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Главная » 2013 » Ноябрь » 27 » Geology of Glacier National Park And the Flathead Region, Northwestern Montana (p4)
07:27
Geology of Glacier National Park And the Flathead Region, Northwestern Montana (p4)
 
Stromatolites have been recorded from the Grinnell argillite only in 2 localities: 1 along the Going to-the-Sun highway and 1 in Bad Rock Canyon. (Rezak, 1957, p. 136-137) This fact may aid in distinguishing its outcrops from otherwise similar exposures of the Missoula group in which stromatolites are far more abundant and easily recognized.

The Grinnell argillite is mostly in shades of red purple which, where of characteristic color, are distinctive enough to be recognized with confidence by anyone familiar with the formation. While the Missoula group also contains many red-purple beds, colors in that group approach reddish brown in hue. Typical beds in the Grinnell argillite are definitely more purplish than those typical of the Missoula, but the distinction is sufficiently delicate and the range in color is wide enough so that, as noted below, reliance on this feature alone is dangerous.

Even with the aid of color charts, precise designation of color in rocks presents difficulties. The opportunity for confusion is increased when descriptions by different authors are compared. Willis (1902, p. 316, 322), who defined the Grinnell argillite, spoke of it as dark-red shaley argillite, Both the Fentons (1937, p. 1887-1890) and Dyson (1949a, p. 7-8) speak of the red as a conspicuous feature. Probably all these authors used the term "red" in a general sense and ignored the purplish hues. The Fentons at the beginning of their description do say "red or purplish." Even the most distinctly red beds in both the Grinnell and the Missoula have a purplish cast.

The Grinnell argillite shows under the microscope rather less variation than might be expected from the variations in its megascopic appearance. From the appearance in thin section, it is clear that the original rock ranged from a siliceous mudstone or siltstone to a somewhat arkosic sandstone, a conclusion that is in agreement with the analyses on page 55. In the more argillaceous beds individual grains are only a few hundredths of a millimeter in diameter, but in the coarser layers the grain diameter ranges from 0.4 to more than 1 millimeter. Locally, coarse grains are irregularly scattered through a fine-grained matrix. In some argillaceous rocks the bedding is so irregular (fig. 6d) as to suggest that the original mud layers were disturbed while still unconsolidated. The argillaceous rocks now consist largely of quartz and fine flakes of mica, with some feldspar and, locally carbonate. The coarser grained rocks are similar except that micaceous minerals are less abundant and in some beds are nearly absent. Feldspar, largely alkalic plagioclase, is more conspicuous (fig. 6e) in the coarse-grained layers, but this difference may be more apparent than real. The minerals would be difficult to recognize in fine-grained rocks. Some of the grains in the coarser grained rocks are themselves fragments of fine-grained sedimentary rocks, and some of the quartz grains had been subjected to marked pressure before they were incorporated in the present rock. Many of the grains in these rocks are well rounded. Some are rimmed with material added late in the process of consolidation. This feature, shown somewhat indistinctly in figure 6E, is not as well developed as it is in many quartzites, suggesting that the process of recrystallization is far from complete. Carbonate is less widespread than it is in the Appekunny argillite. It is too scanty to be detected under the microscope in many of the rocks and is abundant in very few outside of the transition zone at the top. It is plentiful in some beds in Felix Basin, but those have been hydrothermally altered, and at least part of the carbonate may have been introduced in solution.
FIGURE 6.—Photomicrographs of rocks of the Belt series.



A. (Plane light) Supposed fossil in Altyn limestone near Swiftcurrent Falls, Glacier National Park. Fine-grained groundmass is carbonate, and large clastic grains are quartz and feldspar.



B. (Crossed nicols) Another view of the specimen shown in A, showing grains of quartz and one of striated plagioclase feldspar.



C. (Crossed nicols) Appekunny argillite from east of Lake McDonald, Glacier National Park. shows rhythmic bedding.



D. (Plane light) Edgewise conglomerate in Grinnell argillite, Tom Tom Lookout, Swan Range, Nyack quadrangle, Flathead region. Note disturbed flakes of fine grained argillite in quartzitic rock.



E. (Crossed nicols) Coarse-grained Grinnell argillite containing striated plagioclase feldspar, from the upper part of the formation, Wheeler Creek, Swan Range, Nyack quadrangle, Flathead region.



F. (Crossed nicols) Coarse-grained white quartzite in Grinnell argillite, Park Creek, Nyack quadrangle, Flathead region. Some of the grains show rims of added quartz.


