« PrejšnjaNaprej »
able for determining the age of the sundry small areas of greenstone, mapped and unmapped, in this area. Where large areas of such rocks occur, as upstream from Circle, the lithology of the interbedded rocks may afford presumptive evidence for correlating the formation with similar rocks in near-by areas. But the age of the smaller areas of greenstone can not be assigned more closely than Paleozoic.
GRANITE, DIORITE, AND RELATED ROCKS
Granitic rocks occupy a zone in the southern part of this area 75 miles long and from 10 to 50 miles wide; and smaller outlying masses of similar rocks occur both to the north and south of this great massif. Only one new area, about 15 miles southeast of Eagle, in Yukon Territory, was recognized and mapped in 1925, all the other cartographic work on these rocks having been done in previous
The varieties of acidic intrusive rocks previously identified by the writer consist of muscovite granite, alaskite, muscovite-biotite granite, amphibole granite, tourmaline granite, epidote granite, and a little quartz monzonite. Among the subsilicic types are granodiorite, quartz diorite, and diorite. The basic rocks include gabbro, peridotite, and pyroxenite. The granitic and dioritic rocks, however, are the typical rocks of this group. Locally, primary granite gneiss is developed along the contact of these intrusives with the country rocks, but such gneiss is not to be confused with the older gneisses, which are of early Paleozoic or pre-Paleozoic age. Dikes of rhyolite, dacite, andesite, basalt, and diabase are commonly found near the intrusive rocks, but a part of these are of Tertiary rather than Mesozoic age.
The granitic rocks of this area have always been considered to be of Mesozoic and probably of late Jurassic age, and this generalization is doubtless true for the greater part of such rocks. Of late years, however, evidence has been accumulating that some of the granitic and particularly the monzonitic rocks of interior Alaska are of Tertiary age. Such Tertiary intrusive rocks are highly developed in southwestern Alaska and have been recognized up the Yukon as far as the Rampart, Hot Springs, and Tolovana mining districts. Where stratigraphic evidence of the presence of such Tertiary granitic rocks is lacking, the presence of cinnabar in the concentrates taken with the gold from the placers has been interpreted by the
writers as almost infallible evidence of Tertiary age. It is therefore of interest to record the fact that cinnabar has been found in considerable quantity in the placer concentrates on Canyon and Mogul Creeks, tributaries of the Seventymile River from the south. This fact suggests strongly the recurrence of volcanism in Tertiary time and leads to the belief that granitic rocks of Tertiary age are present in this area, though of course such rocks may not yet have been uncovered by erosion and therefore may be below the surface.
RHYOLITE AND DACITE
A number of flows believed to be of Tertiary age have been mapped in the upper valley of the Charley River. They consist mainly of rhyolite and dacite porphyries, which appear to cover the tops of a number of hills. These lavas are clearly flows, but the orifices from which they issued have not been recognized.
The pre-Cambrian and Paleozoic geology of this region is as yet only imperfectly understood, but enough information is available to sketch at least the main geologic cycles of sedimentation, erosion, mountain building, and volcanism. As no one area in the Yukon and contiguous territory appears to furnish a complete historical sequence, the writer has been obliged in the following outline to draw upon geologic data and experience acquired in other areas.
The oldest rocks known in the Yukon Valley are the quartzite and mica schist of the Birch Creek schist, which are of sedimentary origin. Such rocks constitute, then, the earliest evidence of sedimentation in Alaska, and their character indicates that the ordinary processes of erosion and sedimentation functioned then in quite the same manner as at present. As no fossils of any kind have been found in this pre-Cambrian sequence, it can not be stated with assurance that these arenaceous and argillaceous rocks were of marine or terrestrial origin, but such traces of original texture as may be seen in some of the quartzitic rocks indicate that they originated as well-sorted littoral sediments such as characterize the present eastern Coastal Plain of the United States. Later in pre-Cambrian time the sediments appear to have become more dominantly argillaceous and even to a degree calcareous, the resulting deposits being now represented by graphitic, sericitic, and chlorite schists and also by calcareous schist and crystalline linestone.
