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The Arizona Geological Survey is not responsible for the accuracy of the records, information, or opinions that may be contained in the files. The Survey collects, catalogs, and archives data on mineral properties regardless of its views of the veracity or accuracy of those data. 10 GEOLOGY AND MINERAL DEPOSITS, LINCOLN AND FLATHEAD COUNTIES STRATIGRAPHY- BELTIAN ROCKS 15 lake. The water from the lake, which is very deep, As the climate gradually ameliorated, the ice for the quartzose rocks is between 0.01 and 1.0 altered shell surrounding unaltered pyrite-bearing mm or more, the average grain size being about now flows northeastward down Logan Creek. stopped advancing, and for a relatively long period .. \ carbonate rock attains a thickness of 2 cm or Few of the moraine-dammed lakes provide the lake level was static. Presumably evaporation 0.1 or 0.15 mm; in the argillite the mean diameter more. such conspicuous evidence of drainage diversion offset melting. A prominent terrace was locally is about 0.05 mm. Small angular quartz particles Clastic rocks in the Missoula Group in as do Bull Lake and Tally Lake, but there are other developed at an altitude of about 4,100 feet (Mc are probably of detrital origin, whereas larger clude quartzite, sandstone, argillite, claystone, and Murtrey and others, 1965, p. 12). Continued warm examples. The whole subject of the interrelation grains exhibit fused or welded boundaries indica siltstone. Angular to subangular quartz particles ship of glacial erosion, glacial deposition, and ing accelerated the runoff, the lake filled to an tive of secondary growth. Magnetite octahedra are most abundant. Sericite, magnetite, ilmenite, altitude of about 4,200 feet, even though the ice drainage diversion is too complex for further con of both primary and secondary origin are perva specular hematite, and chlorite are found in the may have been retreating slowly, and at last the sive throughout the Ravalli, the size ranging be sideration here, however. Striped Peak Formation. Ferruginous quartzose water overtopped the ice dam. Once water started tween 0.2 and 1.0 mm. Other minerals in rocks Kettle holes, swales, and hummocky topo rocks of the Libby Formation in western Lincoln flowing over it, the ice dam could not long persist; of the Ravalli Group include secondary sericite, County are sporadically sericitic and contain some graphy characteristic of ground moraine are it may have failed completely within a few days. leucoxene, and rare chlorite, carbonate, a moder noticeable west of the Stillwater River between chlorite. Rocks above the Purcell Lava in eastern Obviously, no shoreline markings could have de ate amount of biotite, and minor potash feldspar, Flathead County contain quartz, muscovite, minor Lost Creek and Kalispell (Alden, 1953, p. 123) veloped as the water level fell. The effects down plagioclase, zircon, apatite, epidote, ilmenite, tour and also east and north of Kalispell. Similar topo sericite, chlorite, iron oxides, feldspar, glauconite, stream can hardly be visualized. maline, and hematite. Dahlem (1959, p. 32) barite, sparse biotite, epidote, zircon, magnetite, graphy was observed in the Little Bitterroot described magnetite particles in quartz, and Hall At its maximum, Glacial Lake Missoula was apatite, tourmaline, and carbonate. Valley 2 miles south of Little Bitterroot Lake and (1962, p. 29) found inclusions of quartz in magne about 2,000 feet deep just above the ice dam and also about halfway up Evers Creek 3 miles east tite granules, attesting to secondary growth of The Shepard Formation is dominantly calcar covered an area estimated at 3,300 square miles. of Johnson Peak. the quartz and of the magnetite, respectively. eous. Local discontinuous impure limestone beds If the maximum altitude of the water surface is Biotite about 0.20 mm in diameter encloses crystals occur elsewhere in Missoula strata in eastern Flat Drumlins are numerous and prominent only assumed to have been 4,200 feet, the 4,200-foot in the valley of the Tobacco River. They trend of quartz, zircon, and opaque minerals (Latus head County but carbonate rocks are relatively contour on topographic maps would outline the uncommon in the Missoula Group in western southeastward up the valley from Eureka to the extreme extent of the lake, except in areas that zynski, 1962, p. 28). Lincoln County. vicinity of Stryker, which is on the low divide were still covered by ice when the lake was In the Wallace Formation, carbonate-bearing between the Tobacco and the Stillwater drainages. drained. Inasmuch as the ice dam was destroyed strata are sporadically distributed through the Ripple marks, mud-crack casts, flow casts, One other elongate drumlinlike deposit was ob soon after the glacial retreat began, most of the unit, but most of the impure limestone was ob raindrop imprints, clay galls, mud-chip breccia, served 'l4 mile south of Tally Lake, about 15 area in Lincoln and Flathead Counties was still served near the central part of the formation. In crossbedding, salt-crystal casts, scour-and-fil! miles northwest of Kalispell. covered by ice, and therefore most of the lakebed the Piegan Group, of which the P I, Siyeh, and Ps channels, graded bedding, and load casts are wide silts that are found in that area were deposited in units are the approximate eastern equivalents of spread throughout the Belt strata. These features PLEISTOCENE GLACIAL LAKES lakes other than Glacial Lake Missoula. the Wallace, the Siyeh Formation is everywhere aid in interpreting the geologic conditions under which the sediments were deposited. Ripple marks, Lakebed deposits that are definitely assign carbonate bearing but the lower and upper Piegan As the mountain glaciers and the lobes of the mud-crack casts, scour-and-fill channels, cross able to Glacial Lake Missoula are widespread in units contain more argillite and quartzite and Cordilleran ice sheet advanced, the small quan bedding, graded bedding, and load casts also pro the Little Bitterroot Valley, most of which is south less carbonate. tities of meltwater were at first free to escape to vide evidence helpful in determining the top and of the study area. A terrace fill consisting of 20 Carbonate strata are almost everywhere im the Columbia River drainage system, but even base of individual beds where overturning by feet of brownish-gray clay and silt was observed pure, the major rock-forming minerals being cal tually the lobe that came down the Purcell Trench structural deformation is suspected. near Little Meadow Creek, which is east of Hub cite, dolomite, quartz, and sericite. Minor con into Idaho blocked the Kootenai Valley northwest bart Reservoir. Scattered erratic blocks of quart stituents make up about 5 percent of the rock. Throughout the Prichard Formation, relative of Troy and later the Clark Fork Valley northwest zite and gneiss observed in the Niarada area below Grain size of carbonate minerals ranges from about sparseness of ripple marks, mud cracks, and cross of Thompson Falls. Thereafter the meltwater was an altitude of 3,800 feet are ice-rafted material 0.05 to 0.4 mm; angular and subrounded quartz bedding may indicate that the water was deeper ponded behind these ice dams and probably some deposited in Glacial Lake Missoula. grains range between 0.05 and 0.3 mm. The P I than during deposition of Ravalli and Wallace others, forming lakes; the largest was Glacial sediments. Ravalli rocks, including the Appekunny • and P3 units contain quartz, muscovite, sericite, Lake Missoula, which also has been the most Elsewhere in the study area, Glacial Lake Mis chlorite, and minor amounts of feldspar, biotite, and Grinnell Formations east of the Rocky Moun extensively studied. Although a major glacial lake soula could have extended far up the Thompson dolomite, zircon, and leucoxene. Cubes and grains tain Trench, exhibit current and oscillation ripple may have been impounded in the same place and Vermilion Rivers but even if these valleys of pyrite and limonite pseudomorphs after pyrite marks, mud-crack casts, crossbedding, and mud earlier, possibly several times, only the last one were not filled with ice, the lake would have over are common in Wallace and Piegan carbonate rock. chip breccia. In the Grinnell Formation, mud is considered here. topped the divide only at maximum stage, and Wampler and Kulp (1964, p. 1423) observed that cracks are more common than ripple marks. Both Along its northern side, Glacial Lake Missoula then only by about 200 feet at most. The long forms of pyrite seen in modern sediments are the Wallace Formation and equivalent Piegan was generally confined at least in part by the narrow depression occupied by McGregor Lake, also noted in lithified sediments, and they sug