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Главная » 2013 » Ноябрь » 27 » Geology of Glacier National Park And the Flathead Region, Northwestern Montana (p7)
07:42
Geology of Glacier National Park And the Flathead Region, Northwestern Montana (p7)
 
STRATIGRAPHY (continued)

PALEOZOIC STRATA

BASIS FOR DIVISIONS USED

Exposures of Paleozoic strata are confined almost entirely to areas in the eastern part of the mountains of the Flathead region. The rocks in these areas consist of marine deposits of Cambrian, Devonian, and Carboniferous age that are northward extensions of units described and named by C. F. Deiss (1933, 1943b, p. 1129-1136, 1943a, p. 216-231). His nomenclature for Paleozoic strata in and west of the Saypo quadrangle is applicable in a general way to those of the Flathead region. Lateral changes appear to exist, but more intensive paleontologic and stratigraphic work are required before these can be evaluated. In any case, they could not be adequately shown on small scale maps such as accompany the present report. Most of the names of Paleozoic formations used by Deiss are his own. The problems of correlation between these formations and those that have long been recognized farther south in Montana await further investigation.

Deiss recognized formations of Middle and Late Cambrian age in and near the Silvertip quadrangle and nine formations of similar age and character in the southern part of the Saypo quadrangle. Although the Cambrian rocks are shown undifferentiated on plate 2 of the present report, such data as were obtained as to the different formations are summarized below. It would be difficult to show the individual formations on a map of the scale of plate 2 even if sufficient data were obtained to permit their identification.

Deiss (1933) named several subdivisions of the Devonian rocks which he regarded as members of the Jefferson limestone, but in his later papers on the Saypo quadrangle, he spoke only of two units of Late Devonian age for which available data were insufficient on which to base formal names.

L. L. Sloss and W. M. Laird (1945, 1946, 1947) speak of three Devonian units. Both their reports and those of Deiss that deal with Saypo quadrangle indicate that such names as Three Forks shale and Jefferson limestone, so familiar farther south, are not at present applicable in the part of Montana under discussion. The present studies have added some facts as to exposures beyond those they covered, but the studies have otherwise contributed little to the data on the Paleozoic rocks assembled by Deiss and by Sloss and Laird.

Paleozoic strata south of the Flathead region

[Adapted from C. F. Deiss (1933-1943a, b)]

