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Главная » 2013 » Ноябрь » 27 » Glacier NP (p6)
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Glacier NP (p6)


Lower part of the Missoula group on the ridge between the North Fork of Belly River and Mokowanis River



[From field notes by Eugene Stebinger and H. R. Bennett, Sept. 20, 1914]





























Feet

Purcell basalt.

Hornfels, deep-greenish, siliceous. A baked mud with contorted structure faintly visible, suggesting that the mud was disturbed by the passage over it of the lava that overlies it, hence the eruption of the lava took place under water10

Shale, green-gray to gray, calcareous 45

Shale, deep-maroon, somewhat calcareous ripple-marked, mud-cracked; with a few quartzite beds132

Shale, gray to dark-green-gray, calcareous; weathers buff147

Limestone, shaly, and shale, slabby gray to green-gray, thin-bedded; weathers buff; contains stromatolites158

Shale, deep-red, ripple-marked, mud-cracked59

Limestone, shaly, and shale, slabby gray to green-gray, thin-bedded; weathers buff; contains stromatolites225

Shale, light- to deep-maroon; with many ripple marks and mud cracks; contains stromatolites88

   Total



864



In the lower part of the Missoula group near Logan Pass, Rezak found thin and individually non-persistent biostromes characterized by Collenia undosa Walcott. They are especially well developed near the east base of Mount Oberlin where eight biostromes of Collenia undosa with occasional heads of Cryptozoon occidentale and Collenia symmetrica were noted. Each biostrome is associated with alternating layers of green argillite and pink limestone. Similar biostromes are exposed 0.2 mile east of the loop on the Garden Wall, 0.6 mile east of the lower end of Lake McDonald and along U. S. Highway No. 2, 1.2 miles south of Walton. Biostromes that are probably to be correlated with these are present along the railroad west of the mouth of Coal Creek (fig. 15), along the Garden Wall near Granite Park, and on the northwestern slope of West Flattop Mountain. Similar forms are exposed in scattered outcrops along the ridge crest south of Baldhead Mountain. All these appear to be below the horizon of the Shepard formation although that unit is not mapped in their vicinity. Presumably they correspond to the stromatolites recorded by Stebinger and Bennett in the section given above.



The part of the main body of the Missoula group above the Shepard formation that remains uneroded in the northern part of Glacier National Park appears to be less than 1,000 feet thick and has been little studied. This is the unit which Willis (1902, p. 316, 324) called the Kintla argillite. The description below is adapted from one by Daly (1912, p. 81-83) and includes data from Willis' report. The name was originally taken from exposures north of the international boundary northeast of Upper Kintla Lake, Mont. The rocks are deep-red argillaceous quartzite and siliceous fissile shale or argillite with some white quartzite and occasional calcareous beds. Casts of salt crystals, ripple marks, and mud cracks are abundant. Some beds in the lower 60 feet at the head of Kintla Creek, in Canada, are lithologically identical with those characteristic of the Shepard formation. Above these is a 40-foot lava flow, which Daly says is lithologically like the Purcell lava and the flow in the Shepard formation but which is much less persistent than the Purcell lava itself. Evidently Daly used the name Purcell only for the thicker and more persistent masses of lava, but it has come to be applied to all flows of similar composition and of approximately similar stratigraphic position. The argillite contains angular grains of clear quartz, fresh microcline and microperthite, cloudy orthoclase, and a little plagioclase in an argillaceous cement containing much hematite. The section below represents, according to Daly, the Kintla formation at the type locality.



Columnar section of Kintla formation



[Quoted from R. A. Daly, 1912, p. 82]





















Feet

Top, erosion-surface.

Argillite relatively homogenous, thin-bedded, bright-red, purplish- and brownish-red, and subordinate quartzitic sandstone460

Argillite (dominant), heterogenous, thin-bedded, red, and sandstone, gray and brownish sandstone, magnesian oolitic limestone, and gray concretionary limestone300

Amygdaloid40

Argillite, thin-bedded, red, with thin intercalations of magnesian quartzite60









860



The Fentons (1937, p. 1901) give a somewhat similar composite section for the Kintla formation at and near the type locality, quoted as follows:

























Feet

Base, conformable top of Sheppard formation.