Group contain abundant ripple marks, clay galls advancing ice, and as the water level rose by small Thompson Lakes, and others may have been an gested that sedimentary pyrite reached its present (ovoid clay concretions), mud cracks, raindrop increments, the valleys of the Clark Fork, Bitter ice-marginal drainage way soon after ice retreat crystalline state at the time of or soon after imprints, and cross-laminations. Mud-chip brec began, but perhaps not until after Glacial Lake root, and Blackfoot Rivers and their tributaries burial, but that disseminated euhedral crystals cias are common in the PI unit but rare in the P 3 were flooded. Each temporary level of the rising Missoula had been drained. probably result from recrystallization during dia unit. Ripple marks are present in the more sandy water is marked by a faint shoreline; these are Near the Idaho boundary, the altitude of the genesis. In the carbonate-bearing strata, chlorite units, whereas mud-crack casts characterize the plainly visible in the valley of the Clark Fork, moraine that forms the divide between Lake Creek is present in small amounts as are feldspar min argillaceous layers. Striped Peak and Kintla especially on the hills east of Missoula. When the to the north and Bull River to the south is about erals. An uncommon weathering feature is asso strata contain well-developed current ripple marks, lake began to fill, the amount of meltwater was 2,350 feet. If this moraine marks the southern ciated with Wallace and Siyeh float and some out mud cracks, mud-chip breccia, and sparse to minimal, and later the area over which the water limit of ice advance, then Glacial Lake Missoula crops-rock at the surface has been altered to abundant salt-crystal casts and raindrop impres could spread increased greatly, hence the increase would have flooded much or all of Lake Creek red earthy pulverulent hematite, probably through sions and sparse crossbedding. The Phillips For in depth was slow. valley. The Bull River valley seems to have a U- oxidation of the ferrous iron in the rock. The mation has moderately abundant ripple marks and 11 GEOLOGY AND MINERAL DEPOSITS, LINCOLN AND FLATHEAD COUNT IES PHYSIOGRAPHY-PLEISTOCENE GLACIAL LAKES 11 partly torn up and locally incorporated as mud most pronounced in the Prichard Formation. Phyl shaped cross section clear to its junction with Although Glacial Lake Missoula may have chips in the basal part of the sediment that was lite interbedded with medium-grained white I the Clark Fork, however. Confluent mountain flooded the valley of Lake Creek for a brief period, being deposited. The finer sediment then settled, as sericitic quartzite crops out on the flanks of the glaciers moving down this valley may have most if not all of the lakebed silt in the valleys of influx subsided. As evaporation again exceeded Sylvanite anticline north of Troy. These phyIIitic confined Glacial Lake Missoula to the area the Yaak and Kootenai Rivers and their other influx, the surface eventually became cracked. rocks exhibit well-developed biotite folia, linear south of the divide. Alternatively, the sheet ice tributaries must have been deposited in a separate Deeper portions of the lake probably remained quartz, and sericite grains; the sericite where ex may have continued down the vaIley of Bull River, smaller lake that may have formed at a fairly late covered by progressively more saline water while posed on planar surfaces imparts a silky sheen to and the moraine that now forms the divide may stage of glacial retreat. A deposit of horizontally shallower parts emerged. The rate of subsidence the rock. Some contact metamorphism related to have been a late recessional moraine formed after bedded lacustrine silt, well exposed northeast of probably approximated the rate of deposition; a buried intrusive may have contributed to the a brief re-advance. Libby, overlies glacial till and attains a thickness although no close balance between subsidence and schistosity exhibited by these rocks. In the eastern sedimentation is required for such deposition, a part of Flathead County, formations in the Mis R 33 W balance is required to account for the persistence soula Group show less metamorphism than their R 34 W of shallow-water depositional structures through Lincoln County equivalents; quartzite units are out the sequence. The carbonate rocks of the interbedded with siltstone, mudstone, and shale T T 331~~-----r------4------+------~~~--~~--~~~------+------~33 Siyeh Formation may represent deposition in a in the Whitefish and Lewis and Clark Ranges. N N deeper but still fairly shallow part of the lake, Detrital and secondary minerals of the Belt where detrital influx was less. The denser, more 21 22 23 20 rocks are extremely variable in grain size, but ... saline water resulting from evaporation would few exceed 1.5 mm in diameter. Pyrite, garnet, accumulate here and begin the deposition of dolo and magnetite crystals are commonly megascopic, mite (Barnes, 1963, p. 55). Harrison and Camp and by secondary crystal growth some grains at , bell (1963) described the basin as a trough trend tain large diameters; the larger crystals are almost ing northwest during deposition of pre-Ravalli, confined to shear planes. Exceptionally large (a RavaIli, and Piegan rocks but trending northeast few centimeters) garnet and pyrite crystals and Lalc« during deposition of Missoula rocks. large (several millimeters) biotite and magnetite No doubt some parts of the Belt basin were grains occur in the lower groups of the Belt. The :>omewhat deeper than other parts. On Yaak Moun principal accessory minerals are ilmenite, zircon, tain, northeast of Troy, the Wallace Formation tourmaline, garnet, apatite, rutile, and epidote, 31 32 includes several thousand feet of impure lime but altogether they make up only a very small stone, but these rocks grade lateraIly into clastic percentage of total rock constituents. rocks containing only sparse carbonate. A local Argillite and quartzite of the pre-Ravalli depression may account for the great thickness of Group (Prichard Formation) contain quartz, seri 2 6 3 ~ these carbonate strata, which are strikingly simi cite, hematite, biotite porphyroblasts, pyrite Terrace lar to impure limestone beds of the Siyeh Forma cubes, and pyrrhotite grains and minute amounts A tion. of microscopic feldspar, carbonate, epidote, clino Rocks in the lower two-thirds of the Belt are zoisite, zircon, ilmenite, leucoxene, rutile, garnet, 2500 Fool apatite, chlorite, sericite, and tourmaline. Sericite Terrace characterized by the predominance of various 10 shades of gray, brown, white, and yellow, where makes up as much as 50 percent of some of the rock. Most of the quartz, zircon, apatite, and ~ as rocks in the upper third exhibit red, purple, 2200 Fool rutile originated as primary clastic minerals. Bio Terrace gray, and green hues. Rocks described as argillite • in this report are more indurated equivalents of tite, magnetite, and pyrite are both primary and siltstone, claystone, mudstone, and shale (Twen secondary minerals. Epidote, carbonate, hematite, 14 Terrace hofel, 1937, p. 95). They contain clay- and silt-size sericite, chlorite, and leucoxene are authigenic B constituents. Grain size ranges from 0.01 to 1.5 particles and were not otherwise differentiated in - T reconnaissance field mapping. Quartzite and sand mm, mean diameter being 0.04 mm. Many quartz ~=====~= 32 grains exhibit interlocking characteristics and Terrace N stone contain clastic grains ranging from very c fine to coarse. In impure carbonate rocks the in fused boundaries and are partly recrystallized; - dividual particles of carbonate minerals range such grains are as much as 40 percent of some from very fine to fine. All Belt rocks contain rocks. In eastern Lincoln County, apatite, zircon, Recent quartz as a constituent in various quantities. and tourmaline occur in prismatic crystals as Floodplain much as 0.1 mm long (Latuszynski, 1962, p. 23). - Sericite in minute micaceous mats, shreds, Carbonate strata in the upper Prichard For Moraine and plates is discernible in all formations except o Im i 28 the uppermost units of the Missoula Group, mation seem to thicken eastward from central I I Old -Yoak Riv Lincoln County. where, if sericite is present, it is sporadic (Libby Chon ne I Formation). This distribution of sericite is indica Interbedded quartzite, argillaceous quartzite, tive of the decreased regional metamorphism of and argillite of the Ravalli Group contain large R 35 W R 34 W younger Belt rocks, particularly the upper units of percentages of quartz, but the purest quartzite Figure 4.-Map showing terraces and flood plain adjacent to Star Creek and the Kootenai and Yaak Rivers. the Missoula Group. Metamorphic features are contains only 86 percent silica. Particle size range • (After Dahlem, 1959.) 