Southwestern Saypo quadrangleCentral Saypo quadranglePentagon Mountain in Silvertip quadrangle in unsurveyed Secs. 18, 13, 14, 23, 24, T. 25 N., Rs. 11 and 12W.Spotted Bear Mountain in Silvertip quadrangle Secs. 23, 25, T. 25 N., R. 15 W.
MississippianHannan limestone: Basal limestone breccia overlain by gray limestone and alternating units of shaly-bedded argillaceous limestone which grades upward into hard tan-gray and pale-gray crystalline fossiliferous limestone: contains much pale-gray chert in nodules and beds as much as 7 in. thick. Upper 200 ft consists of white-gray finely crystalline thin- and thick-bedded dolomite. 1,370 ft thick.Hannan limestone: Basal limestone breccia overlain by 400 ft of black and tan brittle limestone. Medial 600-700 ft light-gray fossiliferous cherty crystalline limestone. Upper 300 ft buff and light-gray dense thin- and thick-bedded finely crystalline dolomite which weathers white-gray. 1,460-1,500 ft thick,Hannan limestone: Upper part of formation absent. Rooney member rather coarse-grained and thick-bedded gray limestone, 550 ft thick. Dean Lake member, blue-gray, gray, and black limestone with chert. 60 ft thick. Saypo member, chocolate and gray limestone, shaly at top and bottom. 141 ft thick. Silvertip member, breccia-conglomerate composed of gray argillaceous limestone 40 ft thick. Total exposed 791 ft.Hannan limestone: Upper part of formation absent. Rooney member, chocolate and gray thin, bedded argillaceous limestone with coarse-gained white thick- and thin-bedded limestone at base. 177 ft thick. Saypo member, gray, white, and pink limestone interbedded with buff and red calcareous sandstone with chocolate limestone below. 220 ft thick. Silvertip member, gray-white breccia conglomerate. 23 ft thick. Total exposed 420 ft.
Unconformity
DevonianDolomitic limestone and shale overlain by tan-gray fine- to medium-gained fossiliferous limestone. Alternate units of drab-brown petroliferous arenaceous thick-bedded dolomite and light-gray hard finely crystalline thick- and thin-bedded limestone. All more or less petroliferous and argillaceous. 980 ft thick.Dolomitic limestone and shale overlain by tan-gray fine- to medium-gained fossiliferous limestone. Alternate units of drab-brown, petroliferous, arenaceous thick-bedded dolomite and light-gray hard finely crystalline thick- and thin-bedded limestone. All more or less petroliferous and argillaceous. 1,000 ft thick.Spotted Bear member, thin-bedded limestone at top and bottom. Massive tan and brown limestone in main part. A little shaly. 282 ft thick. Lone Butte member, drab-brown petroliferous limestone at top. Red-chocolate and brown petroliferous limestone, locally shaly. Thickness 220 ft. Coopers Lake member, thick- and thin-bedded chocolate, tan and gray limestone. 263 ft thick. Total thickness 844 ft.Spotted Bear member, gray thin-bedded partly argillaceous limestone at top and bottom. Thicker bedded gray, tan, and pink limestone in middle part. Thickness 287 ft. Lone Butte member, massive chocolate and blue-gray limestone with clay bands and nodules. 185 ft thick. Coopers Lake member, gray and tan limestone partly siliceous. 298 ft thick. Glen Creek member, dull red shale. 40 ft thick. White Ridge member, tan-gray limestone flecked with pink. 50 ft thick. Total thickness 860 ft.
Unconformity
Upper CambrianDevils Glen dolomite: White-gray to pale-buff-gray finely crystalline thick- and thin-bedded dolomite. Mottled salmon pink and coarse gained in upper part of formation. Weathers white gray and forms shear cliffs. 198-250 ft thick,Devils Glen dolomite: White-gray to pale-buff-gray finely crystalline thick- and thin-bedded dolomite. Mottled salmon pink and coarse gained in upper part of formation. Weathers white gray, and forms shear cliffs. 200-260 ft thick.Devils Glen dolomite: Thick-bedded buff siliceous dolomite with a bed of sandy dolomite below and one of red-pink calcareous argillaceous sandstone at base. 43 ft thick.Devils Glen formation: Thick-bedded white and cream-gray massive dolomite. 460 ft thick.
Middle CambrianSwitchback shale: Green and gray soft shale; locally calcareous and arenaceous; interbedded with flaggy-bedded magnesian limestone which weathers rusty tan. 75-125 ft thick.Switchback shale: Green and gray soft shale; locally calcareous and arenaceous; interbedded with flaggy magnesian limestone which weathers rusty tan. Contains youngest known Middle Cambrian trilobites and others which belong to basal upper Cambrian (Cedaris) fauna. 125-175 ft (?) thick.Switchback formation (type locality): Mostly thick- and thin-bedded cream-gray massive argillaceous limestone with green-gray shale and shaly limestone at base. 106 ft thick.Switchback formation: Brown calcareous and shaly sandstone underlain by cream-gray limestone. 70 ft thick.
Steamboat limestone: Pale-gray and tan fine-grained thick- and thin-bedded limestone overlain by interbedded tan-gray irregularly bedded fossiliferous limestone and green fissile shale. Shale in units 2-8 ft thick separated by much thicker units of limestone. 275 ft thick.Steamboat limestone: Pale-gray and tan fine-grained thick- and thin-bedded limestone overlain by interbedded tan-gray irregularly bedded fossiliferous limestone and green fissile shale. Shale in units 2-8 ft thick separated by much thicker units of limestone. Base of formation not exposed. 275 ft thickGordon Mountain formation (type locality): Chocolate, tan, and gray mostly thick-bedded limestone, argillaceous near top and with clay and shale at several horizons. 216 ft thick.Gordon Mountain formation: Buff brown argillaceous limestone. 368 ft thick.
Pentagon shale: Buff-gray platy calcareous thick-bedded fossiliferous shale, which grades upward into thin- and thick-bedded limestone. 0-290 ft thick.Absent.Pentagon formation (type locality): Gray, tan, and chocolate limestone; mostly platy and argillaceous. Some shale. 290 ft thick.Pentagon formation and Pagoda foration: Mostly obscured by soil and glacial drift. Interval represents 296 ft of beds.
Pagoda limestone: Green fissile shale interbedded with thin-bedded limestone; overlain by cream and chocolate-gray finely crystalline limestone; thin-bedded In lower and upper parts, and oolitic and thick-bedded in middle part. 200 ft thick.Pagoda limestone: Green fissile shale interbedded with thin-bedded limestone; overlain by cream and chocolate-gray finely crystalline limestone; thin-bedded in lower and upper parts, and oolitic and thick-bedded in middle part. Exposed thickness not known.Pagoda formation: Massive thin and thick-bedded limestone at top with some black fissile shale, followed below by gray to black calcareous shale with thin limestone beds and, near base, limestone conglomerate with red green and gray pebbles. 290 ft thick.
Dearborn limestone: Green fissile shale and intercalated limestone conglomerate; overlain by tan-gray thick- and thin-bedded limestone which contains varying amounts of buff and orange-tan clay as flakes nodules, and partings. 300 ft thick.Lower part of the Cambrian strata not exposed.Steamboat formation: Gray and chocolate-gray platy limestone with buff clay. 74 ft thick.Steamboat formation: Tan gray, and chocolate limestone with flakes of buff siliceous limestone in some beds. 91 ft thick.
Dearborn formation: Thick- and thin-bedded tan and gray limestone with clay bands and nodules. Green-gray fissile shale at base. 213 ft thick.Dearborn formation: Mostly gray and buff argillaceous limestone. Locally has bands of calcareous clay. Near base has beds of lithographic limestone. 238 ft thick.
Nannie Basin formation: Gray, tan-gray, and blue-gray limestone with flakes and nodules of buff clay. 170 ft thick.Nannie Basin formation: Gray argillaceous limestone with bands of buff and gray clay; underlain by pink-gray limestone. 59 ft exposed.
Damnation limestone: Blue and tan-gray thin-bedded fossiliferous limestone with much limonitic clay; overlain by chocolate-gray and tan fine-grained thick- and thin-bedded hard massive limestone, which also contains flakes and blebs of buff siliceous clay. 150 ft thick.Damnation formation: Thin-bedded shaly tan limestone with flakes and nodules of buff clay; weathers bright buff. 21 ft thick.Strata below are faulted out.
Gordon shale: Drab-green-gray and maroon fissile to chunky micaceous shale interbedded in upper middle part with thin beds of sandstone and thicker beds of limestone. Upper beds very fossiliferous. 220 ft thick.Wolsey formation: Sandy micaceous limestone with much green shale at top. Green-gray micaceous fissile shale, with thin sandstone and sandy limestone beds. Contains hematite oolites. 140 ft thick.
Flathead sandstone: Coarse- and fine-grained crossbedded sandstone interbedded with thin units of micaceous and areanceous shale. Most diagnostic feature is presence of white quartz pebbles 1/8-3 in. in diameter. Basal beds commonly pebbly conglomerate. 50-100 ft thick.Flathead formation: White to cream thin- and thick-bedded quartzite, with limonite grains; underlain by red crossbedded pebbly quartzite, with pebbles up to 1-1/2 in. in diameter. 117 ft thick.

