigraphic descriptions of the major units of the Belt series in the area under consideration will be given in a forthcoming paper by C. P. Ross¹ and will not be repeated here.

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¹Ross, C. P., Geology of Glacier National Park and the Flathead region, northwestern Montana-U. S. Geol. Survey Prof. Paper 296 (in preparation).

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Stromatolites are known to exist in each of the major units of the Belt series in the Glacier National Park region except the Appekunny argillite. (See fig. 50.) Eight distinct zones occur in the area. Not all stromatolites, however, are restricted to these zones. Many isolated bioherms and small biostromes occur at random throughout the section. By far the greater number of these fossils is found in the carbonate rocks, but the stromatolites are not restricted to this type of lithology. It has been noted that some argillites and even quartzites contain stromatolites. Each of the 8 zones is characterized by the dominance of 1 or 2 species, and 7 of the zones are known to persist throughout the park and the areas to the south and west of the park.

FIGURE 50.—Stromatolite zones in Glacier National Park.

The names of the zones are taken from the species that occurs in greatest abundance. For example, the Conophyton zone 1 consists primarily of Conophyton inclinatum n. sp., but this is not the only species that may exist in that zone. Collenia frequens Walcott, Collenia multiflabella n. sp, and Cryptozoon occidentale Dawson also occur but in lesser quantities.

STROMATOLITE ZONES

RAVALLI GROUP

ALTYN LIMESTONE

COLLENIA FREQUENS ZONE (C. COLUMNARIS ZONE OF FENTON AND FENTON)

The Fentons (1931, p. 676) describe the zone as a "massive, light gray, limestone in a single bed some 20 feet thick, and virtually the entire mass of stone is formed by the colonies of this one alga." In a later paper (1937, p. 1833) they describe the zone as occurring in the upper part of their Hell Roaring member of the Altyn limestone. According to the Fentons, "the uppermost Hell Roaring beds are massive, light gray, dolomitic limestones interbedded with edgewise mud breccias and lenses of dolomitic sandstone or conglomerate; their thickness averages 85 feet. Massive beds are crowded with Collenia colmnaris (C. frequens) * * *, They form the Collenia columnaris zone." The zone is reported to extend from the vicinity of Appekunny Falls in the Many Glacier area southward to Spot Mountain at the entrance to Two Medicine valley. I have examined the zone at the type site (Appekunny Falls) and on Divide Mountain. At both localities it is very well developed and fits the description given by the Fentons in 1937. Elsewhere the zone is either less conspicuous or entirely lacking. The very narrow outcrop belt of the Altyn limestone and the fact that the formation is cut by numerous minor thrusts associated with the Lewis overthrust may account for the fact that the zone is missing in some localities.

GRINNELL ARGILLITE

Before the summer of 1953, stromatolites were not known to exist in the Grinnell argillite. During that summer two occurrences were discovered.

The first discovery was made at the west end of Bad Rock Canyon along the tracks of the Great Northern Railway. A 3- to 4-foot-thick bed of greenish-gray dolomitic argillite contains small structures that have been doubtfully referred to the genus Newlandia. These fossils are similar to forms in the Prichard formation in the Coeur d'Alene district to the west. However, slump structures were recognized near some of the stromatolites and their organic origin is questionable.

COLLENIA UNDOSA ZONE 1

The second discovery was made on Going-to-the-Sun Highway in St. Mary valley about 0.3 miles east of the bridge across Baring Creek. There, a 10-inch bed of green argillite contains stromatolites identified as Collenia symmetrica Fenton and Fenton, Cryptozoon occidentale Dawson, and Collenia undosa Walcott. The bed occurs about 200 feet above the base of the formation. It is quite similar to the beds of the Collenia undosa zone in the lower part of the Missoula group, except that the biostrome in the Grinnell occurs in a section consisting chiefly of reddish-brown sandstone interbedded with green argillite. It was not possible to determine the areal extent of these zones because they were found late in the last season of field work. However, it is possible that they are as extensive as the other zones in the area and they may serve as useful horizon markers within the Grinnell argillite when more detailed investigations are conducted.