12 GEOLOGY AND MINERAL DEPOSITS. LINCOLN AND FLATHEAD COUNTIES STRATIGRAPHY-BELTIAN ROCKS 13 of 300 feet; other thick deposits of finely laminated formerly somewhat more extensive than it is now. Montana are part of the Belt Series (Wilmarth, east, in the Whitefish Range, the thickness ex silt are exposed east of Troy. In the Kootenai Impounded water extended some distance up Hill , 1925, p. 104), as are similar rocks in northern posed is 17,000 feet, and in the Lewis and Clark Valley northwest of Troy, massive silt deposits burn Creek and other inlets, as shown by lake Idaho, northeastern Washington, and southern Range it is 25,000 feet. Farther south, in the cen are found at altitudes of only 1,850 to 2,100 feet, silt deposited over local accumulations of outwash. British Columbia. In Canada the equivalents of the tral part of the Mission Range, it is 20,000 feet whereas the altitude of the outlet of Glacial Lake Silt deposits that formerly bordered the west shore Belt Series are named the Lower and Upper (Harrison and others, 1969). Missoula after removal of the ice dam was about of the lake have been removed by wave action, as Purcell. In northwestern Montana, strata within the 2,200 feet. The lake that occupied the Yaak and the lake level fluctuates over a fairly large vertical Belt are conformable. Structural deformation pro Kootenai Valleys may have been impounded be range. GENERAL CONSIDERATrONS duced both tight and open folds having general hind morainal material deposited by the lobe of ice Numerous temporary high-altitude lakes per northwest to north trend. Generally the intensity that moved down the Purcell Trench, rather than sisted long enough to leave deposits of laminated The Belt rocks were deposited in a broad of folding decreases eastward from western Lin behind the ice itself. silt and clay in the higher valleys. Silt in the shallow depositional basin as sand, silt, and clay coln County to the trace of the Lewis Thrust in An implication that the lake in the Yaak valleys of Tepee, Squaw, and Harris Creeks, tribu or a mixture (mud) and as carbonate. Regional eastern Flathead County. Igneous rocks, includ Kootenai Valley area was not released in a sudden taries of Fisher River, was deposited in an ice low-grade metamorphism (quartz-albite-epidote ing mafic dikes and sills and acid dikes and stocks, flood by breaching of an ice dam but rather impounded lake that reached an altitude of 3,500 biotite subfacies of the greenschist facies) sub intrude all formations of the Belt Series. Mafic decreased by irregular erosion of a morainal dam feet. Similar material is poorly exposed northeast sequently altered the clastic sediments to quart igneous dikes and sills were not observed in the of nonuniform composition is seen in the series of Loon Lake in Pleasant Valley. The overflow zite and argillite. The fine to very fined grained Missoula Group in Lincoln County but were seen of terraces near Troy. Two major terraces, at may have drained north down Swamp Creek for a carbonate, supposedly deposited in deeper water, in Missoula rocks in southeastern Flathead County. altitudes of 2,200 and 2,500 feet. border the while until the Fisher River was reopened. was recrystallized to dense impure limestone. A volcanic flow within the Belt sequence was Kootenai River west of Troy and also the Yaak Near the head of Seventeenmile Creek, north Additional metamorphic effects noted include the first named by Daly (1912, p. 207) the Purcell River and Pine Creek for several miles above their east of Troy, deposits of stratified silt and clay formation of sericite, muscovite, biotite, and some Lava and later the Purcell Basalt (Wilmarth, 1938, junctions with the Kootenai (Fig. 4). Along the at altitudes of 3,600 to 3,800 feet mark the site magnetite. p. 1746). It extends westward from Glacier Na east side of the Yaak, at least six gravel-veneered of a mountain lake, which probably was im tional Park to the 115°30' meridian and from terraces are ' visible below the 2,500-foot terrace. pounded by ice. The overflow may have spilled The term "Belt Supergroup" was substituted lat 48°30' in the so'utheastern part of the White Two or more dissected terraces above the 2,500- south into Quartz Creek or east into Pipe Creek; for the term "Belt Series" by Childers (1963, p. fish Range northward into British Columbia. foot terrace are less distinct. The series of terraces the divides are low and indistinct. 142). These Precambrian rocks have been sub suggests that the Yaak-Kootenai drainage was Lakebed silt and clay underlie Brown's divided into four major groups; in ascending order All investigators of the Belt rocks describe graded to a lake that remained stable for long Meadows, on Mount Creek about 6 miles south of they are pre-Ravalli, Ravalli, Piegan, and Missoula abundant sedimentary features suggestive of shal periods but periodically was lowered by rapid its junction with Ashley Creek southwest of Kalis Groups (Ross, 1959). low-water deposition and subaerial exposure, the erosion of its outlet. pell. Part of this deposit is postglacial, however. The lower part of the Belt is dominantly most common features being ripple marks, cross The present Flathead Lake is a remnant of a The temporary glacial lake surface reached an clastic; the Prichard Formation (pre-Ravalli) and bedding and cross-lamination, mud-crack casts, somewhat larger ancestral Flathead Lake that was altitude of about 4,300 feet, as indicated by the the Ravalli Group are almost entirely argillite, mud-chip breccia, raindrop imprints, and salt-crys impounded behind the terminal moraines of the lake deposits, rather than 4,700 feet, as postu siltite, and quartzite. The middle part of the Belt, tal casts. Smith (1963, p. 112) interpreted the rocks lobe of Cordilleran ice that moved down the Flat lated by Alden (1953). represented by the Pi egan Group (Wallace Forma as alternating lacustrine or lagoonal sediments head Valley. Lacustrine silt was deposited as far By their very nature, lakes are transitory tion in the western part and equivalent rocks in the deposited in fairly deep quiet water during pre north as Columbia Falls and Whitefish in the main features. It is not possible to categorize every lake eastern part of the area), is dominantly carbonate Ravalli and Piegan time and lacustrine, fluviatile, valley and almost to Marion in Ashley Creek in the area as ice-blocked or moraine-blocked, but its lower and upper parts contain much clastic and flood-plain sediments laid down in shallow valley. As the outlet at Polson was eroded deeper, glacial or recent. Some of those that still contain material. Upper Belt rocks, that is. rocks of the turbulent water during Ravalli and Missoula time. the lake level was lowered; at present it is con water are obviously remnants of somewhat larger , ~ Missoula Group, are again dominantly clastic, but He postulated a partly enclosed basin of Belt de trolled by a dam. lakes that formed during glaciation. Those that subordinate carbonate-bearing strata occur in the position, as sediment is inferred to have originated After the ice sheet retreated beyond the are filled with sediment (as opposed to those that group. from sources to the northeast, south, southwest, Canadian border, glaciolacustrine deposits of fine were drained by downcutting of the outlet) may Thicknesses of all the groups vary, even with and west. Barnes (1963, p. 51) concluded that silt and clay were spread over the morainal and be entirely glacial, entirely postglacial, or glacial in the map area, but the greatest changes observed quartz, feldspar, and clay minerals in Belt rocks outwash material on the broad plain north of to recent. Illustrative of the difficulty of classifica were the northward thinning of the Ravalli and of the Whitefish Range were possibly derived in Eureka and Rexford. As soon as the Kootenai tion is the situation near the head of Clay Creek, Piegen Groups from west-central Lincoln County most part from a high-grade metamorphic and Valley was free of ice the impounded water began south of Yaak. There a deposit of glacial till con to the International Boundary, and the eastward plutonic terrain because he found composite quartz to flow into the Kootenai River, and the Tobacco taining rock flour, clay, silt, sand, and angular thinning of the Ravalli and Piegan Groups from grains having crenulated crystal boundaries, which River began extending its headward reaches to the to subangular fragments of rock is reworked dur the Salish Mountains to the Whitefish Range, are characteristic of quartz derived from meta southeast (Sommers, 1961). ing each spring runoff; clay and silt are deposited across the Rocky Mountain Trench. The Prichard morphic terrains (Krynine, 