In the Flathead and Swan Ranges the Grinnell argillite crops out extensively with the Appekunny argillite below and the Siyeh limestone above, evidently in normal stratigraphic relations uncomplicated by deformation. The formation consists largely of pale- and grayish-blue-green, grayish-purple, and grayish-red-purple siliceous argillite, in part slightly calcareous, and some quartzite. In the Swan Range the lowest and thickest part of the Grinnell is dominantly pale- and grayish-red-purple argillite, nowhere well exposed. Above this is the middle member in which the proportion of red-purple beds decreases upwards, and much of the rock is quartzitic argillite and argillaceous quartzite, with thin argillite partings, generally rather dark red-purple. Some of these partings more nearly resemble parts of the argillite of the Missoula group and of the red argillite in the Grinnell formation farther north than do any of the thicker beds in the Swan Range. The outcrops pictured in figures 4 and 5 belong to this middle member. The uppermost member of the Grinnell argillite commonly consists of grayish-blue-green calcareous argillite and argillaceous limestone, constituting a transition zone below the Siyeh limestone of the Piegan group. This member contains a few red-purple beds, and the unit below it contains some green beds. Nevertheless, the distinction is sufficiently definite so that the transition zone at the top of the Grinnell has been shown on plate 2. Conceivably this transition zone, or some part of it, corresponds to the "Collenia symmetrica zone," which the Fentons (1937, p. 1894) define as the "upper phase of the transition between the argillitic and arenaceous Grinnell to the dolomitic and limy Siyeh" and place at the base of the Siyeh limestone as defined by them. On that basis the uppermost member of the Grinnell as mapped on plate 2 would become the basal unit of the Siyeh limestone. This unit is more argillaceous than any part of the Siyeh limestone of the present report. No stromatolites were found in this unit in the Flathead region. If they should be found, the probability that the transition zone belongs in the Siyeh would be greatly increased.

The Grinnell argillite in the Flathead Range has features closely akin to those in the Swan Range but is even less satisfactorily exposed. Here also, the transition zone at the top is mapped, but the two subdivisions of the formation beneath this zone were not recognized, possibly because the exposures are so incomplete.

Along the northeastern side of that part of the Middle Fork of the Flathead River below its confluence with Bear Creek, Clapp (1932, pl. 1) mapped a rather large area as being underlain by the Grinnell formation, with the Missoula group on the opposite side of the river. This interpretation, which appears to have been accepted by Erdmann (1944, p. 45), would require that a large fault lies approximately along the channel of the Middle Fork. As indicated on plates 1 and 2 of the present report, the rocks on both sides of the Middle Fork below Bear Creek belong to the Missoula group and are in the normal stratigraphic position for that unit. This eliminates the necessity for a fault in the valley of the Middle Fork. No evidence of faulting here was found during the present study, and Clapp's error presumably arose from inferences in regard to color.

In most parts of the park the Grinnell argillite consists largely of reddish-purple argillaceous rocks, which in many localities are interbedded with other wise similar green rocks. Exceptionally, especially near the top of the formation, the green beds predominate. In some places, notably low in the sequence white and pink to reddish quartzite is conspicuous. Among the localities where the color is exceptionally reddish rather than purplish may be mentioned outcrops along the Going-to-the-Sun Highway west of St. Mary Lake. Here the beds might well be mistaken for components of the Missoula group if their stratigraphic position were not known.

During the present investigation the Grinnell argillite was studied more closely in the southern part of Glacier National Park than in the area north of latitude 48°40'. Where examined, the 3 subdivisions noted in the Swan Range are probably present, but the distinctions between the lower 2 are inconspicuous. The transition zone at the top is certainly present in most localities but, where seen, is less conspicuous than in the Swan Range. This may be due to absence of prominent exposures of the unit along the lines of traverse rather than to any fundamental stratigraphic difference. Nevertheless, present data make it impracticable to map the transition zone throughout the park, and it is not shown on plate 1. Some of the men who worked under Campbell distinguished the zone in the field; others did not. Allowance has been made for this so far as possible in the compilation of plate 1, but it may have resulted in places in some inconsistency in the placing of the upper boundary of the Grinnell. Insofar as information permits, that boundary is placed at the top of the rocks that are dominantly argillaceous and below those that are dominantly carbonate rocks.

The Fentons (1937, p. 1887-1890) studied the Grinnell argillite mainly in and north of the northern part of Glacier National Park in localities not examined closely during the present investigation. The three members they propose are, as judged from their descriptions, different from those in the Swan Range. Their lowest subdivision, which they call the Rising Wolf member, contains variable white and pink quartzite beds interbedded with red argillite in layers that range from mere laminae to beds 5 feet in thickness. Ripple marks and mud cracks are common. The thickness is given as 200-700 feet. The Fentons note that this member is not everywhere clearly distinguishable. The colors of both argillaceous and quartzitic beds in it differ from one locality to another. In some places, such as the vicinity of Ptarmigan Lake, green beds are conspicuous.

The thick middle unit is called the Red Gap member by the Fentons, (1937, p. 1889) and their descriptions show it to be of varied character. It is reported to consist of argillite "in thin minor and thick major beds, dominantly red but incidentally brownish or green, interbedded with pink, white, or greenish-white quartzite, brown sandstone, and sandy argillite." A characteristic feature is the thick beds of red argillite with flat mud-crack polygons. The maximum thickness is reported as 2,800 feet, but in places it thins to as little as 650 feet.