Deiss grouped all of the Paleozoic rocks above the Devonian strata as the Hannan limestone of Mississippian age, a convenient designation which is adopted here. Both Deiss and Sloss and Laird in the papers just cited indicate that subdivision of the Hannan is possible. These authorities differ as to the details, and the matter has not yet been submitted to the test of areal mapping. For the kind of mapping embodied in plate 2, the formation is a convenient, easily recognized body without subdivision.

Deiss believes unconformities separate the Cambrian, Devonian, and Mississippian assemblages from each other and from the Precambrian and Mesozoic strata. As angular discordances are nowhere marked, proof of the stratigraphic breaks is furnished mainly by the fossil evidence. Within the Paleozoic sequence this evidence as presented by Deiss seems conclusive except that Sloss and Laird (1947, p. 1420-1421) raise a question as to whether there is an unconformity between the Devonian and Mississippian strata. The seemingly complete absence of Ordovician, Silurian, Permian, and Triassic rocks leaves no doubt that major breaks in the record exist. At the base of the Paleozoic succession, more uncertainty exists, in part because of the absence of diagnostic fossils. Deiss (1935, p. 95-124) has assembled much data favoring the concept of an unconformity below the Flathead quartzite. The few exposures of this horizon in the Flathead region do not give positive evidence on this point. For the purposes of the present report, this question may be left open.

Four stratigraphic sections of the Paleozoic rocks south of the Flathead region are given below, adapted from the publications by Deiss cited above. The correlations between the sections seem implied but were not made in the reports cited. A notable feature is that Deiss found it necessary to propose new names because he believes that few of the well-known formation names of southern Montana are applicable here. The members named in the sections from Pentagon and Spotted Bear Mountains are of local significance only. They have not been formally adopted by the Geological Survey.

CAMBRIAN ROCKS

The sedimentary rocks of Cambrian age are mapped as an undifferentiated unit. Wherever they are well exposed, it is obvious that formational divisions are present; but, in a country as intricately faulted as the eastern part of the Flathead region, mapping of formations would be time consuming and impracticable in many places on the scale of the present map. The following descriptions give such information as is available regarding the distribution of beds that seem to be correlatable with units farther south recognized by Deiss.