1950, p. 48). Cross Lake Mary Ronan, like Flathead Lake, was downstream in a manmade lake. Formation (pre-Ravalli) is the thickest sequence bedding in quartzite beds is supporting evidence in northwest Montana; in the Pend Orielle region of a source area to the north or east. of Idaho its thickness exceeds 20.000 feet. The Belt basin of deposition was broad and STRATIGRAPHY The base of the Belt is nowhere exposed and shallow, and few permanent streams entered it. PRECAMBRIAN-BELTIAN ROCKS of west-central Montana. Previously the rocks the top is an erosional surface or (top of the Mis The surrounding regions were of moderate relief, had been described by Hayden (1869) and Peale soula Group) an unconformity with Middle Cam and the climate was semiarid. Periodic influxes A thick series of fine-grained clastic and (1873); Peale (1893) correctly assigned them to brian or younger strata. The total thickness of of water brought in fine sediment, which spread carbonate rocks of late Precambrian age was the Precambrian, on the basis of his work in the the Belt is therefore not determinable. In the throughout the basin, probably distributed along named by Walcott (1899) the Belt Series for vicinity of Three Forks, Montana. Metasedimen western part of Lincoln County the thickness of a few very broad and shallow channels. The dried exposures in the Little Belt and Big Belt Mountains tary rocks of Precambrian age in northwestern • exposed Belt rocks exceeds 40,000 feet. Farther and mud-cracked surface around the margins was 24 GEOLOGY AND MINERAL DEPOSITS, LINCOLN AND FLATHEAD COUNTIES STRATIGRAPHY- BELTIAN ROCKS 17 detail as consisting of about 70 percent silt and and dark-gray argillite laminae were noted in this ::-~ ,,00 30 percent clay. A small percentage of very fine lower sequence. Overlying these beds are light -'" c " c: ~ c """ "i sand occurs in the basal parts of microscopically gray or light-greenish-gray fine-grained quartzite Olr =~ :;;0.. : z" graded laminae; the upper parts are medium silt. interlayered with medium-gray argillite. This upper '" :" Q. ~ .. Scattered in these laminated beds are some round part is 3,000 feet thick and weathers pale yellow c -;;; c Q. ,. c: "0 g ~ gray. ~ " 0 ed intraformational pebbles of clay-sized material, If) If) -; Q. :I: If) similar in general appearance to the clay between In the northern part of the Swan and Mission dncJ:) OlnOSSIV>J the graded laminae. As seen in thin section, these Ranges, the Appekunny is predominantly light , .. e "0 Q. 0 "0'" '" Q. pebbles seem to be imbricated, as are the accom 0 -;;; 0 gray to purplish-gray and greenish-gray siliceous E " Q. " " E c ~ " ., ,. -;" ~ w Z 0 ~ g 0 panying mica flakes and a few quartz grains, and (f) ~ :I: argillite. Quartzite interbeds are light gray to In
and thick-bedded white and very light gray seri "~'D ~ - 140 ft. ARGILLITE: Green, siliceous in part, A ~El·l. !_ 60 ft. ARGILLITE: Drab greenish gray, crystalline, r~: .. :.~ 900 ft. QUARTZITE and QUARTZITIC ARGIL- --- biotite bearing citic quartzite interbedded with rusty-weathered weathers rusty orange; mud cracks. :: .. ::.:.: :::: LITE: Light purple, light blue gray, and light grayish-brown quartzite, dark- and light-gray ar -iii :: : .: ;.;. :-:-: gray, crossbedded. Distinctive marker beds at 10 to 30 ft. LIMESTONE: Banded light gray and gillite, and phyllitic rocks containing sericite, bio 30 ft. ARGILLITE: Green and purple banded; blue gray, biotite bearing. ~.;;~.: :.~ft;:;·ii :Sima:~ !~~o: t~~e~S;;~gq~~~~ tite, and quartz. Leech (1958, p. 7) described the I J minor light-purple quartzite and dark-gray argil ugillite. Minor dark-purple arxillite (weak), thin 500 ft. ARGILLITE: Light to dark bluish gray, ~-:~ f- lite. ~.: .:. ::.:;:.:: bedded in units 3 ft. thick or ress. crystalline, biotite bearing, scattered FeC03 Fort Steele Formation underlying the Aldridge 70 ft. ARGILLITE: Light-blue-gray, magnetite blebs. Minor units of drab-gray argillite and as consisting of white siliceous quartzite, gray ~~~ I_ bearing, pyrite weathers to limonite. " --" Mud cracks common near base only. light-gray quartzite. Somewhat banded near .. ... top, showing differential weathering. argillaceous quartzite, dark argillite, and gray ~.. : .;:.:.:.:.:-. =--- 340 ft. QUARTZITIC ARGILLITE and QUARTZ -...... and black dolomitic and calcareous argillite and ITE: Banded light gray, minor purple, and blue -...... - dolomite. The Fort Steele and Aldridge Forma in units several feet thick separated by purple ~·;~··:f-mi to blue-gray noncompetent argillite and rue .. ... - tions total 14,000 feet in thickness and they are thin beds of white quartzite. equivalents of the Prichard Formation.
--- South of Warland, the Prichard is medium - and dark-gray sandy argillite and quartzitic ar gillite, and east of Warland it is white argillitic - - - sandstone. ::c::: '.-==-= ...... - In the Ural and Pleasant Valley areas only :.:,.:.:::,~ :.~.:. :-::~ ~ ~ :": ~ the upper few thousand feet of the Prichard is ~~: ? ::- -~-::: - 90 ft. QUARTZITIC ARGILLITE: Gray to olive ...... ~: .~ - --:C"":~-__ green, weathers bleached green; Iight-gray to ...... ;:::: 1,000 ft. + ARGILLITE: Drab olive gray, crystalline, exposed. On Elk Mountain (Fig. 6) about 1,600 :--::_:.:...:-:: -'-::.' purple banded quartzite. Crossbedded, 5-ft. sugary, biotite bearing. feet of gray, blue-gray, and drab olive-gray bio .=;~.7~':-. _ crossbedded light·gray quartzite at base. tite-bearing argillite is present. Strata are inter 210 ft. ARGILLITE: Mottled green, purple, and blue gray; partly noncompetent; minor orange laminated quartzite and argillite layers in tones --- weathering near base due to profuse limonite. of light gray, dark gray, and bluish gray; sur faces and joint and bedding planes display some --- red-brown weathering. A few mud cracks are pres --- ent near the top of the unit. Toward the top of the formation the beds become more quartzitic, but 180 ft. Laminated QUARTZITE: Light bluish gray, below the Prichard-Ravalli contact there is a ----- crossbedded, and ARGILLITE: Drab greenish 180 ft. ARGILLITE: Interbedded green and pur calcareous zone about 500 feet thick containing ...:..:....:...-.:..:. gray and blue gray, quutzitic in part. .. . . ------ple in units 2 to 3 fL thick. Mud cracks silty interbeds of calcareous argillite and lime common. - -- stone. Carbonate rocks in the zone weather light gray and exhibit a pitted surface from leaching of 1=-- calcite and carbonate-bearing laminae. Carbon 160 ft. ARGILLITE: Gray to olive drab, some ate-bearing rocks at this stratigraphic position duk-blue-gray stringers; weathers light blue to light green. 250 ft. ARGILUTE: Banded light and dark green. were recognized in the Thompson Lakes and Ural areas. --- Along Vermilion River in the vicinity of ------390 ft. ARGILLITE: Laminated dark blue gray, Lyons Gulch is a thick white quartzite overlain purple, and pale green, profuse mud cracks, and by rusty-weathering interlaminated gray argillite -=---=---- • QUARTZITE: Light gray and purple, banded, and white quartzite. Similar rocks crop out along - - - makes up Vz of 3-ft. units near base but less prominent upward; absent in top 60 ft. the Yaak River northeast of Pine Creek. Above ~ '. :: 380 ft. QUARTZmC ARGILLITE: Light purple this sequence is thick-bedded light-colored seri =~;._...._:._.:_: aDd tight green; thin beds of purple QUARTZ citic quartzite and argillite that exhibits no rusty = ITE: Green argillite common at base, less so weathering. In both localities, the base of the :;.~;~ d.kk t:'J 3~:' J~ ft.;':' units several feet quartzite is regarded as the contact between the Prichard and Ravalli. Elsewhere in the area dis ~': : ~ ... . cussed in this report, there is a transitional zone, ~: . :: .. ~ --- I ft. pal~y-green quartzite 160 fL below ------~T::' top. Some light and dark-purple argillite• and the contact was placed at the midpoint of --. -.... . _- L--__~_ Base not exposed. ~~.~:.:: the zone, because poor exposures in many areas ~~:~. :-: .-: precluded the possibility of locating the upper Figure 6.-Upper part of Prichard Formation, Pre-Ravalli most argillite bed with any accuracy. In the Yaak Group, section on west slope of Elk Mountain in sec. 16 ~~f1:"":'7":"'r..:..a...s.:.:. River area, the width of the outcrop of the tran .1..!....:..!..•••• and 17, T. 31 N., R. 26 W. (After Galster, U. S. Army Corps sitional zone ranges from 300 to 500 feet, but at of Engineers). Wolf Creek it is much wider, and in the Thompson Lakes area it is 500 to 800 feet wide. In the Ural area, it is 500 to 600 feet wide, and farther east in .. the Elk Mountain vicinity it is also 500 feet wide . 