The upper part of the formation is called the Rising Bull member by the Fentons and is described by them as containing argillite, quartzite and mud breccias, forming the initial transition between the Grinnell and the Siyeh. The mud breccias of the Fentons (1937, p. 1905-1909) correspond to intraformational conglomerate. The rocks include beds of gray, red, green, pink and white. The thickness is reported to range from 600-1,100 feet. In and west of Waterton Lakes Park, a thin flow of amygdaloidal lava is intercalated in the upper part of the member, but no lava has been found in the Grinnell argillite anywhere south of the international boundary.

The wide variations in thickness estimates and in descriptions of the character of the rocks in different localities must reflect much lateral variation in the Grinnell argillite. In the Swan Range the two lower members together, as judged from plate 2, may have a thickness of 4,000-5,000 feet, and the upper member or transition zone is about 500 feet thick in the Flathead region, probably thinning northward. The unsatisfactory exposures in that region introduce a large element of uncertainty into any estimates of thickness. Nevertheless, the formation in the Flathead region must be exceptionally thick. The thickness of the whole formation in the southern part of Glacier National Park, as judged from plate 1, is close to 2,000 feet. Willis (1902, p. 322-323) speaks of the Grinnell in and near the northeastern part of the park as 1,000-1,800 feet thick. The Fentons (1937, p. 1887) give a range in thickness of 1,500-3,500 feet. Dyson (1949a, p. 7) says the thickness varies considerably but is greater than 3,000 feet in several localities.

In 1914 the Grinnell argillite was measured and described at the same two localities as the observations on the Appekunny argillite recorded above. Most of the Grinnell on Red Eagle Mountain was studied by Eugene Stebinger, but the transition zone at the top was studied by Corbett and Williams in connection with their examination of the Siyeh limestone. A thickness of 739 feet at the base of the section measured by Corbett and Williams contains so little limestone that it is here included in the Grinnell. Presumably agreement was reached in the field between Stebinger and Corbett, so that their sections do not overlap. Corbett and Williams did all of the work near Crossley Lake, but their observations were made on 2 different days and may not represent the whole of the Grinnell argillite in the vicinity. On the second day they measured 184 feet of beds which are here placed at the top of the Grinnell. (See following table.) Possibly some beds are missing between these and the section of the Grinnell they had measured earlier.

Grinnell argillite on Red Eagle Mountain

[After Eugene Stebinger, G. S. Corbett and C. R. Williams, field notes, 1914]











































Feet
Beds here assigned to the Siyeh limestone.
Grinnell argillite:


Argillite, gray, calcareous189


Quartzite, gray and green, interbedded145


Argillite, green138


Argillite, green and gray with numerous thin beds of coarse-grained sandstone and light-gray limestone125


Argillite, deep-red, shaly, intercalated with a somewhat larger amount of gray to reddish-gray coarse-grained crossbedded quartzite in beds 1-2 ft thick440


Argillite, alternate red and green; in beds 1-12 in, thick10


Argillite, deep-red, shaly; alternating with gray to reddish crossbedded quartzite in the proportion of about 60 percent of argillite to 40 per cent of quartzite73


Quartzite, reddish-gray to white; interfingered with an irregular layer, 1-3 in. thick, of red argillite6


Argillite, deep-red, shaly; alternating with gray to reddish crossbedded quartzite in the proportion of about 70 percent of argillite to 30 percent of quartzite753


Argillite, deep-red, shaly44


Argillite, red to maroon, shaly; intercalated With 20 percent of gray to white quartzite in beds 1/2-2 ft thick; mostly crossbedded315


Argillite alternate green and red, with a few thin beds of white quartzite46


Argillite, green; alternating with red coarse-grained crossbedded sandstone in beds 4 in. thick8


Quartzite, white42


Argillite, green, with beds of white quartzite up to 4 in. thick




Quartzite, white, massive23


Quartzite, white, and argillite, green22


Quartzite, white, massive20


Argillite, green19


Quartzite, white, in beds up to 6 in, thick16


Argillite, deep-red, shaly, intercalated with about 60 percent of gray to reddish-gray coarse-grained crossbedded quartzite in beds 1-2 ft thick257


Quartzite, pink, and deep-red argillite, in about equal, quantities, in beds 1-3 ft thick27


Argillite, deep-red, shaly; intercalated with a somewhat larger quantity of gray to reddish-gray coarse crossbedded quartzite in beds 1-2 ft thick47


Quartzite, pink, coarse-grained5


Quartzite, alternating pink and gray, alternating with maroon and gray argillite in layers from 2 in to 3 ft thick8


Quartzite, reddish-gray to pink, with several thin red argillite layers20


Argillite, red, with a thin layer of green argillite4


Argillite, green and red, interbedded with white quartzite3


Quartzite, white, coarse-grained; crossbedded With a shaly layer2


Argillite, red, thin-bedded, with a layer of green argillite39


Argillite, red and green, in equal proportions, in beds 1/2-4 in. thick, with several thin quartzite beds50