Large masses of Paleozoic rocks are exposed in the northern part of the Silvertip quadrangle (unpublished geologic map by C. F. Deiss). A small body of Cambrian strata extends from this mass across the southern border of the Flathead region into the area between the two Twin Creeks. The Flathead quartzite was not found within this body although boulders suggest that it is present a short distance to the south. The lowest beds that are present are on the lower slopes of the east side of lower Twin Creek below Tanner Creek. These beds are mainly dark-greenish micaceous shale, with some quartzite. The apparent thickness is over 500 feet, but this is undoubtedly exaggerated by crumpling and perhaps also by minor thrust faulting. Exposures are inadequate for determining structural details, but in nearby cliffs intricate deformation is well displayed. An undetermined thickness of dark-gray highly oolitic limestone with chert in its upper part overlies the shale and is succeeded upward by other limestone beds, mostly light gray but in part mottled. The blebs and flakes of clayey material that distinguish much of the Cambrian limestone of the region are nowhere conspicuous along lower Twin Creek.

The general character of the rocks just described and their relations to Paleozoic rocks in the Silvertip quadrangle and to Precambrian rocks north and east of them are all in accord with a Cambrian assignment. Presumably the shaly rocks on the lower slopes belong, at least in part, to the Wolsey formation (or shale) described by Deiss, even though the apparent thickness is great. The limestones above would then belong to formations higher in the Cambrian sequence, possibly ranging upward into the Pagoda formation. Deiss (1935, p. 37-38) describes his Pagoda formation as variable in thickness and character. In one locality the Pagoda formation, like the rocks here discussed, contains conspicuous oolitic beds.

Eastward, the next exposures of Cambrian rocks are on both flanks of the valley of Dolly Varden Creek. On the southwest slope, massive beds of limestone form steep cliffs and are overridden by beds of the Missoula group along a steep thrust fault. The upper cliffs expose white dolomite which presumably belongs to the Devil Glen dolomite. On the ridge from Gable Peaks to Cruiser Mountain, east of Dolly Varden Creek, the Cambrian beds are less disturbed, although parts of them are covered by landslide debris. The table below records observations made on and near Gable Peaks. The two lowest formations here are surely the Flathead quartzite and Wolsey shale. The Wolsey shale appears to be essentially equivalent to the formation which Deiss spoke of as the Gordon shale in the Saypo quadrangle. The Damnation limestone, which Deiss reported as only 21 feet thick a few miles to the south, was not recognized on Gable Peaks and may be absent there. It seems a reasonable assumption that the 130 feet of poorly exposed beds above the Wolsey belong to the Nannie Basin formation of Deiss, although, if so, they are more shaly than his description shows the formation to be farther south. However, shale appears to be more conspicuous throughout the Cambrian rocks on Gable Peaks than it is in the localities in which Deiss measured his sections. The other highly tentative correlations in the table below are based largely on the fact that the Pagoda formation is the only one in which Deiss reports that, locally, oolitic beds are conspicuous. No diagnostic fossils were found on or near Gable Peaks, except that Masursky found Elrathiella cf. E. plana Deiss in limestone on Cruiser Mountain some distance to the south. A. R. Palmer, who examined Harold Masursky's specimen, suggests that it may have come from a horizon high in the Pagoda or low in the Pentagon formation, which would be consistent with the tentative correlations in the table.

Both in the table below that summarizes data for Gable Peaks and in the correlation table for Cambrian rocks given above Deiss's name Nannie Basin formation (1933, p. 36) is retained for local use even though in his later papers Deiss dropped that name and included the rocks in his Dearborn limestone.

In addition to those on Cruiser Mountain, a patch of Cambrian rocks was recognized by Masursky in the fault complex north of Lodgepole Creek. The formations that are represented in that area were not identified.

Cambrian strata on and near Gable Peaks












Approximate

thickness

in feetCharacterTentative

correlation


Gray massive limestone with light-yellow streaks.


230Buff-weathering fossiliferous limestone with thin beds of hard chocolate limestone and nearly black shale.Pentagon formation.
275Gray oolitic limestone in beds 1-3 ft thick.Pagoda formation.
40Covered. Largely greenish-black micaceous shale.Steamboat formation.
130Light-chocolate limestone with yellow mottlings and partings.Dearborn formation.
130Poorly exposed greenish micaceous and sandy shale with thin beds of oolitic limestone.Nannie Basin formation.
90Covered. Largely shale.Wolsey shale.
75Poorly exposed red and green micaceous shale.
25Reddish and white quartzite with beds of grit and fine conglomerate (pebbles up to an inch in diameter) crossbedded.Flathead quartzite.
Total 995