The pre-Cambrian also had its periods of volcanism, as indicated by granitic and dioritic intrusive rocks of gneissoid character and by various metamorphosed basic and ultrabasic rocks, such as amphibolite, hornblende schist, and related rocks, which doubtless represent both surficial and deep-seated basic volcanism. A definite paragenetic assignment of the igneous rocks of the pre-Cambrian can, of course, not be made, but the gneisses have invaded the amphibolitic rocks, and the evidence at present available suggests that the acidic intrusives originated at a somewhat later date than the basic eruptives. It should be remembered, however, that some of the igneous rocks at present mapped with the pre-Cambrian Birch Creek schist are possibly of Paleozoic age.
5 Mertie, J. B., Jr., The occurrence of metalliferous deposits in the Yukon and Kuskokwim regions : U. S. Geol. Survey Bull. 739, p. 157, 1923.
The earliest record of a pronounced discontinuity in sedimentation is seen at the top of the Birch Creek schist, in the Fairbanks quadrangle. The field evidence is not that afforded by a visible unconformity but consists of three lines of circumstantial evidence that are believed adequate to prove the point; these are a sudden and pronounced change in the character of sedimentation, a slight but apparently real difference in the course of trend lines in the preCambrian and overlying formations, and a marked difference in the degree of metamorphism. It is believed by the writer that at or about the end of the period of sedimentation represented by the Birch Creek schist the usual diastrophic sequence of elevation, stream rejuvenation, and accentuated erosion produced a great discontinuity in deposition, which was followed by renewed marine sedimentation. The unconformity that marks this break in deposition represents the lower stratigraphic limit of Alaskan geologic knowledge. What happened geologically before this diastrophic epoch is largely a matter of surmise and is likely to remain so for many years to come. The geologic record of succeeding events becomes progressively more intelligible, and additional work in Alaska should continue to contribute evidence.
It seems worth while to suggest the possibility at least that the gneissoid granitic and dioritic rocks of pre-Cambrian age may have originated contemporaneously with and have been one of the manifestations of the diastrophic events that produced this early unconformity. No positive evidence can be cited, but these gneissoid rocks invade some of the pre-Cambrian sedimentary rocks as well as some of the amphibolitic rocks, so that the evidence as far as it goes does not negate this as a possible hypothesis.
The sequence of events following the deposition of the Birch Creek schist is as yet somewhat uncertain. The Middle Cambrian rocks along the boundary lie unconformably above the Tindir group; and the hypothesis has been formulated that the Tindir group may possibly be correlated with the lower part of the Tatalina group, which overlies the Birch Creek schist in the Fairbanks district. These associations suggest the possible presence of a group of rocks of either Lower Cambrian or Algonkian age, or both, lying between the Birch Creek schist and the lowest known Paleozoic rocks. If this interpretation is accepted as a working hypothesis, the conclusion follows that both Archean and Algonkian rocks may be present in Alaska; that the unconformity at the top of the Birch Creek schist in the Fairbanks quadrangle represents a stratigraphic hiatus and period of deformation that separates the Archean from the Algonkian; and that another and similar unconformity exists between the Algonkian rocks and the earliest Paleozoic rocks.
The history of events that occurred during the formation of the rocks of the Tindir group is obscure, but these rocks, which in their type locality are little metamorphosed, indicate processes of sedimentation similar to those of later geologic periods. One distinguishing characteristic is the occurrence at certain horizons of hematitic beds, as mentioned by Cairnes and as described also by the writer in connection with the red beds on the Tatonduk River. The origin of these red beds has not been determined, but they evidently represent a specialized type of sedimentation, different from the ordinary processes of erosion and deposition, that characterized one or more epochs during the Lower Cambrian or Algonkian. Volcanic action is also recorded in the dikes, sills, and irregular intrusive masses of greenstone that appear to constitute an integral part of the Tindir group.