Argillite, maroon, with several green bands14


Argillite, red, shaly, with scattered green layers and a bed 1-1/2 ft thick, of greenish-gray coarse-grained crossbedded argillite at the top59


Argillite, red and green, thin-bedded, with a little coarse-grained grayish-white quartzite86


   Total

3,055



Grinnell argillite north of Crossley Lake

[After field notes of C. S. Corbett and C. R. Williams, 1914]



























































Feet
Argillite, green, gray, calcareous, thin-bedded (weathering buff) in units up to 3 ft thick, with a few thin beds of quartzite103
Argillite, gray, shaly, with a few beds of quartzite up to a foot thick44
Argillite, gray, shaly, and quartzite, gray; in units up to 3 ft thick24
Sandstone, gray, thin-bedded, argillite, gray, and quartzite, light-gray (Section interrupted here and some may be missing. Beds above this horizon were grouped with the Siyeh limestone by Corbett and Williams.)


Quartzite, gray, with red pebblelike pieces of argillite and 2 beds of red argillite, 6 and 8 in. thick, respectively5
Argillite, red2
Quartzite, red and grayish-white, thin-bedded, crossbedded. The latter color bed contains numerous pebblelike pieces of red argillite50
Argillite, red and green, in units up to 6 in. thick40
Argillite, green5
Argillite, red, with thin green argillite seams and a gray sandstone bed 2 ft thick4
Argillite, red, with thin green argillite layers and a bed of gray sandstone 2 in. thick4
Argillite, green3
Quartzite, white5
Argillite, red, and quartzite, white; in beds up to a foot thick17
Quartzite, white, thin-bedded5
Argillite, red, with numerous beds of white quartzite up to 6 in, thick66
Quartzite, reddish-gray, thin-bedded35
Argillite, red, in beds up to 3 ft thick and with thin layers of green argillite, interbedded with gray quartzite in beds up to 1-1/2 ft thick80
Quartzite, light-green, with some green argillite13
Argillite, red, with numerous layers of buff argillite, 2 in. thick23
Quartzite, red, thin-bedded; interbedded with much red argillite35
Argillite, green, with numerous beds of white quartzite, 1-3 in. thick in the upper 25 ft73
Argillite red3
Argillite, green3
Argillite, red, with occasional beds of white quartzite up to 6 in, thick26
Argillite, green and red, intercalated, the former predominating13
Argillite, red, with small seams of green argillite and white quartzite7
Argillite, green3
Argillite, red5
Argillite, green5
Argillite, vivid-red4
Argillite, green1
Argillite, vivid-red, interbedded with much green argillite and white quartzite up to 8 in. thick374
Quartzite, red, massive2
Argillite, vivid-red, shaly, with numerous beds at irregular intervals of buff argillite and white quartzite, 1/2-3 in. thick (Break in measurements here which Corbett says may introduce an error of about 10 ft.)63
Quartzite, red-tinted, in beds 2 or 3 in. thick; interbedded with some red argillite62
Argillite, light-green, thin-bedded3
Quartzite, red, argillitic crossbedded in beds up to 6 in. thick18
Argillite, green, quartzitic in places, with some red argillite in units up to a foot thick40
Argillite, green8
Sandstone, gray, crossbedded1
Quartzite, greenish-white5
Argillite, green9
Argillite red3
Argillite, green3
Argillite, red, with a few very thin seams of green argillite18
Argillite, green1
Argillite, red2
Argillite, green2
Argillite red10
Argillite, green, intermixed, and white quartzite1
Argillite, red, with a 3-in, layer of white quartzite9
Argillite, green, thin-bedded2
Argillite, red, thin-bedded6
   Total

1,361

PIEGAN GROUP

The terms "Piegan group" and "Siyeh limestone" have been used in different ways by different authors. These terms are here redefined in such a way as to make the units convenient to map. In a general way, both the Fentons (1937, p. 1890-1900) and Clapp (1932, p. 22, pl. 1) recognize 3 major subdivisions within the middle group of the 3 into which the greater part of the Belt series is divided. This group is the Piegan group as the Fentons defined it (1937, p. 1890-1892). Their name has been retained, but as already indicated its application has been restricted. The 2 upper subdiviions formerly included in the Piegan group or its equivalents are here excluded from it. The lower unit of the original Piegan group is the Siyeh limestone of the Fentons and of the present report. It is the only 1 of the 3 that constitutes a homogeneous, readily mappable unit throughout Glacier National Park and the Flathead region. Argillaceous beds that have been included by some at the base and top of the Siyeh have been separated therefrom in the present report. As pointed out in the description of the Missoula group, doubt exists as to the proper correlation of the 2 upper units of the Piegan group as originally defined by the Fentons, but these 2 units seem best regarded as parts of the Missoula group.