The following table illustrates the relative stratigraphic positions of the zones as compared with the stratigraphic column described by Fenton and Fenton (1937, p. 1880-1890).

Ravalli group

PIEGAN GROUP

SIYEH LIMESTONE

COLLENIA SYMMETRICA ZONE 1 (COLLENIA SYMMETRICA ZONE OF FENTON AND FENTON)

This zone, the thickest in the area, extends from the base of the Siyeh limestone to the base of the Conophyton zone 1 (fig. 50). Its thickness ranges from 2,500 to 3,000 feet. The zone differs from the others in that it does not contain widespread biostromes. The stromatolites occur at random throughout the zone as isolated colonies or thin, discontinuous biostromes. Collenia symmetrica is the dominant form in the zone, but occasional colonies of Collenia multiflabella n. sp. may also be found. The stromatolite beds seem to be restricted to the fine-grained black-and-tan-laminated argillaceous limestones. Associated with the stromatolites are beds of edgewise breccia containing fragments of algal colonies. (See pl. 22, fig. 5.)

Many beds in the Collenia symmetrica zone 1 and other limestones in the section contain so-called segregation structures that lend an unusual appearance to weathered surfaces. (See pl. 20, fig. 7.) Some of the structures consist of irregular lenticles of relatively pure limestone contained in a matrix of dolomite. Hilary Bauerman (1885, p. 26) first described the structures and named them "molar tooth" because of their resemblance to the markings on the molar teeth of elephants. Daly (1912, p. 73-76) also used the term "molar tooth" and stated that they were formed by segregation along cleavage planes at high angles to the bedding surfaces. The Fentons (1937, p. 1927-1928) agreed with Daly in their idea that segregation occurred long after lithification. The structures probably assumed their present appearance at the time of the Lewis overthrust.

Fenton and Fenton assigned this zone to the lower 500 to 900 feet of the Siyeh limestone and separated it from the Conophyton zone 1 with their Goathaunt member. In the present investigation I have found that the Fentons' Goathaunt member contains Collenia symmetrica Fenton and Fenton in about the same frequency of distribution as the Collenia symmetrica zone 1. Consequently, the zone is here extended to include the Goathaunt member of the Fentons.

The type site of the zone is on the ridge forming Cut Bank Pass, between the Cut Bank and Dry Fork valleys, Glacier National Park. The zone is extensive and may be seen wherever the lower part of the Siyeh limestone crops out in the park area. It has been traced as far west as the upper access road to Hungry Horse Dam.

The following table illustrates the relative stratigraphic positions of the zones as compared with the stratigraphic column described by Fenton and Fenton (1937, p. 1890-1900).

Piegan group

CONOPHYTON ZONE 1 (COLLENIA FREQUENS ZONE OF FENTON AND FENTON)

This is the most conspicuous zone in the park. (See pl. 18.) It forms a massive layer of dark-gray crystalline to fine-grained limestone that may be seen from a great distance. The early workers in the area referred to the zone as the "massive bed" before its algal origin was suspected. Fenton and Fenton called it the Collenia frequens zone, thinking that the conical form so abundant in the zone is what Walcott described as Collenia frequens. Where the zone shows the densest growth of stromatolites, Conophyton inclinatum n. sp. is most abundant, with Cryptozoon occidentale Dawson, Collenia frequens Walcott, and Collenia multiflabella n. sp. occurring in subordinate amounts. Along The Garden Wall the zone may be subdivided as follows:

Subzone B thins gradually toward the west and is completely absent in the Whitefish and Flathead Ranges.