Argillite, thin-bedded and homogenous, bright-red to purplish- or brownish-red; minor beds of quartzite and sandstone440-460

Argillite, thin-bedded, bright-red to brownish, interbedded with red, brown, or brownish-gray sandstone, the latter increasing northward175-200

Limestone, gray to pinkish-gray; weathers pink. Bears two algal bioherms composed of Collenia clappii n. sp20-30

Argillite, thin-bedded, bright-red, with layers of argillaceous red sandstone95-100

Lava, amygdaloidal, green to purplish40-50

Argillite, thin-bedded, bright-red, with dull-red magnesian quartzites60









830-900



The descriptions given above taken in connection with plates 1 and 2 show that most of the main body of the Missoula group has been eroded from Glacier National Park. Even in the Flathead region there are few places where Paleozoic beds remain resting on the Missoula. None of the sections measured represent more than a fraction of the original thickness of the group. Inspection of plate 2 indicates that the total thickness of the group is surely over 10,000 feet and may well be nearly twice that figure. Thus the aggregate thickness of the main body and of units mapped separately in the Flathead region is probably fully as great as the 18,000 estimated by Clapp and Deiss (1931, p. 677) for the type locality of the group.



Greenish Calcareous Argillite



The foregoing descriptions apply to those parts of the thick Missoula group that have not been separated as yet into formations or other subdivisions. The components of the group differ from place to place. In the Flathead region a nearly continuous basal zone, transitional with the Siyeh limestone below, is termed "greenish calcareous argillite." Similar rocks are present locally in Glacier National Park but have not been mapped separately there. The lithologic designation chosen for the unit reflects the fact that much of it reacts readily to dilute acid. However, the carbonate-rich beds in it, as in other components of the Belt series, do contain significant quantities of magnesian carbonate. Those parts of the unit that are especially rich in carbonate resemble the Siyeh limestone in appearance. It is correlated with the Missoula rather than the Piegan group because it is dominantly argillaceous and many of its beds are lithologically identical with some of those high in the Missoula group. Nearly everywhere there is a sharper change in lithologic character at the base than at the top of the greenish calcareous argillite. In places, in fact, the upper boundary is necessarily somewhat arbitrary as green beds are interbedded with the reddish ones higher in the sequence.



The unit is exposed on both sides of Quintonkon and Wheeler Creeks and on and northwest of Pioneer Ridge. It is thus nearly continuous at the base of the Missoula group in the Swan Range. Even in the valley of Graves Creek, where it is not mapped, some beds may be present. In the Flathead Range it has been mapped only in the vicinity of Sheep Creek and opposite the mouth of Harrison Creek. Some green beds are present near the base of the Missoula between these localities, and more detailed study might result in mapping some of the basal unit here.



The basal green calcareous argillite is well displayed in the Belton Hills but has not been traced farther north in Glacier National Park. It was not recognized by members of Campbell's parties and was not sought in the northern part of the park during the fieldwork in 1950. Such data as are at hand indicate that in most places north of the latitude of the Belton Hills it is thin or absent. Calcareous green rocks that might be correlated with it are present near the base of the slopes at the south end of Flattop Mountain, but the three measured sections of the lower part of the Missoula group include relatively few beds that resemble it. Where well-displayed in the Flathead region, the greenish calcareous argillite is estimated to be 500-800 feet thick, but in many localities its thickness is much less. It seems probable that there are few places in the northern part of the park where its thickness exceeds 100 feet.