As far as Glacier National Park and neighboring areas are concerned, the name "Piegan group," as restricted in the preceding paragraph, might well be dropped altogether. However, in a preliminary attempt at broad correlation of the subdivisions of the Belt series (Ross, 1949), Piegan group was adopted as a convenient name for the thick sequence of beds in Montana and adjacent areas that differ in details from place to place but are characterized everywhere by a high carbonate content and the group lies between the Missoula group above and the Ravalli group below. In the present report the designation Piegan group is retained because of its usefulness in correlation throughout Montana and because the thick Siyeh limestone is expected to be divided into several units of formational rank when more detailed mapping is done. When this is accomplished, the name "Siyeh limestone" is expected to be restricted in its application or abandoned altogether.

Siyeh Limestone

The Siyeh limestone underlies broad areas in the median parts of the Flathead and Swan Ranges. These areas extend almost the entire length of the parts of both ranges that are within the Flathead region and that in the Flathead Range persists to the foot of Lake McDonald.

According to Clapp's map (1932, pl. 1) and Deiss's unpublished geologic map of the Ovando quadrangle, the Siyeh limestone is continuously exposed as far south as T. 18 N., R. 15 W., and at intervals beyond this. Study of these exposures and the relations of the limestone in them to overlying beds in the light of data in the present report should yield significant information as to the interrelations of the Piegan and Missoula groups and their components. Such a study should do much to settle the problem of the upper limit of the Piegan group in the Glacier Park region. Additional mapping will be required to determine what relation the so-called Spokane formation in Glacier National Park may bear to the Spokane argillite of the Spokane Hills, east of Helena. Part of the work needed is understood to have been already accomplished (Knopf, 1950), although the maps have not yet been published.

In Glacier National Park the Siyeh limestone forms the core of the mountain mass, although capped in places by later units. It extends from near the intersection of longitude 113°30' with latitude 48°30' northwestward past the Canadian border, widening northward. Its general character can be seen in figures 3, 6, 17, 27, and 28.

The Siyeh limestone is a crystalline carbonate rock that contains varying amount of magnesia, silica, and other impurities. The analyses listed in the table on page 55 show that it contains from 20 to nearly 50 percent of silica and significant quantities of alumina, so that it is a distinctly impure carbonate rock. Some beds are more argillaceous than those selected for analysis, but no aggregates of argillite or of distinctly argillaceous limestone of mappable dimensions are known to be present.

Nearly all of the Siyeh limestone is thick-bedded or massive as viewed from a distance, but much of it shows thin, wavey laminations on fresh fracture surfaces. Close inspection commonly reveals a fine lamination. Some of the limestone is oolitic. The color of the Siyeh limestone on fresh fracture surfaces is dusky blue or, more rarely, greenish gray, varying in value and hue (Goddard and others, 1948) apparently with variations in composition. The rock weathers in orange and brownish tones and commonly shows irregular etched markings on weathered surfaces (figs. 7, 8). These features are distinctive and easily recognizable, but they are quite as characteristic of the limestone here regarded as within the Missoula group as they are of the Siyeh limestone. They correspond to differences in the calcium carbonate content of the rock, but the origin of these small-scale differences is not understood. They include the forms termed "molar tooth" structures by Daly (1912, p. 72-76). His term is derived from resemblance to the markings on a molar tooth of an elephant and is vividly descriptive of some of the structures. However, there is infinite variety in the details of form assumed by the structures, and many have no resemblance to molar-tooth markings. The Fentons (1937, p. 1927-1929) speak of them as segregation structures and, in accord with Daly, think that segregation occurred long after induration. Such an explanation is probably correct for some of the structural features, but the characters displayed and the relation, or lack thereof, to bedding and parting planes of all sorts are so different in different exposures that it seems evident that no single explanation will fit all of these features. Perhaps some have direct or indirect relation to the life processes of primitive organisms in the original lime muds from which the present limestone is derived. Figures 7 and 8 show two examples, but the variations are too numerous to be adequately pictured or described here.
 
fig7 Molar-tooth structure in Siyeh limestone in roadcut along Going to-the-Sun Highway. Glacier National Park
fig 8 Irregular banding in Siyeh limestone in roadcut along Going-to-the-Sun Highway. Glacier National Park. These irregularities may correspond to movement in the limestone before consolidation, but they are in essentially undeformed beds.
 
Textural variations in the limestone on a microscopic scale are quite as diverse as would be expected from the descriptions already given. Two examples are shown in figure 9. Where special features are absent the rock is laminated, and in many laminae the grains average only a few thousandths or a few hundredths of a millimeter in diameter. Beds from one to a few millimeters thick composed largely of subrounded quartz grains are widely distributed but commonly are not abundant. A little sericite is present in these beds and in some of the limestone beds. Most thin sections exhibit irregularities in the texture of the carbonate, related to the "molar tooth" and other structures that are so conspicuous in the outcrop (fig. 9A). Most of the rock does not contain well-defined oolites, but in many there are irregularities in grain that may be derived from oolites now thoroughly recrystallized. Where oolites are preserved, some are roughly elliptical in section and 1 or 2 millimeters long (fig. 9B). These are fine-grained and preserve no internal features characteristic of oolites. The same rock may contain oolites that are circular in section and have well preserved concentric structure. Some of these are broken, and others are invaded in the outer layers by grains of clastic quartz. Most such oolites seen under the microscope are less than a millimeter in diameter and are themselves components of pebble like masses embedded in the fine-grained limestone.
 
fig9 FIGURE 9.—Photomicrographs of rocks of the Belt series and associated igneous rocks.



