Most of the fragmental débris is silicified limestone that resembles in part a white quartzite and in part chert. The matrix of this upper conglomerate is brown rather than red. From this point to the top of the ridge the rock is a brown-weathering quartzite, at places somewhat conglomeratic, mingled with some brown-weathering limestone. This section, as given, ranges downward in the geologic column in going up the spur, and the conglomeratic beds are therefore at or near the base of this section. Doubtless other phases of this formation are present in this neighborhood. Cairnes 25 observed the same formation in much the same general locality and describes it as follows: This conglomerate is at least 700 or 800 feet in thickness and consists dominantly of a firm, somewhat dense, finely textured reddish argillaceous matrix, in which are embedded angular to subangular pebbles and boulders ranging in size from microscopic to 3 or 4 feet in diameter. The matrix appears to have approximately the composition of a boulder clay, and the greater number of the pebbles and boulders are composed of limestone or dolomite, but some were noted composed of other sediments such as sandstone, conglomerate, and shale. The prevailing red color of the matrix is due mainly at least to the considerable amount of iron contained in the matrix, which has in places the general appearance of a hematite ore. The conglomerate, where exposed on a small tributary of Tatonduk River, is quite unstratified and has the general appearance of a consolidated and iron-stained boulder clay. The pebbles and boulders are irregularly distributed and are often quite isolated and completely surrounded by the matrix, instead of resting upon one another as in the case of a normal conglomerate. This conglomerate is thus undoubtedly of terrestrial origin-the term terrestrial being used to imply deposition on the land in contrast to deposition in the sea or on the seashore. Land deposits may, however, be formed in numerous ways, mainly by the action of lakes, rivers, winds, glaciers, and volcanoes, as well as by weathering, creepage, or sliding. Of these, considering the thickness of this conglomerate, its unstratified condition, and the irregular distribution, composition, angularity, and size of the pebbles and boulders, the only two modes of origin which appear to at all satisfy the field observations are glacial action and creepage or sliding. In general composition this conglomerate appears to much more resemble a boulder clay than it does slide material, but on the other hand its prevailing reddish color and the fact that this conglomerate has not been previously described as occurring in Yukon or Alaska, so far as the writer is aware, and is thus probably not very extensive would tend to disprove the glacial theory of origin. Also, no striated pebbles were found; this may, however, be due to the fact that since the pebbles are dominantly composed of soft materials such as limestones and dolomites, the scratches, even if they ever existed, might readily have become obliterated. Further, due to peculiar circumstances, the writer was able to examine this conglomerate in only one very limited area and could devote only a few hours to the examination; thus striated pebbles may well occur and not have been found. Pebbles having faceted surfaces, much resembling "soled" pebbles, were, however, noted to be somewhat plentiful. Due to its prevailing color, also, this conglomerate "Cairnes, D. D., op. cit., pp. 91–92. could be seen to extend for several miles to the west of the area examined, showing it to be somewhat extensive for slide material. This, taken in conjunction with the thickness of the conglomerate, rather favors the glacial theory again. Thus, until more evidence has been obtained, the origin of this conglomerate must remain an open question. Cairnes apparently, from the above quotation, was rather inclined to regard this formation as glacial in origin but admitted that the evidence was inconclusive. An enthusiastic searcher for tillites might lean even more strongly toward the glacial hypothesis, for some of the characteristics of glacial origin appear to be present, such as faceted pebbles and the heterogeneity of assemblage of the fragmental material comprising some of the beds. On the other hand, faulting, landslide activity, soil creep, or fluviatile ice action in winter might also account for the soled pebbles observed by Cairnes. Some peculiar conditions of sedimentation appear to have existed to form beds of the character of some of these. The generally restricted character of this formation, however, is regarded as rather strong evidence against the glacial hypothesis of origin. It seems to the writer that one or more of a variety