« PrejšnjaNaprej »
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 lime
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
of the land represented by the Lower and Middle Triassic time interval must have been of the plateau-forming type, the land moving upward and downward, with an absence or minimum of tilting or deformation in the earth's crust. Possibly an oscillatory movement of the strand line due to variation in the ocean level might better fit the facts. At any rate, a stratigraphic discontinuity exists at this horizon not only along the Yukon but at other localities in Alaska, and this discontinuity appears to characterize the transition from the Paleozoic to the Mesozoic. In some parts of Alaska, particularly in southern Alaska, great extravasations of basaltic lava accompanied the regional uplift and subsequent depression. These eruptions attained their greatest development in the interval between the Permian and the Upper Triassic. Along the Yukon, however, no lavas of Pennsylvanian, Permian, or Triassic age have yet been recognized. At the end of the Triassic period this region was again extensively uplifted above sea level, and it appears to have remained so during all of Jurassic time. Some time during the Jurassic also occurred the great batholithic intrusions of granitic and related rocks, of which the batholith in the basin of the Charley River is a most striking example. The Lower Cretaceous conglomerates show little or no granitic material derived from such intrusive rocks, so that it is likely that the intrusives were injected late rather than early in the Jurassic and had not been uncovered to any large extent by erosion at the time of the formation of the Lower Cretaceous rocks. Concomitantly with these Jurassic granitic injections came the earliest period of mineralization of which there is any definite record in interior Alaska; and during this period were formed many of the gold-quartz veins of interior Alaska from which the present placers were subsequently derived.
By subsequent sinking of the land marine sedimentation was again begun in Lower Cretaceous time, resulting in the formation of the sandstone, slate, and conglomerate that now constitute the Kandik formation. After this epoch of marine sedimentation the land was again elevated, and a great series of fresh-water deposits, which constitute the assemblage of rocks here grouped as Upper Cretaceous and Eocene, was laid down. It is likely that this mid-Cretaceous regional elevation was accompanied by deformational movements that folded the Lower Cretaceous rocks prior to the deposition of the Upper Cretaceous rocks, thus resulting in an unconformity between these two groups of rocks. The structural relations in the Rampart district indicate that such an unconformity exists, but along the upper Yukon the two groups of rocks are not in contact, so that the hypothesis can not be absolutely proved. Nevertheless the unconformity is accepted as the probable condition.
As pointed out in the discussion of the Upper Cretaceous and Eocene sequence of rocks, there appears to have been no stratigraphic or botanic hiatus at the end of the Mesozoic era, so that as well as can be ascertained at present the Cretaceous deposition seems to have merged gradually into the Eocene through a transitional epoch. After Eocene time, however, the region was again uplifted, and it has not been subsequently depressed below sea level. Hence no Tertiary marine strata are present in interior Alaska. Along with this regional uplift in post-Eocene time came further intrusions of granitic rocks of the same general character as those injected in the Jurassic. These intrusives, however, appear to have consolidated fairly close to the surface and at some localities reached the surface, forming rhyolitic and dacitic lava flows, of which those at the head of the Charley River are examples. These post-Eocene granitic intrusions also gave rise to the second known period of mineralization in interior Alaska, forming the later gold quartz veins and cinnabar deposits and at some localities reopening and enriching the Jurassic vein systems.
The post-Eocene history of Alaska is recorded mainly in ancient fluviatile gravel, in the sediments deposited during the Pleistocene epoch, and in the Recent stream alluvium. The interpretation of this record in terms of elevations and depressions of the base level of erosion is largely a physiographic problem the solution of which has hardly yet been begun. One fact in particular should be emphasized, namely, that interior Alaska, except in some isolated areas along the Yukon-Tanana divide, was not glaciated during the Pleistocene epoch. Yet it was bordered on the north, east, and south by great glaciers, and these, together with the climatic conditions which produced them, must have had a potent influence upon the nature of sedimentation in interior Alaska. Great bodies of black peaty silt were found deeply burying the pre-Pleistocene alluvial material, and such deposits, locally known as muck, themselves constitute an important problem in sedimentation and are as well one of the most hopeful sources of data bearing on Pleistocene conditions. The fine nature of these sediments shows that ordinary abrasion and movement of detritus by stream action was not the most typical process. Probably the mean temperature was low and precipitation also low, resulting in unusual conditions of erosion.
Although the Pleistocene is referred to as a glacial epoch, it is believed that interglacial stages formed a part of that epoch in Alaska, as in the States, but the glacial geology of interior Alaska has as yet received but little attention. For such study, as well as for post-Pleistocene glacial study, detailed topographic maps will be required, and little detailed topographic mapping has yet been done in Alaska.
The principal source of exportable mineral wealth in this region is gold. The Eagle-Circle district and more particularly the Fortymile and Circle mining precincts, which lie to the south and west of the area here described, have long been known as sources of placer gold. Deposits were discovered at Franklin Gulch, in the Fortymile district, in the fall of 1887; at Birch Creek, in the Circle district, in the summer of 1893; and at American Creek, south of Eagle, shortly after the discovery on Birch Creek. All these old placer diggings are still producing gold.
No other mineral deposits are known in this region which give promise of possible commercial development in the near future. Some low-grade coal deposits exist, and attempts have been made to utilize these locally for fuel, but without success. Oil shale also is known at several localities, but this also does not seem possible of exploitation at the present time.
The accompanying map shows that all the creeks hereafter mentioned whose gravel has been mined for its content of gold lie southwest of the Yukon River, also that the granite intrusive bodies occur on the same side of the Yukon. The country northeast of the Yukon has not yet, to be sure, been much explored geologically, but the gravel from the streams draining that region does not indicate the presence there of any extensive granitic intrusives close to the river. The smaller degree of metamorphism seen in the rocks northeast of the Yukon also tends to confirm this idea. It is believed that all the placer gold in this region came originally from these granitic masses, although the proximate source may be gold quartz veins, mineralized shear zones, or ancient placer deposits. This distribution of gold placers is therefore only what might be expected from what is now known of the regional geology. This statement is not intended as a general proscription of all the region northeast of the Yukon as a possible seat of gold mineralization and gold placers, for practically all of the triangle between the Yukon and Porcupine Rivers and the international boundary is as yet unmapped geologically, and it may contain granitic bodies that have mineralized the surrounding rocks. But along the northeast side of the Yukon in the zone between Eagle and Circle the known geology does not encourage the hope for finding commercial gold deposits.
The two most productive gold placer camps in this region are the Fortymile precinct, lying south of Eagle, and the Circle precinct,