20 GEOLOGY AND MINERAL DEPOSITS, LINCOLN AND FLATHEAD COUNTIES STRATIGRAPHY-RAVALLI GROUP (BELT)-WESTERN PHASE 21 ~Bb- net, and Bitterroot Mountains, and extends west 225 ft. ARGILLITE: Greenish gray, weathers MC~ - 240 ft. ARGILLITE: Deep purple and gray, minor In the McGregor Lake area, however, it is 700 ::::-:-~ bleached green and minor orange, limonite green; weathers pale green to purplish gray; to 800 feet wide. The average width of outcrop ward to Coeur d'Alene Lake and eastward to 1__ _--_· -_I bearing. Some beds of light-gray to buff quartz- gray argillite is less .competent; some light '. - ite. Magnetite bearing, profuse at base. of this gradational zone is about 575 feet, which Ravalli. 7-=. purple quartzite. is greater than the true thickness of the sequence. ---~ -- --- In the Coeur d'Alene district of Idaho, the --- Transitional zones at the top of the Prichard Ravalli Group includes the Burke Formation, --- Formation have been described from a minimum Revett Quartzite, and St. Regis Formation (Cal ------of 300 feet to over 2,000 feet wide in the Coeur kins and MacDonald, 1909, p. 37). Gibson (1948, 605 ft. QUARTZITE: Light to medium_gray to d' Alene district and adjacent areas (Hobbs and p. 12) described the Ravalli Formation in the .buff, hard; green argillite and sandy argillite and 240 ft. ARGILLITE: Greenish gray, weathers others, 1965, p. 34). In the Coeur d'Alene district Libby area as gray and white quartzite, which he minor light-gray-green quartzitic argillite, all 'bleached green, magnetite bearing, minor orange magnetite bearing. FeC03 blebs near base. weathering and QUARTZITE: Light purplish the contact between the Prichard and overlying stated could be identified as the equivalent of the AIgillite weathers bleached green and orange. gray, in units I ft. thick, mostly in upper half. Burke Formations is arbitrarily placed at the top Revett Quartzite of the Coeur d'Alene district. ------of the highest laminated argillite in the zone. West of the Rocky Mountain Trench, undif ferentiated Ravalli quartzite and argillite (West If the rate of Belt deposition was 10 cm per ------1,000 years as for the Green River Formation ern phase) consists of a homogeneous sequence (Bradley, 1929, p. 99, 108), as suggested by Smith of gray, white, green-gray, and bluish-gray quart 400 ft. ARGILLITE: Purple and blue, stringers of - . _- (1963, p. 114), the time required for deposition zite and argillite, in which detailed mapping may :.:------._ :-.::-_. :::'-.- deep purple. Subordinate green argillite, promi of the gradational zone was 1 y:! to 1 million permit a threefold division as suggested by Shel --- nent near base. Profuse mud cracks and clay % --- galls. Weathers light blue. Some thin (to 8 in.) years, or half again as long as the Pleistocene den (1961, p. 16), Hall (1962, p. 28), and Page units of light-purple-gray quartzite. epoch. (1963, p. 9). ------Prichard beds exposed southwest of the Undifferentiated Ravalli strata as here de Leonia fault are believed to be correlative with scribed are confined, within the map area, west part of the Fort Steele Formation of the Western of a line that approximately parallels the west Canadian Rockies. Prichard strata exposed west of side of the Rocky Mountain Trench from the Yaak River are believed to be correlative with vicinity of Rollins, past Blacktail Mountain, west both Aldridge and Fort Steele Formations. Most of Johnson Peak, to the International Boundary :::_c::::~::::: J 60 ft. ARGILLITE: Dark to medium green, weath- F=~::'~--~' . ers light blue green; and quartzite, biotite bear- --- remaining Prichard strata in the map area are (PI. 2). The strata are well exposed along the ~~ ing, magnetite and limonite present. SO ft. ARGILLITE: Blue-green, weathers bright correlative with the upper part of the Aldridge Vermilion River road northeast of Lyons Creek, ~.~ J 920 ft. ARGILLITE: Greenish gray, weathers light green; limonite bearing, profuse mud cracks. of southern British Columbia and with the upper along the Yaak River road northeast of Pine Creek, blue green; buff to light-gray quartzite, biotite and magnetite bearing, sericitic. Rare thin units I=@- member and part of the middle member of the along Montana Highway 37 northwest of Dunn ::c.c -:-_--:-_--:. 150 ft. interbedded li~t-blue argillite and light- Prichard Formation in the Pend Oreille area as Creek, east of Little Bitterroot Lake, and in the of light-gray quartzite. Sparse ripple marks. gray to buff quartzIte, stringers of soft, deep purple argillite. described by Harrison and Campbell (1963). vicinity of Brush Pass on the crossover road from ...... _.... Sheppard Creek to Wolf Creek. Carbonate-bearing strata in the Pleasant Valley area within the uppermost Prichard may The thickness of the Ravalli beds is variable. ... --- 535 ft. ARGILLITE: Interlaminated, blue green, nndicate a westward wedging out of the car In the Libby district, Gibson (1948, p. 12) reported ~een gray, and purple, general green weather bonate-bearing Altyn Formation of Glacier Na the Ravalli to be about 10,000 feet thick. In the --- mg. Rare units of light-gray quartzite and soft dark-blue-gray argillite (thin units). tional Park. The Newland of the Canyon Ferry Yaak River vicinity about 7,000 feet of Ravalli ---- area and the Belt Mountains is correlated with beds crop out northeast of Pine Creek, and about ~ - -- Prichard strata by M. R. Mudge (personal com 11,500 feet of Ravalli was graphically scaled --- munication) . where the formation crosses Seventeenmile Creek. In the Thompson Lakes area the Ravalli is 7,070 feet thick. A 15,000-foot section south RAV ALL! GROUP-WESTERN PHASE east of Warland is not representative; it results 1=--_._- Walcott (1906, p. 7) first used the term either from local thickening or from fault repe ----- tition. To the north (north Purcell Mountains) the "Ravalli Series" for a thick sequence of purple, ---- gray, and greenish-gray beds underlying his Black Ravalli Group thins to about 4,500 feet thick. In ------foot Series in the Mission and Swan Ranges and Pleasant Valley, the Ravalli Formation JS about _._ -- adjacent areas. The Ravalli was named for the 10,000 feet thick, east of Little Bitterroot Lake ------town of Ravalli, where the rocks crop out in hills between 9,000 and 11,500 feet crop out. On Elk ---_. -_.. - along the Jocko River in southern Lake County Mountain, Richard Galster measured about 8,400 ------feet of beds that he assigned to the Ravalli Group. (Wilmarth, 1938). Calkins and MacDonald (1909, _._-- --- 75ft. ARGILLITE: Gray, weathers blue gray green, p. 37) used the term "Ravalli Group" to desig (Fig. 7). --- sandy in part, some light-gray quartzite. nate a thick sequence of light-colored rocks rang Within central and western Lincoln County - -- ing in composition from pure white quartzite to the Ravalli Group is medium-gray, bluish-gray, ----.- siliceous argillite containing some gray, green, and white quartzite, argillite, and quartzitic ar purple, and red beds. This group underlies the gillite. In north-central Lincoln County and west A Wallace and overlies the Prichard Formation in ern Flathead County light-colored strata are most Figure 7.-Ravalli Group undifferentiated, section on Elk Mountain in sec. 10 and 16, T. 31 N., R. 26 W. (After Galster, their map area, which includes the Purcell, Cabi- common, but green-gray and medium-dark-gray .. U. S. Army Corps of Engineers) . 32 GEOLOGY AND MINERAL DEPOSITS, LINCOLN AND FLATHEAD COUNTIES STRATIGRAPHY-PIEGAN GROUP (BELT)- WESTERN PHASE 25
thickness are exposed. The Libby is the upper MISSOULA GROUP-NORTH-CENTRAL PHASE gray coarse-grained argillite, grayish-red-purple and he placed an overlying one (P3 unit in this most formation of the Belt Series in southwestern fine-grained mud-cracked argillite, interlaminated report) in the Missoula Group. In this report the Lincoln County, and the top has been removed PURCELL LAVA greenish-gray and blue-green and grayish-red western phase of the Piegan Group is represented by erosion. The Purcell Lava was recognized by Willis purple argillite, interbedded with light-greenish by the Wallace Formation, as defined by Gibson (1902), Daly (1905), and others as a marker gray and greenish-gray siltstone. Light-gray fine in the Libby quadrangle (1948, p. 13), but the The Libby Formation, which overlies the to medium-grained quartzite and quartzitic sand eastern phase includes three mappable units, the Striped Peak Formation, is likewise exposed in the unit as early as the beginning of the century. It was named the Purcell Lava by Daly (1912, p. stone are interbedded with the argillite (Barnes, Siyeh Limestone and the transitional beds below trough of the syncline that crosses the Kootenai 1963, p. 11). and above the Siyeh. River west of Libby, and the outcrop extends 207). Clapp (1932) identified the flow as basalt. In northern Lincoln and Flathead Counties the In the Mission and Swan Ranges, the Grinnell