of terrestrial conditions of accumulation might just as easily explain the origin of this formation. Little is really known, however, about terrestrial accumulation of sediments in early geologic time, when the conditions of the atmosphere and lithosphere may have been materially different from those observable at present. It seems best, therefore, to defer judgment as to the origin of these red beds pending the accumulation of more general data about such matters and more specific information about these particular beds. STRUCTURE AND THICKNESS The upper part of this formation is well exposed in red bluffs along both sides of the Tatonduk River, but the lower part can be seen only imperfectly on the hillside slopes of the valley. Where seen along the valley floor these rocks strike in a general northerly direction and dip about 20° W.-that is, downstream. As a result of this relatively low inclination they project as a downstream wedge into the overlying rocks. The accompanying map is not on a sufficiently large scale to show this geographic expression of their structure. Near the eastern geographic limit of the formation, as seen on the spurs above the valley, both the strike and dip are inconstant, although the dip appears to be prevailingly westward. At the contact with the undifferentiated limestone to the east (upstream) the red beds and the limestones are all greatly disturbed. It is conceived by the writer, therefore, that a fault of considerable magnitude forms the boundary line between these red beds and the limestone area to the east. Cairnes 27 did not recognize this zone of faulting, but his observations appear to confirm the general westward-dipping structure of the red beds, for he states: This conglomerate overlies the Devono-Cambrian limestone-dolomite beds and appears also to overlie Carboniferous shales and the Devono-Ordovician shalechert group and to correspond stratigraphically to the Nation River formation, but of this the evidence is not conclusive. In other words, the red beds apparently dip to the west. Cairnes, of course, in assigning these beds to the "Permo-Carboniferous," did not know of the presence of Middle Cambrian formations still farther down the Tatonduk apparently overlying the red beds. The red beds form a belt that is about a mile wide where it crosses the Tatonduk River, below the boundary. The structure shows only at the west side of this belt, along the walls of the valley, but if it is assumed that the structure is the same all the way across the strike, the resulting thickness may be about 1,700 feet. This figure in reality means very little. Deduction for reversals in dip at the east side of the formation might materially reduce it, but on the other hand, if the fault postulated at the eastern border has concealed some of this formation, its thickness may be greater. It will perhaps suffice to state that the rocks of this formation actually visible in the Tatonduk River, with the reversals in dip evaluated, aggregate possibly 1,200 feet. AGE AND CORRELATION The evidence regarding the age of this formation is far from conclusive. Cairnes suggested a "Permo-Carboniferous" age because he believed that these rocks overlie his group of Devonian-Cambrian limestones. As previously pointed out, however, this eastern contact is believed by the writer to be a fault contact; and the structure at the western limit, so far as known at present, leads to the belief that these beds not only underlie the Middle Cambrian limestone but are separated stratigraphically from it by another group of rocks. The sequence, then, is as follows: Middle Cambrian limestone (top of section). A limestone-argillite formation. A red-bed formation (bottom of visible section on the Tatonduk River). One significant fact should be emphasized. The limestone-argillite formation and the red-bed formation are well and continuously exposed at their contact on the Tatonduk River, and it is reasonably sure that although materially different in their lithology these two formations grade into each other. Field conditions indicate that the limestone-argillite and the Middle Cambrian limestone also grade into each other, but their contact is not so well exposed, and the evidence is therefore somewhat weaker. In other words, if any stratigraphic break should be postulated in this sequence of three formations it should more reasonably be assumed to exist at the base of the Middle Cambrian limestone. However, as no fossils have been collected in the two lower formations, and no definite structural data have been collected by the writer that will serve to prove or disprove the existence of any discontinuity of sedimentation in these three formations, they can merely be represented on the geologic map as a