Purcell Basalt



The name "Purcell lava" was originated by Daly (1912, p. 161-163, 207-220) for a large body of lava which he regarded as a persistent horizon marker. He says that the lava has been found in the vicinity of the international boundary from the border of the Great Plains to the eastern summits of the Purcell Range. He places the Purcell lava immediately above his Siyeh formation and units that he equated with that formation. As the composition appears to be that of an altered basalt, the unit is best termed "Purcell basalt" (Wilmarth, 1938, p. 1746).



Daly remarks that "magnesian strata characterized by the peculiar molar tooth structure" become prominent "at a horizon about a thousand feet or more below the Purcell Lava." Because the top of the Siyeh limestone, as here restricted, is in places almost as far as that below the base of the lava, the correspondence is reasonably close. As noted above, Daly apparently intended to exclude such minor flows as those in Glacier National Park from his Purcell lava, but common usage has extended the name to include the minor flows of similar composition and approximately similar stratigraphic position. Nevertheless, it should be noted that the presence of lava that looks like the Purcell basalt is not in itself sufficient for stratigraphic correlation of associated beds. In the park, most of the lava is immediately below the Shepard formation, but in the vicinity of Boulder Peak, some is well above that formation, and the small masses west of the Flathead River may be even higher stratigraphically. On the other hand, similar lava is reported (Fenton and Fenton, 1937, p. 1887-1888) to be intercalated in the Grinnell argillite in Waterton Lakes Park, Canada. This lava is not regarded by the Fentons as belonging to the Purcell basalt, and its exact character is not recorded. There appears, nevertheless, to be sufficient resemblance so that confusion would be possible in localities where stratigraphic relations are not otherwise clear.



Lava flows correlated with the Purcell basalt are exposed at Granite Park, around the periphery of Flattop Mountain, and thence at intervals northward to the Canadian border. They are present on Cathedral Peak, Mount Cleveland, Porcupine Ridge, near Kootenai Peak, near Brown Pass, Boulder Peak, and north of Upper Kintla Lake. On Boulder Peak the flows are at two horizons, separated by argillaceous and calcareous beds. Two small exposures of similar rock have been recorded on the west side of the Flathead River west of the Apgar Mountains.



In Glacier National Park the lava is an irregularly amygdaloidal rock most of which has conspicuous pillow structure. The color on fresh fracture is greenish gray to almost black, locally somewhat purplish. Weathered surfaces are stained brownish. According to the Fentons (1937, p. 1903-1904), in the vicinity of Granite Park the formation includes two major flows—





"the first 30-42 feet in thickness, the second 18 feet thick. The basal 20-25 feet of the lower flow contains ellipsoidal pillows, 10-25 inches in diameter, separated by cherty inclusions; the lavas surround detached masses of modified argillite 2-12 feet thick, the base being irregular. The upper 10-17 feet is a massive, ropy lava. The second main flow is massive and amygdaloidal, containing mud inclusions, steam tubes, and irregular cavities—evidences of subaqueous extrusion."



Near Fifty Mountain Camp the pillows are especially conspicuous and range up to several feet in maximum diameter. The Fentons (1937, p. 1903-1904) speak of 150 feet of lava on Mount Kipp, 175 feet on Cathedral Peak, 200-220 feet in Hole-in-the-Wall Basin, and 275 feet of lava west of Boulder Pass. Above Pocket Lake on Boulder Peak, they counted 8 flows in the upper 100 feet of the lava.



Daly (1912, p. 207-220) says that the lava is much altered but evidently a basalt. In the outcrops in Canada that he saw, much of the rock is a devitrified glass with numerous feldspar phenocrysts. It contains labradorite and much chlorite, leucoxene and an undetermined micaceous mineral. The analysis below is from the freshest of the specimens collected during Daly's investigation. It comes from a thick body of lava in the McGillivray Range about a mile south of the international boundary. This range is west of the Kootenai River and therefore much west of Glacier National Park. Analyses of basalt from Glacier National Park are not at hand, but they would probably be broadly similar to this one.