Beds of Cambrian age crop out in the eastern part of the region mapped (fig. 24) in a band that extends from the North Fork of Dupuyer Creek to the vicinity of Heart Butte, a distance of roughly 14 miles. The most conspicuous and widespread unit is a limestone with numerous yellow, tan, and brown flakes of argillaceous material in it. The clayey flakes appear to be like those reported by Deiss from beds of Middle Cambrian age in several localities farther south, particularly in the Steamboat limestone. Only a few poorly preserved fossils were found, mostly by Masursky. The only collection of any diagnostic value is from the ridge north of Swift Reservoir. This collection was examined by G. Arthur Cooper and later by A. R. Palmer. It contains Solenopleurella sp. and Elrathiella cf. E. plana Deiss. Although the Solenpleurella resembles S. pagodensis Deiss, it is not, according to Palmer, identical with that species. Palmer is inclined to think that the collection may have come from the lower part of the Pagoda limestone of Deiss. In this connection it may be remarked that Deiss (1933, p. 38) in his original description of the Pentagon shale says: "It is a significant fact that the Pentagon formation is shale and shaly limestones only in the immediate vicinity of Pentagon Mountain, but that its equivalent in all of the other sections is generally massive limestone." He adds that the limestone has a small amount of clay disseminated as flakes. The quotation is modifed by the fact that later Deiss (1943a, p. 221-222) tentatively assigned some shale and limestone in the southern part of the Saypo quadrangle to the Pentagon shale. The rocks in the vicinity of Swift Reservoir are limestone, and many contain clay flakes. Shale was noted among the Cambrian rocks in only a few places in the eastern part of the Flathead region. The principal exposures recorded are green and maroon shale outcrops along Whitetail Creek west of Feather Woman Mountain and in the ridges east of Mount Richmond.

DEVONIAN ROCKS

Beds believed to be of Devonian age have been mapped in discontinuous bands from the southern border of the Heart Butte quadrangle east of the North Fork of Teton River north to Feather Woman Mountain and Heart Butte. There are smaller patches southeast of Half Dome Crag, on Spotted Eagle Mountain, Family Peak, Mount Dreyer, and in several other localities in and near the Heart Butte quadrangle, mostly near its southern boundary. Several of these scattered masses were mapped from distant vantage points on the basis of color and relations to the Hannan limestone, so that revisions are to be expected when detailed mapping is done, especially as few Devonian fossils have been found anywhere in the area. There are exposures of Devonian strata on and near Big Lodge and Tent Mountains in the Marias Pass quadrangle, but none are known farther west.

The Devonian rocks in this part of Montana have not been divided into named formations. Deiss (1943b, p. 1134-1135) reports that 1,000 feet of Upper Devonian strata are present in the central part of the Saypo quadrangle and that they can be mapped as one and locally two formations. Sloss and Laird (1945, 1946, 1947) have measured sections in and west of the Saypo quadrangle. They divide the Devonian rocks into three major subdivisions, termed "units A, B, and C," and indicate that the thickness in different localities ranges up to somewhat over 1,500 feet.

The Devonian rocks in the area described in the present paper must be broadly similar to those farther south described by Deiss and by Sloss and Laird. They are characterized by much dark limestone that weathers brownish and most of which is commonly distinctly fetid. Some beds contain blebs of different kinds, but few contain numerous yellow, tan, and brown argillaceous flakes and blebs similar to those that are so widespread and conspicuous in the Cambrian rocks. Few complete sections of Devonian rocks were seen during the present study. The data tabulated below result from two traverses made by Harold Masursky in which thickness estimates were based on aneroid barometer and Brunton clinometer readings.

Section of Devonian rocks with top about at benchmark 7540 between Phone Creek and Lucky Creek

[Calculated from Harold Masursky's notes]

Approximate

thickness

in feet
Limestone, gray; resembles a coquina with crinoid stems and bryozoa2
Shale, brown, calcareous; weathers pink and buff2
Limestone, black; brecciated and cemented with calcite veinlets230
Limestone, black, fine-grained, thick-bedded, with some calcite veinlets34
Limestone, dark-gray; weathers cream and tan17
Dolomite, gray; weathers to a lighter gray48
Limestone, black, in beds 2-5 ft thick; weathers gray17
Shale, brown, limy68
Limestone, gray; weathers gray4
Limestone, brown, with corals; weathers cream5
Limestone, gray; weathers gray5
Limestone, black, laminated4
Limestone brown, with calcite blebs and veinlets; has strong fetid odor128
Limestone, gray, with calcite veinlets8
Limestone, brown, fetid88
     Total, estimated

660
Covered below, but total thickness of supposed Devonian rocks exceeds 1,000 ft.



Section on the southeast flank of Feather Woman Mountain

[Calculated from Harold Masursky's field notes]



Feet
Mississippian system:
Black limestone, thin-bedded; weathers gray and tan; contains chert and crinoid stems; forms cliff200
Buff limestone, with pink and tan clayey flakes; talus covered; possibly in part shaly285
Devonian system:
Limestone, gray and buff, alternating70
Limestone, gray3
Limestone, buff17
Limestone, gray, resistant10
Limestone, buff, thin-bedded, increasingly shaly upward; weathers tan and yellow in alternating bands115
Limestone, gray5
Limestone, black, thin-bedded, resistant; weathers brown105
Limestone, gray5
Limestone, black, fetid; weathers brown with irregular white blebs600
Total thickness of Devonian strata930
Cambrian system:
Limestone, black, thin-bedded25
Limestone, black, massive, thick-bedded, with yellow blebs and laminae50
     Total at bottom of canyon

