Slike strani
PDF
ePub

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.

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 Ordo

vician (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

Mertle, 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 geography of northwestern Alaska: U. S. Geol. Survey Bull. 815, pp. 144-145, 1930. 'Mertie, J. B., Jr., op. cit. (Bull. 773), p. 244.

emergence from the Silurian sea and the regional deformation of the Silurian and older rocks were accompanied in some areas, though apparently not along the international boundary, by the intrusion of granitic rocks. The unconformity resulting from these processes is regarded as one of the critical tie points in the geology of interior Alaska.

Marine sedimentation began again early in the Middle Devonian epoch and continued without major interruptions, though possibly with minor ones, to the end of Devonian time, resulting in the formation of the beds described as a part of the Woodchopper volcanics. The extrusion of basic lava began sometime in the early Devonian, but its first definitely recognized products occur in the late Middle Devonian, as exemplified by the Woodchopper volcanics. A part at least of the Woodchopper flows were of submarine origin. Surficial outpouring of basic lavas seems to have continued intermittently into the Upper Devonian epoch, and it is believed by the writer that certain ultrabasic rocks of deep-seated origin in interior Alaska may also have originated at the same time, but of this belief there is no absolute proof. The Middle Devonian rocks at the head of the North Fork of Shade Creek were deposited either contemporaneously with or somewhat later than the Woodchopper volcanics.

The relation existing between the representatives of the Devonian and Carboniferous systems is not entirely clear. A considerable sequence of marine Middle Devonian rocks is known along the international boundary and in interior Alaska, but Upper Devonian rocks have not yet been identified except in northern and southeastern Alaska. Along the Yukon and the international boundary, therefore, the Upper Devonian, in the light of present knowledge, appears to be represented by a depositional discontinuity, though not necessarily by a structural unconformity.

The earliest known events in the Carboniferous are the extravastation of the Rampart group and Circle volcanics and the deposition, more or less contemporaneously with these lava flows, of the lower Mississippian chert and shale. The chert formation is believed to be of marine origin, but numerous matters relating to its origin as well as its correct stratigraphic placement are as yet unsettled. Some of the lavas of the Rampart group are probably of subaqueous origin, but some of these rocks are clearly of intrusive origin. The complete history of this early Carboniferous volcanism on the Yukon can not yet be written.

The deposition of the chert was followed, without any stratigraphic hiatus or deformational movements, by the deposition of a marine upper Mississippian formation, known on the Yukon as the Calico Bluff formation and in northern Alaska as the Lisburne lime62744-3011

stone. These upper Mississippian rocks constitute very important horizon markers in interior and northern Alaska. After the deposition of the Calico Bluff formation, marine sedimentation continued but changed gradually along the Yukon and the international boundary to a terrigenous type of sedimentation, culminating finally in the deposition of a great thickness of fresh-water sediments known as the Nation River formation. These deposits were then submerged below the sea, and upon them was laid down the marine Permian limestone.

Although several marine formations and one terrestrial formation were laid down during Carboniferous time, no major structural unconformity in the sedimentary sequence is recorded during this interval. This lack of angular unconformities is regarded as one of the typical features of the late Paleozoic history. The transitional beds between the terrestrial Nation River formation and the overlying marine Tahkandit (Permian) limestone may be seen on the Yukon opposite the mouth of the Nation River, with no evidence at all to suggest any interruption in sedimentation or intervening deformational movements; and, although a discontinuity in sedimentation has been recognized at the base of the Nation River formation, no angular discordance in the beds was observed. It can not, of course, be maintained on the basis of such negative evidence that sedimentation continued in this region without interruption during the entire Carboniferous period. Indeed, the transition from marine to terrestrial and back again to marine conditions must certainly have been accompanied by movements of the strand line of considerable magnitude. Unconformities without angular discordance may therefore exist in this sedimentary sequence, but they are not very evident; and the presence of an angular unconformity of any considerable magnitude seems much less probable.

After the deposition of the Tahkandit limestone all of Alaska apparently was elevated above sea level, and as no Lower or Middle Triassic rocks have yet been found anywhere in Alaska it is believed that a land mass existed in much or all of Alaska until the sea again invaded it in Upper Triassic time. One of the vagaries of this regional elevation and the following submergence is that the Upper Triassic rocks of the Yukon are fine-grained sediments, shale and limestone, which appear to lie conformably upon the Tahkandit (Permian) limestone. Fossiliferous beds of early Permian and Upper Triassic age lie practically in contact with one another without divergence in attitude. Therefore a very considerable discontinuity in sedimentation does in reality exist, but it is hard to conceive of the conditions of sedimentation that might have produced such a result. Evidently the elevation and subsequent depression

« PrejšnjaNaprej »