north into the Quartz Creek-China Mountain area WALLACE FORMATION northwest of Libby. A belt of Libby rocks extend lava extends eastward from long 115° 31' W. to is predominantly gray-red-purple to grayish-red ing northwestward from the upper reaches of the Glacier National Park; it does not extend south argillite containing intercalated beds of calcareous Ransome (1905, p. 282) named the Wallace Vermilion River in northern Sanders County dis beyond the Whitefish Range or the central part grayish-green argillite. Thin beds of coarse-grained Formation for exposures near Wallace, Idaho, and appears beneath valley fill in Swamp Creek and of Glacier Park. Where exposed northwest of white feldspathic quartzite are intercalated with described it as compriSing three parts, a lower Lake Creek valleys. Rexford it consists of two distinct groups of argillite in the upper part of the Grinnell Forma gray-green calcareous argillite, a middle sequence flows separated by as much as 100 feet of gray tion. of ferruginous blue-green argillite, limestone, and Gibson (1948, p. 18) reported at least 6,000 and green quartzitic argillite (Johns, 1962b, p. The Grinnell grades through a transitional light-gray and white calcareous quartzite, and an feet of Libby rocks exposed in the syncline cross 190). In parts of British Columbia the value of zone several hundred feet thick into the overly upper laminated blue-gray, gray-green, and white ing the Kootenai River west of Libby. In the the Purcell Lava as a marker is questionable be ing P, unit, but on the basis of weathered sur argillite. The upper unit is fairly homogeneous and Thompson Lakes region the thickness was graph cause of possible confusion with any of several faces, sedimentary structures, and color, the con calcareous. ically scaled as 7,600 feet. Within the syncline other flows higher in the sequence. tact can be placed within a "few tens of feet" in Gibson (1948, p. 14) described the Wallace at the head of Vermilion River the exposed part Near long 115° 31' and just south of the the eastern part of the Whitefish Range (Barnes in the Libby area as the most heterogeneous for of the Libby is about 5,000 feet thick. In the Canadian boundary Purcell Lava crops out in a 1963, p. 17). mation of the Belt Series. There the main con Pend Oreille area, where the Libby is about 1,800 broad U-shaped pattern; several fingers extend stituents are calcareous or dolomitic argillite and feet thick, Harrison and Jobin (1963) divided it The conformable contact between the Grin southward in the vicinity of Robinson Mountain. shale, but the formation contains also some sand into three members. nell Formation and the overlying p] unit is drawn From Rexford to Eureka and beyond, faulted where predominantly gray-red and gray-red-pur stone and small amounts of dolomite and dolomi In the type area, the most common rocks in Purcell Lava crops out in an arc, the lava over pIe argillite is overlain by predominantly gray tic limestone. The upper third of the formation is the Libby Formation are dark- and light-gray and lying P 3 strata and underlying the Shepard For green, pale-blue-green, and pale-green slightly sandy sericitic and ferruginous shale a few hun greenish-gray argillite in beds 1 to 3 feet thick. mation. Within the Whitefish Range, the lava calcareous argillite. dred to a thousand feet thick. This part resembles is thickest in the north-central part, thinning the overlying Striped Peak rocks but can be dis The rocks are somewhat sandy, sericitic, and Ravalli rocks are believed to be correlative calcareous, and some dark-gray ferruginous and southeastward and disappearing near the junction tinguished from them by a less consistent lith of Canyon and Kimmerly Creeks (lat 48° 30' 30"). with the Appekunny and Grinnell Formations of ology. oolitic magnesian limestone is locally present. eastern Flathead County, with the Creston Forma Farther southeast, in the area west of Thompson Sheldon (1961, p. 46) described the Purcell tion of southeastern British Columbia, and with In western Lincoln County, basal Wallace Lakes, yellowish tints appear, and some yellow Lava northwest of Rexford as comprising a lower the Spokane and Empire Formations in the Lewis rocks include argillite interbedded with quartzite brown and grayish-yellow impure limestone is flow of basic rock 450 feet thick and an upper and Clark Range. On the basis of lithologic grading upward into argillite containing subordi interlayered in the sequence. At the headwaters flow of acidic rock not exceeding 100 feet thick, descriptions and stratigraphic position, the Burke nate limestone and dolomitic limestone. Locally, a of the East Fisher River near Miller Lake, Libby separated by thin-bedded greenish-gray argillite Revett and St. Regis Formations of the St. Regis thick sequence of magnesian limestone near the rocks exposed on the east limb of a north-plunging and quartzite 75 feet thick. Superior area and the Burke, Revett, and St. Regis center of the formation is exposed on Yaak syncline consist of dark- to light-green, gray Between Rexford and Eureka the Purcell Formations of the Pend Oreille area are equated Mountain. Upper Wallace beds are characterized black, yellowish-brown, grayish-brown, and yel Lava (Sommers, 1961, p. 24) is 700 feet thick and with Ravalli rocks in Lincoln and Flathead Coun by sericitic argillite and limestone; minor quartzite lowish-gray argillite. Some strata contain profuse consists of two flow units separated by a I-foot ties, but the Burke and St. Regis lithologies were is local. Algal zones and molar-tooth structures rodlike to irregular molds or casts of concre zone of argillite. On the Burma road northeast of not recognized as distinct mappable units in Flat are associated with the Wallace. Secondary cubic tions (?) on bedding planes. Eureka, an agglomerate of Purcell pebbles over head and Lincoln Counties. pyrite and limonite pseudomorphs after pyrite are common in the more calcareous parts of the for The Libby Formation of southwestern Lincoln lies the basalt flow. PIEGAN GROUP-WESTERN PHASE mation. County and the Pend Oreille area is believed to be In the northern part of the Whitefish Range a thickness of 450 feet is reported for the lavas on Fenton and Fenton (1937, p. 1890) proposed Wallace strata are extensively exposed correlative with most of the Roosville Formation throughout western Lincoln County (PI. 1) where of the Whitefish Range, and it has been pre Mount Ksanka and Green Mountain, 300 feet on the name Pi egan Group for a sequence of lime Poorman Mountain, and 390 feet on Bluesky stone, dolomite, and dominantly argillaceous clas they conformably overlie the Ravalli and underlie viously correlated with the Bouchard Formation Ridge (Smith, 1963, p. 95). South of Yakinikak tic rocks lying between the Missoula and Ravalli the Striped Peak Formation. Southwest of Troy, of the St. Regis-Superior area, which in turn is Creek, Barnes (1963, p. 28) reported a thickness of Groups, as redefined by them, and designated a upper Wallace strata are in fault contact with believed to correlate with the Garnet Range For 400 to 500 feet and he found no evidence of dis type locality at Piegan Mountain in Glacier Park. Prichard strata along the Leonia fault. mation of Nelson and Dobell (1961). The Sloway conformity at the base of the flow. In most places The thickness was stated to be 2,780 to 14,300 Several northwest-trending belts of Wallace is described as having no stratigraphic equivalent the underlying argillite (Piegan P3 unit) is altered feet. Ross (1959, p. 33) redefined the group to rocks are exposed south and southwest of Libby to a depth of 6 inches or less, but Sommers (1961, include only the Siyeh Limestone. He retained and northwest of Thompson Lakes. Lower and in the Libby, Pend Oreille, and Trout Creek areas; p.24) described a lO-inch baked zone of laminated the group name to facilitate broad correlation of middle Wallace rocks west of a line just east of its absence is attributed to nondeposition or to greenish-gray argillite that seems to have been this group, which is characterized by its carbonate Mt. Henry are not subdivided, but farther southeast removel by erosion (Harrison and Campbell, crumpled by the lava, attesting to the unconsoli content. Ross assigned an underlying transition the equivalent strata can be subdivided into the 1963). dated nature of the sediment at the time of ex- , zone (PI unit in this report) to the Ravalli Group, three units of the eastern phase of the Piegan 2G GEOLOGY AND MINERAL DEPOSITS, LINCOLN AND FLATHEAD COUNTlES STRATIGRAPHY-MlSSOULA GROUP (BELT)-WESTERN PHASE 3l
