QUATERNARY SYSTEM Unconsolidated sediments ranging in age from Pleistocene to Recent are present in all the stream valleys of this area, and the physiographic history of their deposition constitutes a geomorphologic problem of great magnitude, which has not yet been attacked in this area. The Alaska Range, together with southern and southeastern Alaska, was intensely glaciated during Pleistocene time and probably during early Recent time. In fact, some of this territory has not yet really emerged from the glacial epoch, being still covered with great ice caps. The Brooks Range, which crosses northern Alaska, was also glaciated in Pleistocene time, though not nearly so severely or extensively as the Alaska Range, and it is now nearly free from glaciers. The great stretch of country lying between these two ranges, however, has not been glaciated except in certain groups of high mountains, where local valley glaciation of the alpine type has occurred. While all this glaciation was taking place, both north and south of the Yukon Valley, climatic conditions of a peculiar type must have existed in the interior of Alaska. The mean annual temperature of this region, bordered on the north, east, and south by ice caps, must have been much lower than at present, and it is probable also that the annual precipitation was even less than at present. Rivers like the Tanana, which headed in large part in a glaciated area, handled and reworked great volumes of outwash material from the glaciers, which was subsequently redeposited to build up great alluvial plains like that in the present Tanana Valley, north of the Alaska Range. Streams that headed in essentially nonglaciated areas, however, like those between the Yukon and Tanana Rivers, went through a physiographic cycle which is as yet only imperfectly understood. Their deeply buried preglacial stream gravel, where uncovered by mining operations, is seen to be covered with great deposits of black muck, composed largely of silt and peaty material, with some beds of sand and layers of gravel. These are the sediments which were deposited during this physiographic cycle, and it was during the time of their deposition that numerous forms of preglacial life, such as the bison and mastodon, became extinct in this region, while other more adaptable animals and plants were modified to conform with the new environment. Where small areas were subjected to local glaciation, the emerging alpine glaciers built up a few morainal deposits. Two such morainal deposits in the southwest corner of the area covered by this report have been shown separately on the accompanying geologic map. All the other alluvial deposits, of both Pleistocene and Recent age, have been grouped together as a single cartographic unit. As the glaciers began to retreat, climatic conditions began to change and to approach more nearly those of the present time. Stream erosion and sedimentation of the normal type were renewed; and the formation of alluvial deposits of silt, sand, and gravel which began then has continued to the present time. Evidence exists at numerous localities that this era was ushered in by a lowering of the base-level of the master stream of this region, which produced many changes in the disposition of preexisting drainage channels. Many interesting problems, including stream superposition, reversals of stream flow, "inlaid" gravel deposited on preexisting deposits of muck, terracing, and a multitude of kindred phenomena, are here represented and should sometime be studied in their physiographic and geomorphologic aspects. IGNEOUS ROCKS Five mappable units of igneous rocks are shown on Plate 12. These are undifferentiated Paleozoic greenstone; Middle Devonian greenstone interbedded with sedimentary rocks; lower Mississippian greenstone interbedded with sedimentary rocks (Circle volcanics); Mesozoic and Tertiary granite, diorite, and related rocks; and Tertiary lava flows, mainly rhyolite, but including some dacite. The pre-Cambrian igneous rocks associated with the Birch Creek schist are described in connection with that formation on pages 15-16 and are not separated from the Birch Creek on the map. The Middle Devonian and Mississippian volcanic formations, although essentially basaltic, have been described on pages 77 and 85 in connection with the sedimentary rocks, with which they logically belong because of their bedded character. So far as the other igneous rocks are concerned, no additional work was done by the writer during the season of 1925. The petrology of these eruptive rocks was described in considerable detail by the writer in 1911. Several hundred thin sections were studied and described at that time, and all that seems necessary for the present report is a brief summary of that work. UNDIFFERENTIATED GREENSTONE OF PALEOZOIC AGE DISTRIBUTION The principal areas mapped as undifferentiated greenstone are the Mount Sorenson massif, at the head of the Seventymile River, some Prindle, L. M., A geologic reconnaissance of the Circle quadrangle, Alaska, with a chapter on the igneous rocks by J. B. Mertie, jr.: U. S. Geol. Survey Bull. 538, pp. 37-48, 1913. scattered areas farther east, and the greenstone that forms the bluff at Eagle and extends northwestward up the north side of American Creek. Certain small areas along the international boundary, mapped by Cairnes, are also included in this grouping. PETROLOGY The greenstone at Mount Sorenson is an ultrabasic rock of peridotitic character, which weathers reddish brown. In relatively unweathered specimens it is seen to be essentially a dunite consisting entirely of olivine, much of which is altered in part or wholly to serpentine. This mass of serpentine is undoubtedly intrusive. Similar serpentine is exposed on American Creek, forming the bedrock from the forks downstream for a mile or two. This serpentine evidently forms a narrow belt trending northeast, but it is included in the undifferentiated Paleozoic rocks because its extent in the direction of maximum elongation has not been determined. Other small areas of serpentine as well as greenstone of original basaltic character occur at a number of localities along the Seventymile between the Mount Sorenson massif and Mission Creek. The greenstone that forms the bold bluff at Eagle and continues northwestward up the north side of Mission Creek is essentially a basaltic greenstone, with which are interbedded flow breccia and tuff, also of greenstone habit, as well as more or less quartzite and crystalline limestone. This body of greenstone is mainly of effusive origin, though it may also contain some intrusive rock. AGE Greenstones are known at many horizons in the Paleozoic and prePaleozoic sequence of Alaska, and most of these rocks resemble one another greatly. Theoretically, the present grouping of undifferentiated greenstone includes all the Paleozoic basic and ultrabasic rocks of greenstone habit. The serpentine, however, which forms a large part of these rocks, is known to intrude the Silurian and Middle Devonian rocks of this region but has not been seen anywhere in rocks of later age. This does not definitely establish its age, for it may have been intruded at depths below the surface too great to reach the later Paleozoic rocks. But a stratigraphic discontinuity and perhaps angular unconformity of considerable magnitude is believed to separate the Middle Devonian rocks of this area from the overlying Carboniferous sequence, and with the idea in mind that such rock deformation is usually accompanied by volcanism, it seems possible that these ultrabasic rocks are Upper Devonian. The greenstone at Eagle may also be of Devonian age, but if so, it is more likely to be correlative with the Woodchopper volcanics and therefore to be late Middle Devonian. No data are avail 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 DISTRIBUTION 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 seasons. PETROLOGY 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. 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 writer" 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. GEOLOGIC HISTORY 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 Mertie, J. B., jr., The occurrence of metalliferous deposits in the Yukon and Kuskokwim regions: U. S. Geol. Survey Bull. 739, p. 157, 1923. |