Slike strani

Devonian age, and as the Birch Creek schist antedates the rocks in which those fossils were found it was designated originally preDevonian. Subsequently, in 1909, Prindle 13 discovered Middle(?) Ordovician fossils in the White Mountains, in a limestone that is younger than the Birch Creek schist. A great thickness of rocks, however, lies between the known Ordovician beds and the Birch Creek schist, and Prindle suggested at that time the probability that the Birch Creek was pre-Cambrian. Blackwelder collected Lower Ordovician fossils from the White Mountain district in 1915, further strengthening this conviction. Meanwhile Cairnes, 14 in 1911 and 1912, had found Upper Cambrian fossils along the international boundary, and during the summer of 1925 and again in 1928 the writer collected Middle and Upper Cambrian fossils along the Yukon below Eagle. The Birch Creek schist is therefore now definitely known to be older than Middle Cambrian, and as two other formations that are not a part of the Birch Creek schist appear to lie between it and the Middle Cambrian beds, assignment of the Birch Creek schist to the pre-Cambrian seems now to be fully justified.

Some of the igneous rocks associated with the Birch Creek schist, notably the gneissoid rocks of granitic affinity, are definitely intrusive in character and may or may not have been formed prior to the Cambrian period. Evidence that at least some of the gneisses are of Paleozoic age was discovered by the writer 15 in the Chandalar district of northern Alaska in 1923. In that region the Paleozoic rocks up to and including the Silurian are metamorphosed to a greater or less degree, and in them the intrusive gneiss was found within a few feet of Silurian (?) fossils. This gneiss is believed to be of late Silurian or early Devonian age. Similarly, a chlorite schist believed to have been originally a lava flow of Silurian age was found adjoining the great middle Silurian limestone of this region. Doubtless, rocks of similar character are included with the Birch Creek schist as mapped and will some day have to be separated from it.

The Yukon-Tanana region is the type locality of the Birch Creek schist, for it is in this region that these rocks are best exposed and in this region only that their pre-Cambrian age has so far been demonstrated. There has been a tendency to correlate schistose rocks seen elsewhere in Alaska and Yukon Territory with the Birch Creek schist, merely on the ground that such rocks are greatly metamorphosed. Many such metamorphic rocks, however, are probably of Paleozoic age and have been altered to their present condi

13 Prindle, L. M., A geologic reconnaissance of the Fairbanks quadrangle, Alaska : U. S. Geol. Survey Bull, 525, pp. 38-39, 1913.

14 Cairnes, D. D., The Yukon-Alaska international boundary between Porcupine and Yukon Rivers: Canada Geol. Survey Mem, 67, pp. 63-65, 1914.

16 Mertie, J. B., Jr., Geology and gold placers of the Chandalar district: U. S. Geol. Survey Bull. 773, pp. 243-244, 1925.


tion either by intense local dynamic disturbances or by contact metamorphism. Age assignment on degree of metamorphism alone, without contributory evidence, leads directly to such fallacies. One of the most striking examples of relative differences in degree of metamorphism may be seen along the Tanana River near Hot Springs, where slightly metamorphosed pre-Silurian rocks may be seen on the east side of Baker Creek, and semischistose Lower Cretaceous rocks on the west side of the same creek. Contact metamorphism has been the cause of this anomaly.

Various names have been proposed to designate the metamorphic rocks of Alaska and Yukon Territory. Spurr 18 in 1896 proposed the name

Birch Creek series " for the oldest rocks and the name “Fortymile series " for an assemblage of metamorphic rocks which he considered to overlie the Birch Creek schist. This dual nomenclature, however, has not survived, and the term “Fortymile series " has been abandoned, the rocks comprising that so-called " series” being now assigned in part to the Birch Creek schist and in part to the Paleozoic.

Brooks, 17 in 1898, used the name “Nasina series" to describe the schistose rocks of the lower White River and the term “Tanana schist” for the schistose rocks of the upper Tanana and differentiated a group of granitic gneisses which he believed to be older than either of these. In 1899 Brooks 18 used the term "Kotlo series " to .

“ designate the metamorphic rocks of the White River Basin and apparently included under this name both the “Nasina series " and the “Tanana schist.” He indicated, however, that the “Kotlo series" probably included metamorphosed lower Paleozoic as well as preCambrian rocks. None of these three names have persisted in the United States Geological Survey nomenclature, but the beds to which they were applied include in part at least the Birch Creek schist, though they may be in part younger.