1,490

Discrimination between the Devonian strata and those that overlie and underlie them is based primarily on lithologic characteristics, as few fossils were collected. Atrypa missouriensis was identified by G. Arthur Cooper in specimens from near Hurricane Mountain and from a small fault block on Spike Creek, a small tributary of the Middle Fork of Birch Creek. In the field the upper 268 feet of the section near Phone Creek was thought to belong to the Hannan limestone, but it seems more probable that the brecciated beds in this part of the section correspond to the breccia near the top of the Devonian sequence farther south. These breccias are reported in the papers cited above as resulting from solution of extensive anhydrite deposits. It is somewhat strange that no such breccia was recorded from the section on Feather Woman Mountain in which the Devonian rocks are overlain by beds of definitely Mississippian characteristics and that similar breccias are absent or inconspicuous in the exposures of Devonian rocks seen in other localities.

Evidently the exposed part of the section near Phone Creek corresponds essentially to the upper half of the Devonian section on Feather Woman Mountain. There are numerous differences in detail between these two sections, and similar differences exist between all recorded sections of Devonian rocks in this part of Montana. For that matter, there are differences among the various recorded sections of other Paleozoic units in the region also. These differences result in part from lateral variations in the beds. They can be evaluated and understood only after more complete and detailed geologic mapping has been accomplished.

UPPER PALEOZOIC ROCKS

The Paleozoic rocks above the Devonian rocks are here grouped as the Hannan limestone in accord with Deiss' usage (1943a, p. 227-231). This formation is of Mississippian age and, according to Deiss, is partly correlative to both the Madison and the Brazer limestones. It includes all the beds in the region under discussion which lie between the Devonian and Jurassic rocks. In the eastern part of the Flathead region, the Hannan constitutes the principal formation exposed in a band of northwesterly trend that extends from the southern border of the mapped area to Mounts Baldy and Pablo, a distance of about 28 miles (figs. 22, 23). The northeastern border of this band throughout its extent is at or near the boundary between the mountains and the plains, and the width of the band is about 10 miles. Another line of outcrops of Hannan limestone subparallel to this band and almost as long but much narrower appears immediately east of the Lewis overthrust near the middle of the region covered by plate 2.

The Hannan limestone constitutes one of the most easily recognized and best exposed units in the region. It is composed of thick- and thin-bedded gray limestone, in part dolomitic. Many of the beds contain abundant chert (fig. 17). Thin beds are especially conspicuous near the base of the formation in most localities. Some of the beds throughout the formation are healed breccias. Shale is not plentiful but is reported to be abundant at some horizons in the Hannan south of the region here reported on. Fossils are fairly common, and crinoid stems and corals are the most conspicuous. Data on the fossils in the Hannan farther south are given by Deiss in the paper just cited and by Sloss and Laird (1945).
 
fig17 Closeup of Hannan limestone with chert layers, gorge of Gateway Creek, Heart Butts quadrangle, Flathead region
Only a few collections from the Hannan were made during the present investigation. These were examined by James Steele Williams and Helen Duncan, of the Geological Survey, who report that the fossils are not positively diagnostic but have a Mississippian aspect and contain nothing incompatible with assignment to the Hannan limestone.

While the evidence of the fossils, taken alone, may thus not be conclusive, the field relations leave no doubt that the unit is to be correlated with the Hannan limestone of Deiss. Deiss (1943a, p. 230) gives a list of fossils collected from the Hannan limestone south of the Flathead region. The collections made during the present investigation and identified by J. S. Williams and Helen Duncan include the forms listed below:

Brachythyris? cf. B. suborbicularis (Hall)

Brachythyris? sp. indet.

Crinoid columnals

Spirifer of the S. rostellatus Hall type

Other spiriferoid shells

Rynochopora? sp. indet.

Orthoceroid cephalopod?

Fenestellid bryozoans, 2 species

Polypora sp. indet.

Rhomboporoid bryozoans, indet.

Sulcoretepora sp. indet.

Camaro toechia? sp. indet.



In the two papers by Deiss that deal with the Saypo quadrangle, the thickness of the Hannan is about 1,400 feet. The formation was not measured during the present examination, but the thickness within the area mapped is thought to be at least as great as that in the Saypo quadrangle. A few of the sections drawn by Sloss and Laird show thicknesses of Mississippian rocks in excess of 1,500 feet, but most are thinner. It seems from their sections and description that breccias are more conspicuous in the mountains west of Choteau than in any of the outcrops in the Flathead region.
 
