« PrejšnjaNaprej »
The larger animals include caribou, moose, bear, and sheep. Many thousands of caribou live in this region and form an important source of food for the white and native population. The caribou assemble in large bands in August and begin to migrate across country. One of the impressive sights of the region is the spectacle of such a passing band, whose transit sometimes takes several days. Moose also are fairly plentiful, but their numbers would be counted in hundreds rather than thousands. Bears also are fairly numerous, the black bear being more common. In the higher hills, however, the great brown grizzly bear, with the light-colored back, is seen. Sheep are found only in the highest mountain ranges and are relatively scarce.
The fur-bearing animals include chiefly fox, lynx, marten, muskrat, squirrel, weasel, beaver, mink, land otter, wolf, and of late years coyote. Other animals, such as porcupines, rabbits, tree and ground squirrels, and mice, are also found.
The native game birds are ptarmigan and grouse, but in summer ducks, geese, and other waterfowl inhabit the country. Other birds include the loon, tern, gulls, owl, hawks, kingfisher, raven, swallows, sparrows, junco, thrushes, warblers, waxwing, jay, and shrike.
Grayling are found in nearly all the streams and trout in a few. Salmon run up the larger streams, and other fish, such as whitefish, pike, pickerel, and lake trout, are also present in the lakes and rivers.
BIRCH CREEK SCHIST
Proterozoic or pre-Cambrian rocks have so far been recognized definitely only in the southern half of the area covered by this report. The distribution of these rocks as shown on the accompanying geologic map (pl. 12) should be regarded only as a provisional mapping that may be materially changed by later work. One reason for this uncertainty is the presence near by of metamorphosed lower Paleozoic rocks, which in reconnaissance work are difficult to distinguish from the pre-Cambrian rocks. It is possible and even probable that the band of rocks mapped as Birch Creek schist around the Glacier Mountain massif, as well as the bands on the Seventymile River and on the Middle Fork of the Fortymile River, may later be found to be Paleozoic rocks that have suffered contact metamorphism as a result of their proximity to the great mass of granitic rocks which they border. The rocks on the South Fork of Birch Creek and thence westward, however, are almost surely of pre-Cambrian age.
The name Birch Creek schist, as used in this report, is a designation for all the definitely pre-Cambrian sedimentary of rocks of this region. The term was introduced originally by Spurr 11 in 1898 as Birch Creek" series," to characterize the oldest rocks of sedimentary origin in the Birch Creek and Fortymile districts, although Spurr stated that “there are also found, although rather sparingly, schists of igneous origin, being dikes which have intruded into the sedimentary series previous to the shearing.” As the name is used at present, however, the igneous rocks, although not separated from the sedimentary rocks on the map, are not considered a part of the Birch Creek schist. It is expected that the mass will eventually be subdivided, when more detailed work is done. Even at present the more distinct lithologic units are recognized, but no attempt has yet been made to delimit these units on a geologic map. It is possible that the Tindir group may prove to be of pre-Cambrian age, and in that event the Birch Creek schist would represent only the basal part of the pre-Cambrian sequence.
Most of the rocks of the Birch Creek schist are either schistose or gneissoid. They include quartzite, quartzite schist, quartz-mica schist, mica schist, graphitic schist, crystalline limestone, and calcareous schist. The associated metamorphic igneous rocks include granitic and dioritic gneiss, amphibolite, hornblende schist, and a certain proportion of sericite and chlorite schists. Nearly all these rocks are recrystallized, but in some of them traces of the original sedimentary or igneous fabric can still be seen.
Among the schistose rocks quartzite schist is perhaps the most common, followed closely by quartz-mica schist, quartzite, and mica schist. The quartzite schist and quartzite occur for the most part in beds 1 foot to several feet thick and weather into blocky talus piles. Plate 3, A, shows a typical exposure of quartzite in the Birch Creek schist. In many places these quartz-rich rocks are covered by a black lichen, which gives the outcrops a dark, forbidding appearance from a distance. Both muscovite and biotite occur with the quartz in these rocks, but the biotite is perhaps the more common.
The presence of these micas affords one criterion for distinguishing these pre-Cambrian rocks from the metamorphosed lower Paleozoic rocks, for the latter, even where schistose, are likely to contain a larger proportion of the hydromicas, brittle micas, and chlorite. At several localities, as for instance at the head of Woodchopper Creek and in the headwaters of the Salcha River, the difference in the character of the micas constituted the main basis for the separation of the Birch Creek schist from younger rocks.
11 Spurr, J. E., The geology of the Yukon gold district: U. S. Geol. Survey Eighteenth Ann. Rept., pt. 3, pp. 140-145, 1898.
Graphitic and calcareous schist and crystalline limestone are found to some extent throughout the Birch Creek mass, but for the most part such rocks are localized in their distribution in such a way as to suggest that they characterize the upper part of the sequence. No large bodies of limestone occur in the Birch Creek schist; the beds or zones range in thickness from a few inches to 100 feet. One of the unsolved problems of these older rocks is the definite identification of the carbonaceous and calcareous members as integral parts of the unit. Even without an exact determination of age, sufficient lithologic differences exist between these rocks and the older pre-Cambrian rocks to afford the basis for a cartographic differentiation whenever more detailed work can be attempted.