According to Cairnes, the Middle (?) Cambrian limestone of Jones Ridge lies unconformably above the Tindir group. Hence it should follow that a period of regional deformation occurred after the rocks of the Tindir group were formed, which was followed by an era of regional depression below sea level, when the Cambrian sequence of rocks began to be laid down. The writer's hypothesis, however, is that the early Cambrian sea, though possibly widespread at other localities to the southeast in the Rocky Mountain provinces of Canada, was restricted in Alaska to a relatively narrow embayment more or less coincident with the present Ogilvie Mountains but possibly extending some distance northward and westward. This localization of marine sedimentation would account for the apparent absence of early Cambrian rocks elsewhere in interior Alaska.
No evidence has yet been found in Alaska or in Yukon Territory that suffices to prove or even to suggest strongly any marked interruption of sedimentation in Cambrian or Ordovician time. Rocks of Middle and Upper Cambrian age and of Lower Ordovician age are known along the international boundary just north of the Yukon, apparently in conformable sequence; and in other parts of interior Alaska rocks of Middle Ordovician (Mohawkian) and Upper Ordovician (Richmond) age are also found. Hence it would seem that the Cambrian and Ordovician seas spread gradually over Alaska, reaching their maximum extent in the Upper Ordovician epoch, for such rocks have been found well down into southwestern Alaska and in Seward Peninsula. Minor unconformities undoubtedly exist, as, for example, the discontinuity represented in the White Mountains north of Fairbanks, where middle Silurian rocks rest with apparent discordance upon Middle Ordovician rocks; but no great stratigraphic hiatus, accompanied by regional deformation, has yet been identified in the Cambrian and Ordovician seuqence. Apparently volcanic activity was slight in the Cambrian and early Ordovician but became progressively greater in late Ordovician time, as indicated by the late Middle Ordovician basic volcanic rocks of the White Mountains and the Upper Ordovician greenstones in the Nowitna Valley of southwestern Alaska.
No rocks of lower Silurian age are known in interior Alaska, and this fact might be used as presumptive evidence that between the Ordovician and Silurian rocks a stratigraphic hiatus, possibly an angular unconformity, exists. The contact relations between the Ordovician and Silurian rocks, however, have not been studied, and the writer is not in a position either to affirm or to deny this hypothesis. If such an unconformity can be shown, it should prove a convenient horizon in the stratigraphic column with which to correlate the Totatlanika schist, a formation of metamorphosed rhyolitic flows and interbedded sediments of Silurian or Devonian age found between the Tanana River and the Alaska Range. Middle and upper Silurian rocks, however, are well developed along the international boundary and at numerous other localities in interior, northern, and southwestern Alaska; and these rocks, which include a large proportion of limestone, indicate strongly another long period of undisturbed marine deposition comparable with the Cambrian-Ordovician deposition. These marine beds appear to have been affected to a minor degree by volcanism.
Lower Devonian rocks, also, are unknown in Alaska, but the meaning of their absence is better understood than that of the absence of lower Silurian rocks. The hypothesis has been proposed and evidence therefor given by the writer & that a great structural unconformity exists between the Silurian and Devonian rocks of northern and interior Alaska. Perhaps the direct evidence so far adduced for this hypothesis is better in northern Alaska than in the interior, but it is believed by the writer to hold for all of Alaska. Equally good is the evidence presented by the writer ? to show that the
6 Mertie, J. B., Jr., Geology and gold placers of the Chandalar district : U. S. Geol. Survey Bull. 773, p. 234, 1925. Smith, P. S., and Mertie, J. B., Jr., Geology and geog. raphy of northwestern Alaska : U. S. Geol. Survey Bull. 815, pp. 144-145, 1930.
"Mertie, J. B., Jr., op. cit. (Bull. 773), p. 244.