A. (Plane light) Irregularly textured Siyeh limestone near head of Hidden Creek, Glacier National Park.



B. (Plane light) Oolitic Siyeh limestone south of Red Eagle Pass, Glacier National Park.



C. (Plane light) Red argillite of the Missoula group on Argosy Mountain, Nyack quadrangle, Flathead region. A quartzitic argillite, with fragments of fine-grained argillite in it. Grains are subangular and do not appear to have been recrystallized.



D. (Crossed nicols) Purcell basalt, Flattop Mountain, Glacier National Park.



E. (Crossed nicols) Limestone in the Shepard formation, Flattop Mountain, Glacier National Park. Shows oval masses of coarse-grained carbonate in a fine-grained carbonate matrix. Oval bodies may be recrystallized oolites.



F. (Crossed nicols) Metagabbro from an irregular intrusive mass near the Spotted Bear airfield, Flathead region. Note interstitial micropegmatite
 
The stromatolite zones at several horizons in the Siyeh limestone locally provide means of subdividing that thick formation, but none of these has yet been mapped throughout plates 1 and 2. In the following descriptions the zones are designated by the names of the principal stromatolites found in each, and the nomenclature proposed by Rezak is used. This fact should be borne in mind when comparing the descriptions with those of previous writers, who used different names for the stromatolites, and consequently for the zones. For discussion of the differences in nomenclature, see Rezak's (1957) paper entitled, "Stromatolites of the Belt Series in Glacier National Park and vicinity, Montana."

No mappable subdivisions of the Siyeh limestone have been recognized in the Swan or Flathead Ranges; however, some stromatolite zones are known. These and other features are expected to permit subdivision when thoroughly detailed mapping is undertaken. By far the most conspicuous masses of stromatolites found are in the vicinity of Clayton Lake. The ridge crest surmounted by Tongue Mountain, west of the lake has almost continuous exposures of these fossils for a distance of more than a mile along the trail (fig. 10). Rezak visited this ridge in 1952 and concluded that the most abundant forms belong to Collenia symetrica Fenton and Fenton, although C. frequens Walcott is also present. On Graves Creek, southeast of Clayton Lake, outcrops of stromatolites are present near the trail but are less abundant and much more imperfectly exposed than on Tongue Mountain. These probably are C. symmetrica. An outcrop of stromatolites on the ridge above the head of Forrest Creek surely represents some form of the genus Collenia, possibly Collenia symmetrica. The Siyeh limestone in several other localities in the Swan Range has wavy laminae that may represent stromatolites, but these are much too small and indefinite to be named on the basis of present in formation. Most of the scattered outcrops of Collenia noted in the Swan Range are at or near the same stratigraphic horizon. Together they may mark a zone corresponding more or less to the Conophyton zone 1 of the park (the Collenia frequens zone of the Fentons).

Most of the mapping in the Swan and Flathead Ranges was done in 1949 before the significance of the stromatolites was appreciated and before special studies of them had been initiated. In the course of this mapping no stromatolites were recorded from the Flathead Range although some are probably present there. Nordeng in 1950 found some in highway and railroad cuts at the north end of the range. Rezak regards these as dominantly Collenia symmetrica.

In Glacier National Park the Conophyton zone 1, named by Rezak (the Collenia frequens zone of the Fentons), is so persistent and easily recognized even at a distance (fig. 3) that it has been shown on plate 1 throughout the area of the Siyeh limestone. On this basis the Siyeh limestone in Glacier National Park might be thought of as having three mapped components: this zone and the beds above and below it. However, formal division of the formation is not here attempted, partly because of the difficulty with present incomplete data in carrying it throughout the region covered by plates 1 and 2. Detailed mapping will result in discrimination of map units other than the three indicated above, at least in parts of the park. Because of lateral variations it seems probable that some of these units will not be found persistent enough to be mapped throughout the park, and it may be that few of them can be carried far beyond the limits of Glacier National Park. That is, it is anticipated that future work will result in establishing local units, some of which will be valid only over a few scores or hundreds of square miles, rather than throughout the broad region underlain by the Siyeh limestone.