Among the igneous rocks associated with the Birch Creek schist granitic and dioritic gneisses, particularly the former, are conspicuous. In general, these rocks appear to invade the Birch Creek schist but like it have undergone sufficient metamorphism to acquire a gneissoid and in places a schistose texture, with the resultant formation of new minerals. Albitization of the feldspars, with the production of calcite, and alteration of the dark minerals to epidote, chlorite, and secondary hornblende or mica are the more common secondary processes. One of the more striking types of these metamorphosed intrusive rocks is augen gneiss, in which feldspar augen as large as 2 inches in diameter have been observed. Plate 3, B, shows a typical exposure of the augen gneiss. Sericite and chlorite schists of uncertain origin are also included with the Birch Creek schist as mapped. Some of these schists are of igneous origin, derived probably in part from lavas and fine-grained intrusive rocks of acidic and intermediate character, and therefore do not properly constitute a part of the Birch Creek schist. Others are undoubtedly derived from argillaceous sedimentary rocks and are closely related to the sedimentary mica schists. These rocks present a difficult problem, for their cartographic differentiation will have to be made on lithologic rather than genetic differences. These schists, like the carbonaceous and calcareous metamorphic rocks, belong in the upper rather than the lower part of the sequence and would have to be included there in any except the most detailed type of geologic mapping.
The associated amphibolite and hornblende schists, produced in large measure by the metamorphism of intermediate, basic, and ultrabasic igneous rocks, form another possible mapping unit. These rocks in some localities, as at the falls of the Seventymile River, occur with crystalline limestone and quartzite in such a way as to suggest at least the possibility of a sedimentary origin. They are also found at many places, however, in close association with the gneissoid rocks and are locally intruded by those rocks. The origin of all these rocks can not be stated with assurance, but their field relations to the gneisses and to adjacent sedimentary rocks suggest that most of them are recrystallized dike and sill rocks that were related genetically to the ancient granitic magma that produced the gneisses.
STRUCTURE AND THICKNESS
The pre-Cambrian rocks have been subjected to diastrophism during many periods and have unquestionably been intensely deformed in several stages. They therefore probably reflect in their present structure the combined effects of close and open folding, together with thrust and normål faulting, repeated several times and accentuated by proximity to intrusive rocks. Original bedding planes, except in the more massive quartzites and quartzite schists, are quite obliterated, and the present visible structure exhibits a multitude of diverse cleavage planes and of close or even recumbent folds, which give little idea of the original sequence of deposition. It is believed that the Birch Creek schist has a structure almost if not quite as complex as the pre-Ordovician schists of Seward Peninsula, and neither of these groups of rocks is likely to be understood structurally for many years to come.
In addition to the regional metamorphism, portions of the preCambrian rocks have been subjected also to intense contact metamorphism caused by granitic intrusions of at least three eras. The pre-Cambrian (?) granite gneiss represents the earliest of these, the Mesozoic granite batholiths the second, and the Tertiary granitic intrusive rocks the third. It is difficult without detailed work to assign to each of these intrusive periods its proper share in the contact-metamorphic effects now visible, but the Mesozoic granitic rocks have unquestionably produced a large part of the contact metamorphism.
Garnetiferous schists are common near the contact with the great granitic batholith that stretches from Glacier Mountain westward to the Salcha River, and the Mesozoic granite itself at places shows the effects of its intrusion by a primary gneissoid fabric. At the heads of Woodchopper and Coal Creeks, near the contact with the Mesozoic granitic rocks, Prindle 12 and the writer in 1911 noted also a stauro
22 Prindle, L. M., A geologic reconnaissance of the Circle quadrangle, Alaska : U. S. Geol. Survey Bull. 538, p. 24, 1913.
litic garnetiferous schist, in which crystals of staurolite as much as half an inch in length were especially abundant. Mica, particularly biotite, is also prominent in the schist near its contact with the granite. It is apparent that the schists have received by injection a considerable amount of material from the granitic intrusions, for the schists near the contacts are locally feldspathic and have even in some places themselves been changed into augen gneiss much like the older metamorphosed granitic rocks. The schistosity has also been accentuated by contact metamorphism. These contactmetamorphic effects, however, are sporadic rather than universal and can not therefore be said to characterize the schist-granite contacts as a whole. As the Mesozoic intrusions must have affected the Paleozoic as well as the pre-Paleozoic rocks, it is altogether likely that contact metamorphism has been mistaken for regional metamorphism at some localities, resulting in the mapping of altered Paleozoic rocks as pre-Cambrian.
Veins of quartz in greater or less number are of course present in nearly all the geologic formations of this region, but the Birch Creek schist, because it is the oldest terrane, contains more vein quartz than any of the younger formations. The quartz is diverse in character, owing to differences in age and mode of formation. Much of it is a white vitreous quartz that ranges from tiny seams to veins several feet thick, but some, particularly in the smaller seams, is almost colorless and transparent. Little of the porous quartz with crystal outlines that is so characteristic of the gold lodes of the Fairbanks district has been found. It was the presence of so much quartz that led mining men as well as the earlier geologic workers in this region to believe that areas of Birch Creek schist were the most favorable localities for prospecting. Locally, to be sure, the quartz is mineralized with sulphides, chiefly pyrite, and also gold, but most of the quartz in the Birch Creek schist is barren. As it is now known that the granitic rocks are the ultimate sources of the gold, the Birch Creek schist can no longer be regarded as the mother lode, except in so far as it has been directly mineralized.
As the Birch Creek schist is the oldest terrane recognized in this region, its base has not been seen, and at present its upper limit is also indeterminate. These facts, together with the intensely complicated structure, make it impossible to hazard any exact estimate of the thickness. It suffices to state that at least several thousand and perhaps many thousand feet of strata are represented in this complex of metamorphic rocks.
AGE AND CORRELATION
The Birch Creek schist is here classified as pre-Cambrian in age. The oldest fossils found in this region by Spurr in 1896 were of