Purcell basalt



[From R. A. Daly, 1912, p. 209, analyst Prof. M. Dittrich]

































Silica (SiO2)41.50

Titanium oxide (TiO2)3.33

Alumina (Al2O3)17.09

Ferric oxide (Fe2O3)3.31

Ferrous oxide (FeO)10.08

Manganese oxide (MnO)Trace

Magnesia (MgO)12.74

Calcium oxide (CaO).97

Soda (Na2O)2.84

Potassium oxide (K2O).22

Water at 110°C (H2O).21

Water above 110°C (H2O)6.99

Carbon dioxide (CO2)None

Phosphoric oxide (P2O5)1.08









100.36



All the lava in the park appears to be too altered for precise determination. Finlay (1902, p. 350-351) says that the lava he saw had normal diabasic texture and was composed principally of augite with less abundant idiomorphic plagioclase with the habit of labradorite. He was unable to determine the feldspar accurately. The small amount of olivine originally present in the rock was altered to serpentine and chlorite.



At the south end of Flattop Mountain, the rock is even more thoroughly altered. Traces of diabasic texture remain, and there are remnants of phenocrysts that may have originally been augite and olivine. Some of the rock (exemplified by fig. 9D) contains feathery plagioclase laths with the habit of a calcic plagioclase but with indices of refraction close to that of Canada balsam. These are crowded with specks of a secondary mineral of high index and low birefringence, presumably a chlorite. The main part of the laths now approximate oligoclase or albite-oligoclase in composition but may well have resulted by recrystallization from an originally more calcic plagioclase. Much of the rock consists of a fine-grained nearly opaque aggregate of secondary minerals that include chlorite, sulfides, calcite, quartz, and possibly serpentine. The writer's examination of this highly altered rock was kindly supplemented by Howard Powers.



L. D. Burling (1916) says the Purcell lava near Shepard Glacier is 150 feet thick and—





"composed of 6 or more flows, each of uneven and more or less ropy surface, separated by small and more or less local accumulations of shale. The lower 25 or 30 feet of the flow is composed of a conglomeration of dense, homogenous, spheroidal masses averaging 1 to 2 feet in diameter. They preserve their shape in the lower layers, being separated from each other by chert or drusy cavities, and many individuals have displaced considerable portions of the mud upon which they were rolled or shoved, even to the extent of complete burial. The bottom of the flow is therefore exceedingly irregular. Toward the top of this bed the individual spheroids yield more or less to the pressure of their fellows, and they unite to form an upper surface of moderate unevenness."



The upper part of the formation is composed of a bed about 20 feet thick which is massive but very porous. Vesicles are plentiful in the lower parts of several of the individual flows.



Shepard Formation



The Shepard formation in Glacier National Park is essentially coextensive with the Purcell basalt just described. That is, it is exposed at intervals from Granite Park northward past the international boundary. Originally Willis (1902, p. 316, 324) applied the name "Sheppard quartzite" to what he regarded as yellow ferruginous quartzite resting on the Purcell basalt along the crest of the Lewis Range in the vicinity of Mount Cleveland and Shepard Glacier between Belly River and Flattop Mountain. Because of a decision of the U. S. Board on Geographic Names (Wilmarth, 1938, p. 1980) and because much of the formation was found not to be quartzite, the name was changed to Shepard formation. The field notes of members of M. R. Campbell's parties, descriptions published by the Fentons, and observations during the present investigation are in agreement that both in the type locality and in other places in the park quartzite is subordinate to dolomitic rocks.



The formation is characterized by the fact that most of the beds in it weather to a distinctive color that ranges from pale yellowish brown to grayish orange. A typical specimen proved on analysis to be a fairly pure dolomite, as is shown in the table of analyses of Belt rocks on page 55, but many of the beds in the formation are more argillaceous or siliceous than the sample analyzed. West of Continental Creek, the Shepard contains beds of conglomerate containing lava pebbles.



As can be seen from figure 9E, the dolomite analysed is a peculiar rock. The matrix consists of dolomite grains about 0.01 millimeter in diameter, but embedded in it are numerous oval masses up to 4.0 millimeters long that consist of carbonate grains up to 0.2 millimeter in diameter. Perhaps these ovals are thoroughly recrystallized oolites.