 
MESOZOIC ROCKS

JURASSIC ROCKS

The only beds of Jurassic age known are thought to belong to the Ellis group (Middle and Upper Jurassic). If the Morrison formation exists, it was not recognized during the fieldwork and may have been grouped with the Kootenai formation on the map. The distribution of the Ellis is similar to that of the Hannan limestone just described, but the Ellis generally occurs as narrow, inconspicuous bands, which are masked in places by hill wash and talus, in strong contrast to the great cliffed ridges formed by the Hannan. The Ellis consists of soft fissile nearly black shale interbedded with more resistant beds of sandy limestone that weather in conspicuous shades of yellowish brown and dark yellowish orange (fig. 18). In places the basal beds are yellow or yellowish-orange quartzitic sandstone. As a result of faulting, few of the exposures seen in 1948 contain more than a couple of hundred feet of beds, and most of them contain less; but the Ellis group is nearly 400 feet thick in the Saypo quadrangle (Deiss, 1943b, p. 1136-1140), and in the area here discussed the thickness is probably even greater. At the lower end of Swift Reservoir (NE 1/4 sec. 27 and SW1/4 sec. 26, T. 28 N., R. 10 W.) and other nearby areas, the thickness is reported to be 480 feet (Imlay, 1948).
 
fig18 Contorted shale and limestone of the Ellis group on Badger Creek near the mouth of Red Poacher Creek, Maclas Pass quadrangle, Flathead region. The exposure is in a small mass overridden by a large block of Hannan limestone
 
A fossil collection made from the Ellis group near the head of Crazy Creek by Harold Masursky proved when examined by Ralph W. Imlay to consist of fragments of Belemnites and plant stems indicative of the Swift formation, the uppermost formation in the Ellis. A collection made by Masursky through a vertical range of 20 feet about 300 yards downstream from the first one is reported by Imlay to contain Pleuromya subcompressa (Meek), Pleuromya obtusiprorata (McLearn), Grammatodon? sp., and ammonite fragments. Imlay regards these fossils as probably from the Sawtooth formation because of the presence of Pleuromya obtusiprorata (McLearn). The Sawtooth is the lowest of the three units into which the Ellis group is divided in this region. The middle one of the three, called the Rierdon formation, is probably represented by a collection made by C. P. Ross at the head of Canyon Creek. Imlay reports that this collection contains Trigonia sp., a belemnite fragment, Meleagrinella sp., Gryphaea nebrascensis (Meek and Hayden), and Camptonectes sp. A collection made by C. P. Ross in the west bank of Badger Creek above the mouth of South Badger Creek (fig. 18) was found by Imlay to contain Gryphaea impressimarginata (McLearn) and an ammonite, Gowericeras. According to Imlay, the first named has been found in abundance elsewhere in western Montana in the upper part of the Sawtooth and lower part of the overlying Rierdon formation, but not in association with Gowericeras, which commonly occurs at a slightly higher position. The collection came from a narrow, much deformed fault sliver, and the ammonite may have been moved from its normal position as a result of the deformation. It is possible, also, that Gowericeras has a longer range than previously known.

While more complete stratigraphic studies are needed, the collections listed above serve to indicate that the subdivisions of the Ellis group that have been recognized (Imlay, 1945, 1948; Cobban, 1945) in nearby areas are present in the region here described. It is impractical on the basis of the present incomplete information to subdivide the Ellis group on the map herewith. Many of the exposures are too narrow to show subdivisions on a map of the scale of plate 2 even if full information was at hand.

CRETACEOUS ROCKS

Kootenai Formation

Several narrow exposures of the Kootenai formation (Lower Cretaceous) stretch diagonally across the mountainous southwestern corner of the Heart Butte quadrangle. The widest and most continuous of these is along the mountain front and connects to the northwest with a broad expanse of Cretaceous rocks in the Marias Pass quadrangle. These latter merge westward with a broad band that reaches from the extreme southwest corner of the Heart Butte quadrangle northwest at least as far as Marias Pass and Summit Creek. This band consists mainly of strata belonging to the Kootenai formation and is so designated on plate 2. It is possible that unfolded or infaulted parts of higher Cretaceous units may be present in it and have escaped recognition. On the other hand, beds of Kootenai age are present near the boundary of Glacier National Park northeast of Marias Pass and are presumably exposed farther north as well. Available data in regard to the Cretaceous rocks east of the Lewis overthrust are not sufficiently complete to make it practicable to show subdivisions on plate 1.