Where Conophyton inclinatum n. sp. occurs in biostromes, the colonies are closely packed, and no detrital material is apparent between the individuals. The podlike bioherms range in size from 5 feet to 22 feet in width and up to 6 feet in height. They contain closely packed colonies of Conophyton inclinatum n. sp., measuring from a few inches to 4 feet in diameter. Each bioherm has associated with it a variable thickness of finely laminated black and tan limestone. The laminae are continuous from the colonies on the margins of the bioherms and are apparently offreef algal mats. All the observed offreef deposits dip towards the west and are overlain by other bioherms. An excellent exposure of one of these bioherms and associated offreef deposits may be seen in the road cut 6.4 miles northwest of Logan Pass.

The type site of this zone is on the eastern slope of Swiftcurrent Pass on the Granite Park trail. The zone is on The Garden Wall near Haystack Butte, along Going-to-the-Sun Highway just below Logan Pass on both sides of the pass, and on the highway 6.4 miles northwest of the pass.

COLLENIA MULTIFLABELLA ZONE

Fenton and Fenton (1937, p. 1896) describe their Granite Park member of the Siyeh limestone as: "Magnesian limestones, oolites, argillites, and quartzites [that] represent the final stage of Siyeh sedimentation. Large colonies of Collenia willisii n. sp. are abundant at several horizons.

The Collenia multiflabella zone coincides roughly with the Granite Park member of Fenton and Fenton (1937). It contains Collenia multiflabella n. sp. and Cryptozoon occidentale Dawson in large biostromes that are up to 6 feet thick, Conophyton does not occur in the zone, although Fenton and Fenton (1933b, p. 1136) refer to bioherms of that form in their Granite Park member. The boundary they draw between their Granite Park member and their Collenia frequens zone falls within subzone C of the Conophyton zone 1.

The type site of the Collenia multiflabella zone is on Logan Pass near the point where the north fork of Reynolds Creek plunges into St. Mary valley. The zone is well exposed above the highway on both sides of the pass.

PLATE 18.—OUTCROP OF CONOPHYTON ZONE 1 IN McDONALD VALLEY. McDonald valley and The Garden Wall as seen from the top of Mount Oberlin. The Conophyton zone 1 can be seen crossing Going-to-the-Sun Highway at two points. The 100-foot zone appears as a narrow light-colored band crossing diagonally just below the center of the photograph.

MISSOULA GROUP

COLLENIA UNDOSA ZONE 2

The lower few hundred feet of the Missoula group consists chiefly of alternating red and green argillite and subordinate thin beds of pink limestone. The thickness of the unit varies appreciably over the area described in this report. However, the unit is easily recognized at all localities. In the vicinity of Logan Pass it is about 400 feet thick. The limestone beds average 1 foot in thickness and, although none has great lateral extent, together they are numerous enough to be recognized over a large area. The biostromes are crowded with colonies of Collenia undosa Walcott, Collenia symmetrica Fenton and Fenton, and Cryptozoon occidentale Dawson. Of the three species, Collenia undosa Walcott occurs in the greatest abundance. The colonies are composed of alternating layers of pink limestone and green argillite. On the fresh rock surfaces they present a spectacular appearance with the structure of the laminae made especially conspicuous by the alternating colors. On weathered surfaces the laminae show strong relief, owing to the differential weathering of the limestone and argillite layers.

The type site of the Collenia undosa zone 2 is near the base of Mount Oberlin at Logan Pass. At the type site, 8 biostromes of Collenia undosa Walcott were found. Other localities where the zone is well exposed are 0.2 mile east of the big switchback on The Garden Wall, 0.6 mile northeast of the lower end of Lake McDonald, and 1.2 miles south of Walton, Mont., on U. S. Highway 2.