Although in the Shepard formation as in most subdivisions of the Belt series variations in composition are plentiful, the color of weathered surfaces is so distinctive that, where coupled with the presence of lava below, the formation could be mapped with confidence. Attempts to extend the mapping of the Shepard far beyond the limits of the lava were unsuccessful. The characteristic color either disappears or is found in discontinuous beds at several horizons separated by beds that have none of the special features of the Shepard formation. The Shepard formation is reported to contain stromatolites. When these and other features have been studied in detail, it may prove possible to correlate carbonate-bearing units farther south with those now assigned to the formation.



Willis (1902, p. 316) gives the thickness of the Shepard at the type locality as about 700 feet. Parks, in his field notes, recorded 400 feet on Mount Cleveland, and Stebinger and Bennett, in their field notes, measured the following section of the formation on the slopes south of the North Fork of Belly River, where the top has been removed by erosion. The Shepard formation here overlies 115 feet of Purcell basalt, which in turn overlies the argillaceous beds of the lower part of the Missoula group measured by Willis and by Parks and listed above.



Shepard formation near Belly River



[According to the field notes of Eugene Stebinger and H. R. Bennett]























Feet

Eroded.

Limestone, bluish to green-gray, fine-grained, thin bedded; with molar-tooth markings, and Collenia; weathers dull buff yellow395

Limestone, blue-gray, pure; in laminated beds 2-5 ft thick interbedded with green-gray calcareous shale in 2- to 4-ft beds115

Shale, red, micaceous17

Shale, green-gray, calcareous, thin-bedded64

Purcell basalt.

   Total



591







Limestone







Unnamed more or less lenticular bodies of carbonate rocks of mappable proportions are present in the Missoula group at various horizons. The largest single mass extends along the northeast side of the Middle Fork of the Flathead River from near the head of Cy Creek to a point north of Harrison Creek where it disappears under deposits of Tertiary age. This mass may have a maximum thickness of nearly 3,000 feet, but it thins at both ends. It resembles the Siyeh limestone in general appearance to such an extent that Clapp (1932, pl. 1) mapped it as part of his Siyeh group and Clapp and Deiss (1931, p. 691, fig. 3) referred to it as the upper Siyeh limestone. It is not, as persistent a feature as might be inferred from their publication, based on reconnaissance. The thinner carbonate bodies around Mount Bradley, east of Spruce Park, near Hematite Peak, and in the vicinity of Grouse and Twin Creeks represent the feathering out of the large limestone mass into the main body of the Missoula group. Bodies at higher horizons are mapped in the vicinity of Baldhead and Square Mountains. There are, in addition, detached carbonate masses near the Continental Divide from Mount Cannon to Mount Logan. Some of these last-mentioned masses are so similar in character and stratigraphic position to the Shepard formation that they were correlated with that unit in the field notes of Campbell's men. Perhaps detailed studies may trace a connection between the Shepard as now mapped and carbonate rocks farther south, through argillaceous beds with some carbonate content in intervening areas. Clearly, however, the carbonate rock in the Missoula group extends through such a wide stratigraphic range that much of it cannot be correlated strictly with the Shepard formation.







The carbonate bodies near the Middle Fork of the Flathead are somewhat better known than the outlying masses. The parts of these that are relatively rich in carbonate are closely similar in appearance and composition to the Siyeh limestone. Like much of that rock, it is a limestone with some magnesian carbonate and some clastic impurities, whereas the characteristic rock of the Shepard formation is more nearly a dolomite. The thinner bodies mapped as limestone and parts of the larger ones, especially along their margins, are more impure than typical Siyeh limestone. These impure beds contain different proportions of argillaceous and siliceous material, and some of them react feebly or not at all to field tests with dilute acid. They resemble the green calcareous argillite that in places has been mapped at the base of the Missoula group. All of the rock mapped as limestone in the Missoula group weathers with the colors characteristic of the Siyeh limestone. This is the feature that guided the sketching of parts of contacts in the field.







