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unconformably a limestone-dolomite series in which Cambrian fossils were found.
It is not believed by the writer that a continuous Devonian-Cambrian limestone sequence exists along the boundary. Unquestionably Devonian, Silurian, Ordovician, and Cambrian fossils have been collected from massive limestone and doubtless also at many places where it is impracticable to map separately the different limestone series. But all the stratigraphic data from the Paleozoic rocks of Alaska indicate that such limestone series are separated from one another by thick groups of essentially noncalcareous rocks and also by one or more unconformities. The great unconformity between the upper Silurian and the Middle Devonian rocks, as seen elsewhere in central and northern Alaska, is an example of such discontinuities of sedimentation. Hence the statement that the Tindir group lies beneath a Devonian-Cambrian limestone sequence implies so much doubt with regard to the areal distribution and interrelations of the several limestones within that range of age that such a statement fails to be convincing. The first two reasons above given are therefore discounted at the outset.
At Jones Ridge, however, a different condition appears to prevail. There only Cambrian fossils were found in the limestone along the south flanks of the Tindir group, and Cairnes saw the Cambrian limestone resting unconformably upon the Tindir. Also two recorded strikes and dips on Jones Ridge within the Cambrian limestone area show the limestone striking in a general northerly to northeasterly direction and dipping east to southeast, and this attitude of the beds, if continued to the Cambrian-Tindir contact, would undoubtedly place the Cambrian stratigraphically above the Tindir. Hence it seems reasonably sure that the rocks mapped as Tindir group north of Jones Ridge in fact underlie the Cambrian limestone of Jones Ridge. Cairnes's Cambrian fossils, however, have been determined as essentially Upper Cambrian in age, with one lot questionably Middle Cambrian. But if, as seems likely, the Middle Cambrian is definitely represented at Jones Ridge, and Middle Cambrian limestone rests unconformably upon the Tindir group, it seems probable that the Tindir group is, as stated by Cairnes, Lower Cambrian or older. Cairnes, evidently believing that this unconformity represents a discontinuity of major magnitude, was inclined to stress the possible pre-Cambrian age of all or at least a part of the Tindir group. This interpretation, though by no means proved, is a most suggestive one and is fraught with far-reaching consequences in connection with early Paleozoic and pre-Cambrian correlations. Cairnes suggests, for example, the correlation of the Tindir group with the Tatalina group, of the Fairbanks district, and this corre
lation, if made with the lower part of the Tatalina group, seems indeed to be a very logical one, for the Tatalina includes rocks of Lower Ordovician and probably of Cambrian and pre-Cambrian age. If the Tindir group should finally be proved to be pre-Cambrian in age, it would naturally be correlated with some part of the Algonkian system; and if the Tatalina group should also be found to include rocks of that age, the Birch Creek schist, which has heretofore been regarded merely as undifferentiated pre-Cambrian, would probably come to be regarded as Archean. Much, therefore, hangs upon a definite age determination of the Tindir group in its type locality, and future geologic work along the boundary should have this for one of its most important objectives.
So far as known, the red beds, which are here regarded as underlying the Middle Cambrian, have been seen only at one locality, in the valley of the Tatonduk River about 3 miles downstream from the international boundary. They crop out on both sides of the valley, ending sharply to the east against Silurian and later Paleozoic rocks; to the north or northeast, however, along the strike, they appear to continue for a number of miles into the heart of the Ogilvie Mountains.
These rocks may be described collectively as red beds. As seen in. the bluffs along the Tatonduk River they consist of red sandstone and sandy shale, with layers of almost pure red hematite as well as layers of hematitic breccia. The fragmental material of the breccia consists of poorly assorted subangular to angular pebbles and cobbles that range in diameter from a few inches to a foot. The red shale is well indurated in the outcrops along the valley walls, but it disintegrates rapidly on exposure to the weather and is seen on the gravel bars downstream at many places as lumps or mounds of red clay or mud. On the slopes along the south side of the Tatonduk River it was found that the red beds contained a large proportion of conglomerate. At an altitude of 1,400 feet above the valley floor on this south side of the valley a massive bed of red conglomerate perhaps 75. feet thick was seen continuing up the spur from the creek. It consists of angular pebbles of quartz, chert, slate, and phyllite. This conglomerate is followed by red sandstone and slate, which in turn are succeeded at 1,600 feet above the creek by another bed of red conglomerate about 100 feet thick, composed of subangular to well-rounded pebbles, cobbles, and boulders, the largest of which are 2 feet in diameter.
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 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.
*Calrnes, D. D., op. dt., pp. 91-92.
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 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
** Cairnes, D. D., op. cit., pp. 92-93.