The Kootenai formation, of continental origin, is made up principally of red-purple, reddish, gray, and green sandstone and sandy shale. Some of the shale is carbonaceous and is brown to nearly black. The carbonaceous shale is especially plentiful near the South Fork of Two Medicine Creek. Tiny fragments of black chert are fairly plentiful in the sandstone. Some of the sandstone beds contain numerous cherty and limonitic concretions that weather dark brown. Some of these are fully 2 feet long. In other places the concretions are arenaceous and calcareous. Coarse grit and fine conglomerate are present locally but are nowhere conspicuous. Near Cox and Winter Creeks, and less conspicuously east of Strawberry Creek, several beds of fine-grained limestone occur. These contain fragments of fossils, but identifiable material was not obtained. In other localities, where limestone is absent, some of the sandstone is calcareous. Deiss' (1943b, p. 1143) estimate of maximum thickness of about 1,500 feet for the Kootenai in the northern part of the Saypo quadrangle is probably applicable to the southern part of the Marias Pass quadrangle also. The thickness farther east appears to be somewhat less, which is in accord with Stebinger's (1918, p. 156) estimate of approximately 900 feet for the Kootenai formation in the plains. Direct measurement of thickness in this soft, highly deformed formation would, at best, be subject to a large margin of error.

Colorado Shale

No Cretaceous beds younger than the Kootenai were recognized in the mountains of the Flathead region. If any are present they are mapped with the Kootenai. In the plains east of the mountains, however, many Upper Cretaceous strata are known as a result of Stebinger's work, from which most of the data given below are abstracted. He has mapped several masses of Colorado shale immediately east of the mountains in the Heart Butte quadrangle. Beneath the overburden, these masses probably form a nearly continuous band, which is broken and offset in places by faults.

The basal part of the formation, known as the Blackleaf sandy member, is reported to be present but was not mapped separately by Stebinger within the area of plate 2. It consists of grayish black shale, in part arenaceous, with intercalated dark-gray to black sandstone. Deiss (1943a, p. 238-240; 1943b, p. 1144-1145) has described this member in the Saypo quadrangle in some detail. Some of the upper part of the Colorado shale is also present there. This upper part is generally composed of soft, somewhat fissile, brownish- and grayish-black shale, with subordinate sandstone. Some limestone is included. The Colorado shale, according to Stebinger, has a total thickness in excess of 1,800 feet, but all of this may not be present within the area of plate 2. Unlike the underlying Kootenai formation, the Colorado shale is of marine origin.

Montana Group

Most of the bedrock in the plains of the Heart Butte quadrangle belongs to the Montana group. These rocks extend into the northeastern corner of the Marias Pass quadrangle also, but in most places they are masked by detritus, which is mainly of glacial origin.

Close to the mountains in the Heart Butte quadrangle, Stebinger has distinguished, in ascending order, the Two Medicine formation, Bearpaw shale, and Horsethief sandstone; but in the southeastern part of the quadrangle, he maps the group as undifferentiated. He says that in many places facies changes make the component formations unrecognizable. In and near the northeast corner of the Marias Pass quadrangle, small areas underlain by Virgelle sandstone near the base of the Montana group are distinguished on unpublished maps prepared by Stebinger; and these are shown on plate 2.

As the Montana group was not studied during the 1948-50 fieldwork nothing can be added to published descriptions of its character. In general, the group consists of dark-gray and greenish sandstone and shale beds of marine origin, many of which weather brownish. Fossils are plentiful in some beds. The Virgelle sandstone includes beds in which magnetite is so abundant that Stebinger (1918, p. 165, 1912, p. 329-337) speaks of it as a low-grade, siliceous iron ore. The thickness of the group differs from place to place but is approximately 3,000 feet.

St. Mary River Formation

A few exposures of the St. Mary River formation, which here constitutes the uppermost Cretaceous unit, are shown on plate 2 on the basis of Stebinger's published and unpublished work. The St. Mary River formation is present within the area of plate 1 but is not distinguished on that map. The formation consists, according to Stebinger (1912, p. 330-332), of light-gray fresh and brackish-water clay and sandstone with some red and varigated shale in the upper part. The total thickness of the formation is somewhat less than 1,000 feet. The formation in Canada north of the area of plate 1 has recently been described in some detail (Williams, 1951).

Undifferentiated Cretaceous Deposits

Along the eastern and southeastern borders of Glacier National Park numerous outcrops of beds of Cretaceous age project through the cover of material of Cenozoic age; the Cenozoic deposits are largely glacial. The Cretaceous rocks include all of the formations mentioned above, but the formations are not distinguished on the maps of the present report because of inadequate information.

In the Blacktail Hills and from there northeastward for several miles in the vicinity of the trace of the Lewis overthrust, beds of the Kootenai formation are abundantly exposed. From near Glacier Park Station northward along the mountain border, the Colorado shale is exposed in numerous places and probably constitutes the principal formation present. Near the international boundary in the general vicinity of Belly River, exposures of the lower part of the Montana group were recognized by members of M. R. Campbell's field parties. Farther east in the greatly faulted rocks in and east of the northeast part of plate 1, higher units of the Montana group and also the St. Mary River formation are present
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