So-called segregation structures of one kind or another are conspicuous on weathered surfaces. Some of these are identical in appearance with the molar tooth structure for which the Siyeh limestone is noted. Others are so irregular and angular in outline that they bear no resemblance to the markings on teeth. Far more intensive study than could be given in the various reconnaissance examinations that the region has received will be required before these structural features will be understood.







Stromatolite zones are fairly plentiful in the limestone bodies of the Missoula group but have received less study than those in the Siyeh limestone. During the summer of 1952, Rezak discovered 2 hitherto unknown stromatolite zones in a large limestone lens approximately 6,000 feet above the base of the Missoula group. The larger and stratigraphically higher of the 2 zones is about 100 feet thick. It was mapped by Rezak in and near the southern part of Glacier National Park but has not been looked for farther south. Because it closely resembles the better known Conophyton zone 1 in the Siyeh limestone, it is mapped on plates 1 and 2 as the Conophyton zone 2. Only two species of stromatolites occur in the zone: Conophyton inclinatum and Collenia frequens. The zone is divided into five parts that persist over the entire area in which it was examined. At the base is 23 feet of well-developed C. frequens. The next unit is 35 feet thick and consists of large heads of Conophyton inclinatum. Overlying this is 12 feet of barren black shaly limestone. Above this is 6 feet of C. frequens, and at the top of the zone there is 24 feet of Conophyton inclinatum. The Conophyton in this zone does not occur in the podlike bioherms that are so characteristic in the Siyeh limestone.







The second zone lies about 500 feet below the one described above. It is only about 35 feet thick and contains extremely large heads of Collenia symmetrica. The colonies measure up to 5 feet in height and 6 feet in diameter. Subordinate species that occur in this zone are Cryptozoon occidentale and Collenia frequens.







Both zones are well exposed in the southwestern part of Glacier Park and the adjoining area east of the Middle Fork. They may be seen near the top of Running Rabbit Mountain and along the Great Northern Railroad tracks a few miles to the east of the mountains. The Conophyton zone 2 has a prominent outcrop west of Ole Creek. The other zone, characterized by Collenia symmetrica, is well displayed on and near Mount Furlong.







CHEMICAL COMPOSITION OF ROCKS OF THE BELT SERIES







The table below shows the chemical composition of 14 samples of rocks of the Belt series in and south of Glacier National Park. The table is arranged with the sample at the left representing the stratigraphically lowest formation. The analyses show that the rocks include impure dolomite and limestone, argillaceous quartzite, and siliceous argillite. Most of them are moderately high in silica. The most quartzose beds were not analyzed as the intention was to include material fairly representative of the different formations, rather than that representative of special features. All of the samples contain some carbonate, and only 5 show less than 1.0 percent. All of the carbonate-rich rocks contain both calcium and magnesium carbonates, and the amounts of other carbonates present must be small. When calculations are made based on the assumption that all of the calcium and magnesium occur as carbonate, it is found that 3 of the carbonate rocks show excess of carbon dioxide but 3 show deficiencies of somewhat less than 3 percent carbon dioxide. Obviously in the latter, part of the calcium and magnesium is in noncarbonate minerals. As most of the carbonate rocks have rusty colors on weathered surfaces and some are yellowish on fresh fracture, some iron carbonate is present, but the percentage of FeO recorded suggests that the amount must be small. Probably some of the calcium and magnesium in all of the rocks is in silicate minerals, which would leave a little carbon dioxide available for iron or other carbonates, even in those that seem deficient in that component under the assumption that all of the magnesia and lime are in carbonates. It is a fair guess that the carbonate rocks contain 1-4 percent ferrous carbonate.