Group. The western phase of the Piegan Group yellowish-green and blue-gray carbonate-bearing, MISSOULA GROUP-WESTERN PHASE and was graphically scaled to be 2,100 feet near (Wallace Formation) merges into the eastern laminated, thin- and medium-bedded argillite, The name Missoula Group was first used by Willow Creek, a tributary of Vermilion River, in phase (PI> Siyeh, and Pg ) along a line trending siltstone, and subordinate gray and white quart Clapp and Deiss (1931, p. 677) for a sequence northern Sanders County. A thickness of 3,000 southeast from the International Border at long zite, more sandy and quartzose near the base. of argillite and quartzite 18,000 feet thick over feet or more was graphically scaled for a com 0 115 35' through Mt. Henry, Lawrence Mountain, Middle Wallace rocks are thin- and medium-bed lying the Wallace Limestone in the Missoula plete section of the formation across Squaw Hornet Ridge, and Meadow Peak, then south to ded dark- and light-gray, green, yellow-green, and Helena region. As originally defined the group Creek. the Thompson River. blue-green sandy and calcareous argillite and silt included all Belt rocks above the Wallace Forma Striped Peak rocks in the map area are dis The thickness of the Wallace Formation stone interlayered with dark- and medium-gray tion. tinctive red and gray quartzite, indurated sand and blue-gray medium- and thick-bedded, pyritic ranges from about 8,000 to 15,000 feet or pos In western Lincoln County (western phase stone, quartzitic argillite, and shaly argillite. Beds sibly more. Gibson (1948, p. 13) reported a thick impure magnesian limestone. Molar-tooth and are sericitic and ferruginous. Ripple marks, mud segregation structure is conspicuous in the car of Missoula Group) the Striped Peak and Libby ness of 12,000 feet on Mount Berray and 16,000 Formations make up the Missoula Group. In cracks, and crossbedding are well developed feet on Snowshoe Peak, and suggested that the bonate zones. Upper Wallace strata are dark- to throughout the formation. The quartzose rocks light-green and gray, blue-gray, and yellow-green northeastern Lincoln County and northern Flat unit may be as much as 17,000 feet thick in the head County all units from the Purcell Lava up are medium to coarse grained. The formation is vicinity of Mount Sheldon. non-calcareous and calcareous laminated to med easily recognizable because of its distinctive red ium-bedded argillite, siltstone, and quartzite inter ward are included in the north-central phase of Southwest of Troy, Dahlem (1959, p. 37) color and the striking surface luster produced by bedded with gray and white carbonate rocks con the Missoula Group. In southeastern Flathead observed at least 10,000 feet of Wallace, and far light reflecting from sericite flakes on bedding taming white stromatolite zones 1 to 10 feet thick. County, where the Purcell Lava is absent, the ther north a graphically scaled section along planes. In the Fisher River-Thompson Lakes area Locally, greenish-gray laminated argillite and in basal Missoula unit is the Snowslip Formation, Seventeenmile Creek measured about 14,000 feet the lower part of the Striped Peak is almost all terbedded sequences of grayish-red argillite and and the upper Missoula unit underlying Middle thick. South of the International Boundary a quartzite, but to the north the beds gradually quartzite in these upper Wallace beds could be Cambrian rocks is the McNamara Formation. graphically scaled incomplete section was about change to argillite and shaly argillite interbedded confused with Striped Peak rocks. East of Pipe .. 11,000 feet thick. STRIPED PEAK FORMATION with only minor quartzite. Near the head of Ver Creek Divide near Flatiron Mountain a sequence milion River the Striped Peak Formation is red Graphically measured sections in the Thomp of alternating beds of light-gray and white dolo The Striped Peak Formation was named by dish quartzite and argillite and gray silty and son Lakes area ranged from 11,000 to as much mitic limestone, crossbedded light-gray quartzite, Calkins (Ransome and Calkins, 1908, p. 25) for quartzose argillite. Interbeds of red and grayish as 14,000 feet in thickness. Beer (1960, p. 31) and dark-gray dolomitic limestone and argillite in a mountain of that name in the Coeur d'Alene green laminated quartzitic argillite are common reported an incomplete section (measured several beds ranging from a few inches to 2 feet thick is region in Shoshone County, Idaho. Striped Peak near the center of the unit. Near Willow Creek on miles southwest of the Kootenai River-Fisher conspicuous. Wallace argillite in places contains beds in the Coeur d'Alene region are composed Vermilion River several thin stromatolitic lime River junction) that amounted to 15,050 feet in abundant sericite commonly aligned parallel to of ripple-marked and mud-cracked red, purple, stone beds crop out near the base of the unit. Salt thickness. stratification (Dahlem, 1959) . and green shale and quartzitic sandstone. crystal casts were observed at this locality in the Bedding plane faults and other, discordant Gibson and others traced the formation from argillitic beds a few hundred feet above the base. faults, some too small to plot on the scale used the Coeur d'Alene region to the Libby quad PIEGAN GROUP-EASTERN PHASE for mapping, occur throughout the unit. Repetition rangle in southwestern Lincoln County where its The contact between the Striped Peak and Libby Formations is gradational and was placed of beds by faulting and also overthickening by The Pi egan Group is separable into three thickness ranges between 2,000 and 2,500 feet. folding may exaggerate calculated thicknesses of The formation there is described (Gibson, 1948, where predominantly red and gray quartzite and units traceable through eastern Lincoln, Flathead, argillite are overlain by predominantly laminated the Wallace, but the average thickness is believed and northern Lake Counties. The lower and upper p. 16) as thin-bedded ferruginous and sericitic !dark-red to purplish sandstone and quartzite grayish-green quartzitic argillite or gray, green, to be about 12,000 feet. units are designated P and P g , respectively, and 1 locally containing some stromatolitic dolomite as brown, and yellow ferruginous and locally slightly Lithology of the Wallace Formation is hetero the middle unit is mappable as the Siyeh Forma calcareous sandy argillite of the Libby Formation. geneous multicolored rocks exhibiting subtle to tion. sociated with oolitic dolomite and sandy dolo marked change in mineral composition and tex mitic limestone. Gibson reported crossbedding, Striped Peak rocks of western Lincoln County ture. In the Libby area the most abundant rock is LOWER PIEGAN (PI UNIT) ripple marks, and sun cracks as being common and the Pend Oreille region are tentatively re green-gray calcareous and sandy argillite and throughout the formation. garded as equivalent to the Spruce Formation and siltstone containing dark and light-colored lami The basal member of the Piegan Group (P,) Lupine Quartzite of the St. Regis-Superior area was first mapped and described by Shelden (1961) In the western part of Lincoln County, Striped nae; some of the rock breaks across planes of Peak beds are exposed on both limbs of a syncline and are believed to be correlative with the in the Ural area. It is believed to be equivalent stratification. Less abundant gray and brown that crosses the Kootenai River between Libby Shepard, Kintla, and Phillips Formations of the to the lower part of the Wallace Formation. quartzite and sandstone are interbedded with argil and Troy. Outcrops of Striped Peak strata con Whitefish Range. The Phillips Formation has been lite. Lenticular impure dark- and light-gray lime Northwest-trending belts and irregular out tinue north, and in the Quartz Creek-China Moun correlated (Smith, 1963, p. 108) with the Red stone, magnesian limestone, and white impure crops of lower Piegan rocks are widespread tain area north of Kootenai Falls they merge Plume Quartzite of the Marias Pass area. dolomite commonly exhibiting segregation and throughout eastern Lincoln County and Flathead around the nose of the syncline. Striped Peak molar-tooth structures are characteristic of middle County. In the Whitefish Range, only incomplete strata are also exposed along Vermilion River in UBBY FORMATION Wallace strata but are not confined to it. Upper sections of the P 1 unit are exposed in a complexly northern Sanders County and northwestward Gibson (1948, p. 17) applied the name Libby Wallace beds consist of gray and green calcareous faulted area. across Fisher River (Thompson Lakes area) but Formation to a sequence of light- to dark-gray and mottled argillite and siltstone, lenticular gray Thickness of the lower Piegan (P1 unit) they disappear beneath glacial gravel and silt de argillite cropping out in the vicinity of Libby. A impure limestone, and local sequences of gray ranges from about 1,200 feet to about 3,500 feet. posited in the valleys of Swamp Creek and Libby typical section of the formation is exposed south red argillite and quartzite interbedded with green In north-central Lincoln County, Shelden (1961, Creek. A faulted segment of Striped Peak rocks east of Flagstaff Mountain south of the Kootenai gray argillite. p. 19) and Sommers (1961, p. 21) reported a thick extends across the lower reach of Wolf Creek River near Libby, where dark- to light-gray and Elsewhere in western Lincoln County (Troy, ness of 2,000 feet for the formation. In the White to a point near the mouth of Fisher River. olive-green argillite, sericitic argillite, calcareous Yaak, Rainy Creek, and Thompson Lakes areas) fish Range, Smith and Barnes reported a thickness Thickness of the Striped Peak Formation argillite, sandstone, and minor amounts of shale lower Wallace strata are dark- to light-gray, green, of 1,400 feet. , ranges from 2,000 to 2,500 feet in the Libby area and magnesian limestone totaling 2,250 feet in 30 G EOLOGY AND MINERAL DEPOSITS, LINCOLN AND FLATHEAD COUNTIES STRATIGRAPHY-PIEGAN GROUP \BELT)-EASTERN PHASE 27