COLLENIA SYMMETRICA ZONE 2

The upper half of the Missoula group is exposed only in the southwest part of the park and in the Flathead region to the south of the park. Elsewhere in the area it has been removed by erosion. Along the east side of the valley of the Middle Fork Flathead River, south of Riverview Mountain, a sequence of red and green argillite and quartzite is overlain by a limestone that is very similar in lithology to the Siyeh limestone. The field party of M. R. Campbell mapped these two units as Grinnell argillite and Siyeh limestone. In 1932, C. H. Clapp (1932, pl. 4) published a geologic map that included this area. On his map the units are designated "Grinnell argillite" and "Newland limestone." For both, it is necessary to postulate a steeply dipping fault extending up the valley of the Middle Fork Flathead River and separating the so-called Grinnell argillites on the east side of the valley from argillites of the Missoula group on the west side. Fenton and Fenton (1937, p. 1900) accepted this interpretation. However, in the course of the field work for the present report, no evidence was found to confirm the presence of this fault. Also, stromatolites were found in the argillites on the east side of the valley that indicate a Missoula age for those beds. In addition to this evidence, the stromatolite zones in the limestone differ considerably from those in the Siyeh limestone just a few miles to the east. The entire sequence lies conformably upon argillites of the Missoula group in the Flathead Range.

Collenia symmetrica zone 2 occurs about 6,000 feet above the base of the Missoula group in the area described above. The zone is about 50 feet thick and is made up of 3 biostromes of Collenia symmetrica Fenton and Fenton. Cryptozoon occidentale Dawson and Collenia frequens Walcott occur in minor numbers.

Section of Collenia symmetrica zone 2, measured just west of the tunnel at Singleshot, on the Great Northern Railway tracks along Bear Creek

The following table illustrates the relative stratigraphic positions of the zones as compared with the stratigraphic column described by Fenton and Fenton (1937, p. 1900-1903).

Missoula group

The Collenia symmetrica zone 2 differs from its counterpart in the Siyeh limestone in thickness and the lateral persistence of the biostromes. Zone 2 is much more compact vertically than zone 1. In the Siyeh limestone, no continuous beds of Collenia symmetrica Fenton and Fenton have been found. However, in the Missoula group the species forms widespread biostromes that are easily recognized in widely separated outcrops.

The zone is known to extend from Riverview Mountain on the east side of the Middle Fork Flathead River, southward to a point a quarter of a mile west of the mouth of Cabin Creek on the Middle Fork Flathead River, a distance of about 20 miles. It may also be seen on both sides of the valley of Bear Creek. The type locality of the zone is just west of the tunnel at Singleshot, on the Great Northern Railway tracks along Bear Creek.

CONOPHYTON ZONE 2

A second zone of Conophyton occurs approximately 400 feet above the Collenia symmetrica zone 2. It is similar to its counterpart in the Siyeh limestone when viewed from a distance. The zone averages about 100 feet in thickness and has the same massive character as the Conophyton zone 1. The elements of the zone, however, are considerably different. Conophyton inclinatum n. sp. and Collenia frequens Walcott are the only species that occur in the zone. Their distribution within the zone is constant over a large area. The following section is typical of the zone in the southern half of the mapped area. (See pl. 25.)

Section of the Conophyton zone 2, on hillside above trail opposite mouth of Cy Creek, NE1/4 sec. 35, unsurveyed T. 28 N., R. 15 W., Flathead National Forest, Mont.

The zone has been mapped for 20 miles along the strike and 7 miles along the dip. (See pl. 25.) From the Cy Creek locality to Running Rabbit Mountain the subzones are remarkably uniform in thickness and composition. However, from Running Rabbit Mountain northward to the northwest flank of Riverview Mountain, subzones B and E thin rapidly and subzones A and D thicken. The total thickness of the zone remains about the same. At Riverview Mountain the zone dips under Tertiary alluvium and its northern limits are unknown. The southern limits of the zone were not explored, owing to the inaccessibility of the area and the limitation of time.

The type locality of the zone is at the top of Running Rabbit Mountain, in the southwestern corner of Glacier National Park. The zone on the western slopes of Scalplock, Rampage, and Riverview Mountains may be viewed from U. S. Highway 2, between Essex and Pinnacle. A good distant view is also afforded from the highway along Bear Creek between Java and Fielding. Here the zone is exposed on the south slopes of Running Rabbit, Snowslip, and Blacktail Mountains. An excellent exposure of the zone occurs at the pass between Giefer Creek and Twenty-five Mile Creek, on the west side of Baldhead Mountain.