The Fentons (1937, p. 1892-1897) have set up four subdivisions of the formation within the park. In ascending order these, with Rezak's revisions in nomenclature shown in parentheses, are the Collenia symmetrica zone and Goathaunt member (= C. symmetrica zone 1) Collenia frequens zone (Conophyton zone 1) and Granite Park member. They describe their Collenia symmetrica zone as the "upper phase of the transition between the argillitic and arenaceous Grinnell to the dolomitic and limy Siyeh" and add that it includes quartzite, argillite, and argillaceous dolomite that weather green, brownish, or buff, with subordinate purplish-red argillite limited to the lower 75 feet of the unit. The member is reported to have a thickness of 500-900 feet and to grade upward into the Goathaunt member. It must grade downward into the upper part of the Grinnell argillite. The part that contains the "purplish red argillite" of the Fentons and any part above this in which argillite and quartzite predominate over carbonate rocks belongs with the transition zone at the top of the Grinnell as mapped in plate 2. This matter has been commented on in the description of the Grinnell argillite.

Clearly the Collenia symmetrica zone 1 does not, as the name might imply, consist mainly of a definite biostrome in which C. symmetrica is dominant. Nordeng comments that, on the contrary, it is a zone in which C. symmetrica occurs with varying frequency, with occasional colonies of Collenia clappi Fenton and Fenton. Later studies by Rezak have led him to regard C. symmetrica and C. clappii as synonymous and to drop the latter term. Probably the zone has much lateral variation and would not be recognizable throughout Glacier National Park on the basis of its stromatolite content. Thus, if an equivalent unit is mapped in the future, it is likely to receive some name other than a paleontological one. Rezak reports that C. symmetrica may be found anywhere in the Siyeh limestone below the Conophyton zone 1, mostly in isolated bioherms in dense black and tan laminated argillaceous limestone. Isolated heads of Collenia multiflabella Rezak are present in the bioherms of C. symmetrica. Most such bioherms noted by Rezak are stratigraphically higher than the C. symmetrica zone of the Fentons; that is, these bioherms are within the Goathaunt member of the Fentons.

The Goathaunt member of the Fentons consists of the greater part of the limestone typical of the Siyeh. They say it contains limestone, dolomite, and subordinate quantities of oolite, dolomitic sandstone, and argillite and add that mud breccias, commonly containing coarse sand and pebbles, are abundant in northern exposures. They estimate the thickness as 2,000-3,200 feet, whereas they give the thickness of the entire Siyeh limestone in Glacier Park as 2,900-4,000 feet.

The Fentons speak of their Collenia frequens zone (= Conophyton zone 1 of Rezak) as composed of dark-gray crystalline to amorphous limestone in one or more massive biostromes with thinly bedded calcareous or dolomitic intercalations. They say that the biostromes consist of little except Collenia frequens ( = Conophyton inclinatum Rezak) and Collenia versiformis Fenton and Fenton (= Cryptozoon occidentale Dawson) and assign a thickness of 100-156 feet to the zone. It is clear that the zone is a mappable unit, although very thin in comparison to most of the other map units in the Belt series, and it would appear that the paleontological designation given by Rezak is appropriate. Consequently, this designation is employed in the present report. The zone within the Siyeh limestone is designated the Conophyton zone 1 to distinguish it from the similar zone recently found within the Missoula group and designated the Conophyton zone 2.

The Collenia frequens zone (= Conophyton zone 1) was studied by Nordeng in 1950 and by Rezak in 1951. Their descriptions differ in many respects from each other and from that given by the Fentons. This arises partly from differences in the character of the zone from place to place, but perhaps even more from the fact that classification of the different species of Collenia has not been standardized in the past. Nordeng reports that Collenia willisii Fenton and Fenton (= Collenia multiflabella) is the principal form in the lower part of the zone. As the Fentons note that Collenia willisii (= Collenia multiflabella) is common in their Goathaunt member, which underlies the Collenia frequens zone (= Conophyton zone 1), it may be that Nordeng extended the Conophyton zone 1 somewhat lower stratigraphically than they did. He says that the beds containing C. willisii (= C. multiflabella) are followed upward by a biostrome of Collenia versiformis (= Cryptozoon occidentale), overlain by bioherms and biostromes of Collenia frequens (= Conophyton). Above these he found in most places a biostrome 2-3 feet thick "of what is usually identified as Collenia versiformis but has a stronger resemblance to Collenia columnaris ( = Collenia frequens)." Nordeng noted much variation in the thickness and abundance of the components of the Collenia frequens zone but regarded the biostrome composed of Collenia versiformis as the most persistent and the masses of Collenia frequens as the most conspicuous.