of upper strata of the Wallace Formation in the Whitefish Range the P3 unit is characterized by To the south the lower Pi egan unit thickens the lower Piegan unit. Near Blacktail Mountain Libby area. The Snowslip of Childers is believed laminated and thin-bedded strata, green and gray to 2,700 feet in the Stryker area (Gilmour, 1964, south of Kalispell the thickness was graphically to be absent in the Whitefish Range. color, its argillitic and quartzose composition and p. 34) and 2,500 to 3,500 feet in the Pleasant scaled to be about 3,150 feet. Page (Johns, 1964, grain size, and its sporadic carbonate content. Valley area (Johns, 1962, p. 14; Hall, 1962, p. 32). p 21) reported that the unit i~ 2,400 feet thick The P:; unit is exposed in the north-central Galster (Pl section, Fig. 8) measured 2,320 feet of in the Mission Range and thins eastward Salish Mountains conformably overlying the Siyeh In the northeast part of the Whitefish Range, to about 1,800 feet in the Swan Range. Formation and underlying the Purcell Lava. The Barnes (1963, p. 25) described the lithology as AMN·- 1,100 ft. LIMESTONE: Gray, weathers red, orange, Harrison and others (1969) reported a unit occupies the same stratigraphic position in fine-grained argillite of various pale tints of green yellOW, and light gray. Contains rusty pyrite thickness of 800 feet for the lower unit in the the Whitefish Range. In the north-central Swan interlaminated with coarse-grained argillite throughout Characteristic "birdseye" weather Helena Formation in the central part of the Mis Range it is between the Siyeh and Snowslip of pale tints of gray and green. Very fine grained ing. sion Range. At Badrock Canyon east of Columbia i Formations. Along the west flank of the Rocky quartzite forms rare discontinuous cross-laminated i Falls the lower Piegan unit was graphically scaled Mountain Trench, from a point just south of lenses. Most of the argillite is calcareous, especial as about 1,200 feet thick. Eureka to a point beyond Kalispell, P3 strata have ly the coarser layers, and cementation may be Lower Piegan strata are light-greenish-gray been removed by erosion. Thickness of P3 strata nearly contemporaneous with clastic deposition, as 1 and green-gray fine-grained argillite interlami ranges from 250 feet in the northwest part of the a few fragments of calcareous fine- to medium nated with subordinate light-gray coarse-grained Whitefish Range to about 2,400 feet in the area grained sandstone were found imbedded in coarse, argillite, producing a banded appearance. Light west and southwest of Rexford. In the northwest cross-laminated argillite. Some of the argillite is greenish-gray, greenish-white, and greenish-yel part of the Whitefish Range, the unit (P3 ) ranges feldspathic, but most contains only minor feldspar. low coarse-grained argillite is interbedded with from 250 to 500 feet thick, but it thickens to the Muscovite and chlorite make up as much as 25 finer grained argillite, but makes up only a minor southeast and is 2,100 feet thick in the southern percent of the rock. Near the top, the unit contains part of the unit. A few thin gray limestone beds part of the Whitefish Range (Smith, 1963, p. 92). more dusky-red and grayish-red coarse- to fine In the northeast part of the Whitefish Range the grained argillite than greenish-gray argillite; there TOP ERODED unit is 1,000 to 1,200 feet thick (Barnes, 1963, p. fore, where the lavas are absent, it may be difficult to place the upper contact. It may be arbitrarily 1,000 ft. LIl\IESTONE: Blue gray, "molar tooth" 25). In the north-central Salish Mountains a thick and CALCAREOUS ARGILLITE: Blue gray, ness of 2,400 feet is reported for the formation placed where the argillite grades upward into the : minor noncalcareous blue·gray argillite. (Shelden, 1961, p. 25). In Badrock Canyon at the yellow-brown-weathering dolomitic rocks of the north end of the Swan Range these strata are Shepard Formation. ~, i estimated to be 1,800 to 2,000 feet thick (Johns The Pa unit in the north-central part of the and others, 1963, p. 27). On the Swan divide east E Swan Range overlies the Siyeh Formation and lL of Swan Lake the P 3 unit is about 1,000 feet thick. underlies the Snowslip. The formation consists , J::. ! I I Q) of pale-green and grayish-green thin-bedded and '~ l · ~ The upper Pi egan unit in the north-central i-LT-'-,..-'- i U1 Salish Mountains consists of homogeneous thin laminated argillite, calcareous argillite, and im 1 I I and medium-bedded, greenish-gray and light-gray pure limestone. Mud cracks and mud breccia are : 0:1; 180 ft. CALCAREOUS ARGILLITE: Green. quartzose argillite and quartzite overlain by the moderately abundant. These rocks weather gray Purcell Lava. About 200 feet below the base of ish orange and yellow brown. Locally, weathering .. ; : l ::l has produced solution holes, which give the sur Interbedded with calcareous green argillite in lower the flow, two 8-foot limestone beds are inter 200 ft. and predominating at base. Lower COll calated with the quartzite (Shelden, 1961, p. 25). face outcrop a pocky appearance. tact gradational. II~
On the northeast flank of the Salish Moun The Wallace Formation of western Lincoln ,~ "T 560 ft. LIMESTONE and CALCAREOUS ARGIL LITE: Bluish gray, weathers light gray; inter tains from Rexford to Eureka and continuing County and of areas farther south and west is ! ' bedded with noncalcareous argillite in upper southeast, P3 strata were described by Sommers believed to be equivalent to the three units of the part. (1961, p. 21) as light- to medium-dark-greenish Pi egan Group in eastern Lincoln County and I~- 310 ft. ARGILLITE: Calcareous and noncalcareous, i light to daIk green, weathen pale green, string gray thin-bedded non calcareous argillite lacking Flathead County; to the Helena Formation of the en of Iight·gray limestone. .;'1~li" LIMESTONE;: Bluish gray, dense, pyrite in lower molar-tooth structure. He described beds near Mission Mountains in Missoula County; to the Middle part laminated and contains profuse part. the Purcell Lava as consisting of dark-greenish p8 cg, Siyeh, and p8 ca units (Ross, 1959) in the mud cracks. Flathead region and Glacier National Park; and gray argillite in alternating fine dark and light I-~ layers. About 500 feet below the base of the lava the Kitchener-Siyeh Formation of southeastern the sequence contains structures indicating pos British Columbia. The Pi egan Group is correlated ~ sible channels. with the Siyeh Formation in the Marias Pass area, as the term "Siyeh" was used by Childers (1963). -= In the northwest part of the Whitefish Range ~ rocks in similar stratigraphic position are greenish The gross lithology and sedimentary features 170 ft. CALCAREOUS ARGILLITE and LIME· gray and light-green thin-bedded and laminated of the Piegan Group, and to a lesser extent the STONE: Greenish gray, slight "birdseye" weath ering. 'I'=rc siltstone and fine-grained quartzite, locally cal Wallace Formation, suggest a complete cycle of I \ I careous and dolomitic. Mud cracks, ripple marks, sedimentation commencing with deposition of I I and pyrite cubes as large as 5 mm are sporadically fine-grained calcareous argillite (PI), deposition ->----r--'"" distributed throughout the unit. East of Blue Sky of carbonate-bearing strata (Siyeh), and a return ~ - Creek some red siltstone and quartzite are in to deposition of fine-grained calcareous and sili Figure S.-Lower Piegan (P,) and part of Siyeh Formation, section on east slope of Elk Mountain, in sec. 2 and 10, cluded in the upper part, but throughout the ceous argillite (P3 ). • T. 31 N., R. 26 W. (After Galster, U. S. Army Corps of Engineers) •