Judged by the analyses, the Altyn limestone should be more properly called a dolomite, and the basal calcareous argillite of the Missoula group is likewise dolomitic. As many beds of the basal argillite effervesce with cold dilute acid, it is probable that much of the unit is less magnesian than the sample recorded in the table. The sample from the Shepard formation has a little more lime than the mineral dolomite, but it and most of the argillitic rocks analysed are dolomitic. The two samples from the Siyeh limestone and the one from a large body of similar rock within the Missoula group are limestones with less than 8 percent of magnesium carbonate in them. Analyses of four specimens from the Siyeh limestone quoted by Erdmann (1944, p. 54) agree in showing that calcium carbonate is much more abundant than magnesium carbonate. Obviously these limestones contain much clastic material.







A large part of the sodium and potassium present in all of the rocks analyzed probably is in feldspar grains or in minerals derived from the decomposition of alkali feldspars. Some of the argillaceous rock may contain nearly 10 percent of alkali feldspar.




http://www.nps.gov/history/history/online_books/geology/publications/pp/296/sec2-1.htm


All of the green argillaceous rocks contain more ferrous oxide than ferric oxide, and the reverse is true of those of purplish and reddish colors. In these rocks the coloring matter is presumably ferrous silicate, including chlorite, in the green rocks and hematite in the others. One specimen of purplish Grinnell argillite yielded 5.08 percent of ferric oxide and 2.16 percent of ferrous oxide, but in most the sum of the iron oxides is about 4 percent. One decidedly purplish-red argillite sample from the Missoula group yielded only 0.78 percent of Fe2O3 and 0.76 percent of FeO. Clearly very little ferric oxide, properly distributed, suffices to impart a strong color to a rock. As the ferrous oxide enters into the composition of complex silicates, a distinct green color may require much more of the coloring constituent. The amount of water in all of the samples is low; that given off below 100°C is less than 0.2 percent and in most samples well below 0.1 percent. Three samples of argillite yielded approximately 3.1 percent of water when heated above 100°C, but most of the other yielded less than 2 percent. This paucity of water is one of the indications that the rocks have begun to be metamorphosed. In most other respects they accord more nearly to typical sedimentary than to metamorphic rocks.

 

Tablehttp://www.nps.gov/history/history/online_books/geology/publications/pp/296/images/tablea.pdf
PRECAMBRIAN INTRUSIVE ROCKS

The Belt series throughout the two regions here described is intruded by igneous rocks of peculiar, but on the whole gabbroic, composition. In Glacier National Park north of latitude 48°28', they form narrow sills and dikes which are almost everywhere confined to the Siyeh limestone. The sills are a few score to over 100 feet thick, and most of the dikes are 10-200 feet wide. In most places only a single sill is exposed, and this is commonly a short distance below the Conophyton zone 1. However, the sill is not so strictly conformable with the bedding as to be a horizon marker of reliability closer than several hundred feet stratigraphically. In a few localities the sill departs sufficiently from conformity with the bedding to cut across the Conophyton zone 1 at a low angle. Here and there the igneous rock is not continuously exposed, and in other places a couple of sills close together are visible. The sills form black lines on cliff faces, rendered doubly conspicuous by white border zones of recrystallized limestone (figs. 3, 16). Long, thin, steep dikes in the Siyeh limestone are conspicuous from St. Mary Lake northwestward past Lake Sherburne. Shorter dikes are exposed in other localities, such as the west end of Porcupine Ridge
 
fig 16 View north from the head of Avalanche Basin, Glacier National Park. Most of the rock shown belongs to the Siyeh limestone. The dark peak at the left consists of beds belonging to the Missoula group. The Conophyton zone 1 and a sill of metagabbro are visible in the upper part of the cliffs. Note the pronounced discordance between the nearly flat basin floor at the right of the view and the cliffed gorge to the left. Photograph by Eugene Stebinger.  
 