In Yukon Territory adjacent to Alaska McConnell 19 has divided the metamorphic rocks into two main groups—a lower and older series, called by him originally the Indian River series and later redesignated the Nasina series, a name earlier applied by Brooks 20 to other rocks in the lower valley of the White River; and an upper series called the Klondike series. The Nasina series of McConnell

[ocr errors]
[ocr errors]

18 Spurr, J. E., The geology of the Yukon gold district, Alaska : U. S. Geol. Survey Eighteenth Ann. Rept., pt. 3, pp. 140–155, 1898.

17 Brooks, A. H., A reconnaissance in the White and Tanana River Basins, Alaska, in 1898: U. S. Geol. Survey Twentieth Ann. Rept., pt. 7, pp. 460-470, 1900.

18 Brooks, A. H., A reconnaissance from Pyramid Harbor to Eagle City, Alaska, including a description of the copper deposits of the upper White and Tanana Rivers : U. S. Geol. Survey Twenty-first Ann. Rept., pt. 2, pp. 357–358, 1900.

19 McConnell, R. G., Report on the Klondike gold fields : Canada Geol. Survey Ann. Rept., vol. 14, pp. 10B-23B, 1905.

* Brooks, A. H., A reconnaissance in the White and Tanaa River Basins, Alaska, ip 1898: U. S. Geol. Survey Twentieth Ann. Rept., pt. 7, pp. 465-467, 1900.

consists mainly of quartzite and quartz-mica schist but includes also certain green schists of igneous origin and beds of crystalline limestone. The Klondike series consists principally of light-colored sericite schist and subordinately of darker-colored chlorite schist, both believed to be of igneous origin. Associated with and constituting, in fact, a subdivision of the Klondike series of McConnell is a group of ancient rocks of gneissic character that are believed to have intruded into his Nasina series and are probably the deepseated equivalents of the sericite schist of the Klondike series. These rocks are known as the Pelly gneiss, a name which, according to McConnell,21 was applied originally to them by Brooks, although the name does not appear in Brooks's report on the White and Tanana River Basins. Another cartographic unit recognized by McConnell 22 is the Moosehide diabase, composed mainly of altered diabase and supposed to be nearly contemporaneous with the Klondike series. Of these units, the Nasina series alone is regarded as correlative with the Birch Creek schist, as redefined in this report.

Subsequently Cairnes,23 in 1914, introduced the term Yukon group to include “all these older metamorphosed, schistose, and gneissoid rocks of both sedimentary and igneous origin.” He further defined the Yukon group as pre-Cambrian in age. This term, therefore, would include not only the Birch Creek schist but all the associated metamorphic rocks of igneous origin here mapped with the schist. No general term, similar to Yukon group, to include all the pre-Cambrian rocks, is at present recognized in the United States Geological Survey nomenclature. Unfortunately, as shown on page 33, Cairnes included in his mapping of the Yukon group certain Paleozoic rocks, some of which are as young as Devonian. This, however, should in no wise detract from the use of the term Yukon group as defined by Cairnes.

Cockfield,24 in the most recent geologic work done in Yukon Territory, has used the general term Yukon group, but has divided the group into four subgroups—the Nasina series, at the base; the Klondike series, overlying the Nasina; the amphibolites, unnamed, overlying the Klondike; and the Pelly gneiss, which invades the Nasina and Klondike series as well as the amphibolites. It seems to the writer rather likely that parts of the Klondike series, of the amphibolites, and of the gneiss may later turn out to be of Paleozoic age.

* McConnell, R. G., op. cit., footnote p. 13B. 22 Idem, pp. 22B-23B.

23 Cairnes, D. D., The Yukon-Alaska international boundary between Porcupine and Yukon Rivers : Canada Geol. Survey Mem. 67, p. 40, 1914.

24 Cockfield, W. E., Sixtymile and Ladue Rivers area, Yukon : Canada Geol. Survey Mem. 123, 1921.




The term Tindir group was applied by Cairnes to groups of similar rocks at four distinct localities, as follows: Along the Porcupine River where it crosses the international boundary; along the international boundary between Fort and Orange Creeks; along the international boundary between Tindir, Cathedral, and Hard Luck Creeks; along the international boundary southeastward from the mouth of Eagle Creek. The first two localities lie outside the area covered by this report and will not be further discussed. The rocks of the fourth locality are here called the slate-quartzite group and for lack of definite paleontologic evidence have been assigned to the group of undifferentiated noncalcareous Paleozoic rocks. (See p. 34.) The rocks of the third locality, though not seen by the writer, come within the area shown on Plate 12 and will now be considered.