Rezak thinks of the Conophyton zone 1 as made up essentially of three parts. At the base he reports a biostrome of Collenia frequens which is relatively thin near and east of Logan Pass but thickens westward. Directly above this biostrome he found pod-like bioherms of Conophyton, 5-22 feet wide and up to 6 feet high. The closely packed heads of Conophyton in these range from a few inches to 4 feet in diameter (fig. 10). Each of the bioherms has associated with it undistorted finely laminated black and tan limestone. The laminae are continuous from the colonies on the margin of each reef and are interpreted by Rezak as off-reef deposits. Each dips west and is overlain by other bioherms. The subzone containing Conophyton with the associated off-reef deposits is about 50 feet thick near Logan Pass but only a few feet thick at Heavens Peak Lookout. If, as both Nordeng and Rezak think, the exposure of stromatolites near the lower end of Lake McDonald is the stratigraphic equivalent of the Conophyton zone 1, this particular part of that zone in absent or obscure there. (See the stratigraphic section measured at that locality by Nordeng and given on p. 40). Rezak notes that the uppermost part of the Conophyton zone 1 consists of a layer 1-12 feet thick in which Cryptozoon occidentale predominates but which also contains Collenia multiflabella. The Fentons report that their Granite Park member, the top unit of their Siyeh formation, contains magnesian limestone, oolite, argillite and quartzite, with colors that range through gray, greenish gray and brown. The thickness is given as 280-900 feet. This is one of the transitional zones between formations of the Belt series in which consistency and uniformity of definition and mapping are difficult to attain. In at least some places, the argillite and quartzite grouped with this member by the Fentons are probably included in the calcareous argillite at the base of the Missoula group as mapped on plate 2 and the southern part of plate 1.
fig10Conophyton inclinatum in Siyeh limestone, roadcut above the big switchback on Going-to-the-Sun Highway. Glacier National Park
fig11 Stromatolite on Tongue Mountain, Nyack quadrangle, Flathead region. Presumably Collenia symmetrica or a similar species
 
The Fentons say that their Granite Park member contains many algal bioherms. These include large colonies of Collenia willisii (= Collenia mulitflabella) and also masses of Collenia frequens (= Conophyton) and of Collenia versiformis (= Cryptozoon occidentale). Some of these stromatolitic masses may be low enough in stratigraphic position so that they merge with the Conophyton zone 1 as shown on plate 1. This statement applies particularly to the large biostrome described by the Fentons as 6.4 miles along the highway westward from Logan Pass and 1.4 miles east of the loop, or big switchback, which was included in the Conophyton zone 1 in the mapping done in 1950 (pl. 1). Nordeng notes that a zone at the top of the Siyeh limestone is composed entirely of Collenia willisii (= Collenia multiflabella) and is present over much the same area as the Conophyton zone 1 that has been mapped. He adds that this Collenia willisii zone (= Collenia multiflabella zone) is distinguished by the presence of alternating gray dolomite and brown argillite layers. Rezak also found biostromes up to 6 feet thick and isolated bioherms consisting dominantly of Collenia multiflabella at Logan Pass and along the Garden Wall in the uppermost part of the Siyeh limestone. In addition, Cryptozoon occidentale is present here.

In the Swan and Flathead Ranges the Siyeh limestone has not been measured but has an apparent thickness of at least 5,000 feet, as judged from plate 2. Erdman (1944, p. 48-55, 88-95) supplies details in regard to the Siyeh limestone in the northern part of the Swan Range, including partial sections and chemical analyses. He estimates that the Siyeh limestone on the southeast slope of Teakettle Mountain in T. 31 N., R. 20 W., is 4,550 feet thick (accurate within 5 percent).

The Fentons, as already noted, estimate the thickness in Glacier Park as 2,900-4,000 feet, but these estimates include clastic rocks not here included in the Siyeh limestone and would, therefore, be high by several hundred feet. Comparatively detailed stratigraphic sections of the Siyeh limestone in the park, adopted from the field notes of geologists under M. R. Campbell, followed by a partial section made by Nordeng in 1950 and one measured by Rezak in 1951 follow. The most reliable and detailed section appears to be that pieced together from measurements made by Corbett and Williams in three different localities near Red Eagle Creek. (See p. 41.) As the separate measurements are each tied to recognizable horizon markers, they have been compiled below with confidence that they represent a fairly accurate picture of the entire formation in the part of the park close to Red Eagle Creek. In accord with present concepts, the red and green argillite beds measured on the upper part of Almost-a-Dog Mountain and originally listed by Corbett and Williams as the upper part of the Siyeh are not included. It seems more appropriate to regard them as part of the Missoula group. Similarly, it may be that a small part of the lowest unit that is listed below should be regarded as belonging to the transition zone at the top of the Grinnell argillite and, consequently, eliminated here. Similar modifications have been introduced in copying the two other sections listed below from the field notes of geologists in Campbell's parties. Thus, the three sections as here given represent with a fair degree of accuracy the complete thickness of the Siyeh limestone, as the term is used in the present report, in the northeastern part of Glacier National Park. It will be noted that the average thickness on this basis is close to 2,000 feet. Equally complete measurements in the western part of the park are not at hand, but would probably show a greater average thickness. In comparing the sections listed below with generalized statements as to character of the Siyeh limestone given above and in other publications, it should be borne in mind that nomenclature of color, texture, and other features is so far from standarized that descriptions of the same rock by different geologists may differ markedly.
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