In the Flathead region the intrusions are more irregularly scattered, more diverse in character and mostly in the Missoula group. The most conspicuous sills, are near the top of that group on the eastern slopes of Cruiser and Ringer Mountains, but there are small ones in several other places, such as Scalplock Mountain. In addition there are several irregularly shaped intrusions, such as those on and near Lodgepole Mountain and on the ridge between the South Fork of the Flathead River and Sullivan Creek (Section C—C', pl. 2).

The character of the rock in all of the intrusions is broadly similar although almost every outcrop has peculiarities in color and texture that reflect minor differences in composition. The petrographic notes that follow are based in part on data furnished by Ray E. Wilcox, who kindly examined three of the thin sections. The rocks are composed principally of titaniferous augite, largely altered to hornblende, and zoned plagioclase that ranges in composition from An75 at the core to An25 in some of the outermost zones. Some separate hornblende crystals may be of primary origin. Most of the plagioclase is decidedly calcic. In addition, some potash feldspar, micropegmatitic intergrowths of quartz and alkalic feldspar, minor amounts of quartz, apatite and opaque iron oxides, and such alteration products as sericite, chlorite and calcite are present. Exceptionally, potash feldspar is sufficiently plentiful to color the rock pink. The rock has diabasic texture, obscured, however, by alteration products and interstitial micropegmatite, which commonly constitutes 10-20 percent of the whole and which is locally more abundant. A photomicrograph of this rock is shown on figure 9F.

The best available description of the intrusive bodies in the Belt series close to Glacier National Park is that of Daly (1912, p. 212-255). Although most of his observations were made north of the international boundary, they agree in general with those made south of that line. He noted rather more hornblende as an original constituent than was found in specimens examined during the present investigation. Finlay (1902, p. 349), who examined the intrusive rocks in what is now Glacier National Park in connection with Willis' studies, called them diorite; but Daly, more accurately, spoke of them as "somewhat acidified, abnormal gabbro." Because many details are obscured by alteration products and the origin of such abnormal features as the micropegmatite is still open to debate, it will serve present purposes to speak of the rock as metagabbro. Much similarity exists between this rock and that called metadiorite by Gibson and Jenks (1938), but the abundance of calcic plagioclase and the presence of residual pyroxene in the rock of Glacier National Park indicate that the original rock had the composition of gabbro rather than diorite.

S. J. Schofield (1914, 1915) has given extensive information, including analyses, on sills in a part of Canada just north of the boundary and west of longitude 115° that are much thicker but otherwise somewhat similar to the sills in Glacier National Park. He thinks the sills in his area—


"represent intrusions from a single intercrustal reservoir of a series of magmas—acid magmas—which gave rise to composite sills where rock types vary in the gabbro * * *. The simple sills solidified in the usual manner of such intrusives while the acid material differentiated under the influence of gravity giving rise to composite sills."

Differentiation by gravity would not hold for the thin, steep dikes in Glacier National Park, and the other intrusive masses in that region bear no visible evidence of being stratiform. R. A. Daly thought of the peculiar composition of these rocks as a result of assimilation of quartzite wall rocks in one locality or "metargillite" wallrocks in another, but in Glacier National Park the wallrock for most of the intrusions is limestone, and no significant difference has been found between those and the intrusions in argillaceous rocks; so, his explanation, also, seems inapplicable. N. L. Bowen (1938, p. 71-74, 82-83) cites several examples of diabasic and gabbroic rocks that contain interstitial micropegmatite. He interprets the micropegmatite as one of the last constituents to crystallize in the course of the original consolidation of the rock. He regards olivine and quartz as complementary. It seems quite possible that his explanation may fit the rocks under discussion, although the data at hand are inadequate for a final decision.

R. A. Daly, S. J. Schofield, and the Fentons are agreed that the intrusive rocks and the Purcell basalt are genetically related and, hence, essentially of the same age. This opinion is based on the fact that the two are, in numerous localities, found at neighboring stratigraphic horizons and they have similar compositions. No place is recorded where a sill or other intrusive body of metagabbro has been traced into a lava flow. Even so, a genetic relation between the two is the most logical explanation now available.
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