As mapped by Cairnes, the rocks of the Tindir group extend from the Cambrian limestone at Jones Ridge northward in a solid block for 5 or 6 miles to Cathedral Creek, thence northward in a narrow zone up a tributary of Cathedral Creek and on to the northwest and north of Mount Slipper. A small outlying mass lies on the north side of Tindir Creek about 4 miles north of Mount Slipper. Because of the proximity of these areas to Tindir Creek, it seems best to regard them as the type locality for this group of rocks. Here certainly is the best available evidence for evaluating the age of the

Tindir group


As the Tindir group in its type locality has not been seen by the writer, the description given by Cairnes, 25 referring to the Tindir, Cathedral, and Hard Luck Creek areas, is quoted here:

The Tindir section exposed still farther south along Tindir Creek and between Ettrain and Hard Luck Creeks particularly resembles that observed along Porcupine River, the members including mainly. dolomites, limestones, quartzites, slates, shales, and greenstones. Here, however, the greenstones are developed to a much greater extent than to the north of Orange Creek and along the Porcupine, and the quartzites instead of being dominantly white to grayish include more greenish, reddish, and dark-colored members, even quite black quartzites being prominent in some places.

The dolomites weather characteristically rough and reddish to brownish as elsewhere and are generally bedded, the strata ranging in places from a fraction of an inch to a foot or more in thickness. They also include numerous bands of chert 1 to 2 inches thick. Intercalated with these dolomites also are

* Cairnes, D. D., op. cit., pp. 52–53.

occasional grayish limestone beds and also some of black slate. These dolomites appear to be at least 700 feet in thickness.

Mount Slipper is capped by Devono-Cambrian limestones and dolomites, which are underlain by the members of the Tindir group. Thus around the western and southern faces in particular of this mountain a splendid section of a part of the Tindir rocks is exhibited. There these beds include mainly dark to black calcareous shales, limestones, and quartzites, all invaded by greenstones which occur both as dikes and sills. The quartzites are dominantly thinly bedded and nearly black but weather in places to a dark reddish or reddishbrown color. The limestones are prevailingly also thinly bedded and dark to nearly black in color and grade into very soft, thinly bedded, friable black calcareous shales, the beds of the upper 500 feet at least of the section being very calcareous. The Tindir beds exposed here on Mount Slipper evidently constitute the upper portion of the Tindir group in this locality and very closely resemble the shale member comprising the upper 1,000 feet in the section measured along Porcupine River.

A typical member of the Tindir beds south of Tindir Creek also is a finely laminated rock consisting of alternating white and black bands, there being on an average about 20 laminae to the inch. The light bands consist dominantly of quartz and the dark bands of argillaceous shaly material.

Certain shales and quartzites also exhibit considerable hematite, and in places portions of these beds contain up to 30 per cent or even possibly 40 per cent metallic iron.

The greenstones are dominantly diabases and occur in sills, dikes, and irregular intrusive masses and in places constitute a considerable portion of the entire formation. The sills are in places as much as 100 feet and the dikes 200 feet in thickness. Since these intrusions were rarely noted intruding the overlying Devono-Cambrian limestones and dolomites, it is concluded that they are dominantly at least older than these rocks.

The Tindir rocks here, in the vicinity of Tindir Creek and between Ettrain and Hard Luck Creeks, as farther north, are characteristically much indurated, folded, and contorted, and are also brightly and varicolored, black and shades of red, gray, and yellow being conspicuous. A single side hill in places may exhibit reddish to brownish dolomites, yellow to black quartzites, gray limestone beds, gray, red, or black shales or slates, and black to bright-red iron ore containing beds all ribboned and intersected by irregular brownish-weathering greenstone dikes and sills. The hills on which these rocks outcrop are dominantly lofty and irregularly distributed and are characterized by long, sharp, steep-sided ridges, with smooth slopes covered with a fine talus. The bright and contrasted colors which they exhibit also constitute one of the most striking pictorial features of the landscape of the district.


The assignment of the Tindir group by Cairnes to the Lower Cambrian or pre-Cambrian was made for the following reasons: The Tindir group underlies the Devonian-Cambrian limestones and dolomites between the Black River and Fort Creek (north of the area here considered); the Tindir group underlies the Devonian-Cambrian limestones at Mount Slipper, north of Cathedral Creek; along the north side of Jones Ridge the Tindir beds were observed to underlie

« PrejšnjaNaprej »