succession of beds of which the Middle Cambrian limestone is the top and the red beds the basal part. These structural uncertainties and the lack of fossils make it difficult to assign definite age labels in the explanation that accompanies the map. If a continuous sedimentary sequence exists, all three of these formations are probably of Cambrian age; but if, as thought by Cairnes, an unconformity exists at the base of the Middle Cambrian limestone, it is easy to conceive that the two lower formations are of pre-Cambrian age. This uncertainty has been registered on the map in the designation "Middle Cambrian or older" for the limestone-argillite formation, and by the designation "Lower (?) Cambrian or pre-Cambrian" for the red beds. The interrogation point in the expression "Lower (?) Cambrian or pre-Cambrian" is intended to indicate the possibility that the red beds may be of Middle Cambrian age. So far as correlation with other rocks is concerned little can be said, for Cambrian rocks have not been recognized in Alaska except along the international boundary. No rocks that are lithologically exactly similar in interior Alaska have been described, irrespective of geologic age. The nearest lithologic counterpart consists of certain red slates and phyllites and associated arkosic rocks that constitute a part of the Tatalina group, in the Fairbanks quadrangle. The Tatalina group lies stratigraphically between rocks of Middle Ordovician and pre-Cambrian age, and therefore this correlation seems a possible interpretation. In Cairnes's description of the lithology of the Tindir group, which he assigned to the Lower Cambrian or pre-Cambrian (see extract, p. 22), he mentions the high hematitic content of some of the Tindir shales and quartzites. This is most suggestive and leads to the inference that these red beds will ultimately be found to be a part of the Tindir sequence. The geographic position of the Tindir group, which lies along the strike and 8 miles to the north of the red beds. here described, may be taken as corroborative evidence of this correlation. UNDIFFERENTIATED PALEOZOIC ROCKS DISTRIBUTION The undifferentiated Paleozoic rocks have been divided for cartographic purposes into two groups, of which one includes all the dominantly noncalcareous rocks and the other includes essentially the calcareous and dolomitic rocks. The dominantly noncalcareous undifferentiated Paleozoic rocks are found chiefly in the valley of Slate Creek, a tributary of the North Fork of Fortymile River; in the headwaters of the Charley and Salcha Rivers; in a zone extending from the head of the Seventymile River to the southeast corner of this district; in a bifurcated belt that extends eastward from Thanksgiving Creek to Washington Creek and northwestward into the Crazy Mountains, south of Circle; along the international boundary in an irregular zone that extends from the valley of the Tatonduk River northward to Hard Luck Creek. Some smaller areas included with this group of rocks are a small belt of slate-quartzite rocks that crops out along the north bank of the Yukon above Eagle and extends southeastward up the Yukon Valley; certain areas on the north side of the Yukon below Calico Bluff; and a belt extending along the south bank of the Yukon from Fourth of July Creek downstream intermittently to Glenn Creek. The group that extends southeastward from the head of the Seventymile River continues up the Yukon River beyond the limits. of the area shown on the accompanying map in the direction of Fortymile and comprises some of the rocks included by Cairnes 28 as part of his pre-Cambrian or Yukon group. The reassignment of these rocks from pre-Cambrian to undifferentiated Paleozoic is made on the basis of their lithology and of certain fossils found in them. The undifferentiated calcareous and dolomitic rocks of the Paleozoic, because of their striking lithologic differences from the other undifferentiated Paleozoic rocks, are shown by a separate pattern on the accompanying geologic map. They occur chiefly as follows: A limestone belt crops out along the international boundary midway between Eagle Creek and the Yukon and continues southeastward up the Yukon Valley and northwestward from Eagle up the north side of Mission Creek; two great masses of limestone and dolomite are found along the international boundary, one cropping out in the valley of the Tatonduk River and extending northward to Hard Luck Creek, the other extending from Cathedral Creek northward almost to Ettrain Creek; a belt of limestone lies northwest of Nation, extending from Spring Creek northwestward across Bull Creek to Logan Creek; a small wedge of limestone crosses Woodchopper Creek about 3 miles from its mouth. 25 Cairnes, D. D., op. cit., map 140A. |