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ient is correct, is that it may show a gradual stratigraphic transition downward into the marine Calico Bluff formation, of upper Mississippian age. Brooks,64 in an earlier paper, without reference to any particular group of rocks, felt that an intermediate formation might well exist between the Calico Bluff and Nation River formations. He states:
It is not improbable that detailed mapping may reveal a considerable thickness of strata lying between the Calico Bluff and Nation River (formations) as here described. Whether such strata, if found, should be included in one or the other of these formations or be mapped as a distinct stratigraphic unit must be left to the future to be determined.
It is conceived by the writer that this so-called transitional formation does, in fact, represent such an intermediate stratigraphic horizon; and the evidence at hand indicates that this transitional formation is more closely related lithologically and stratigraphically to the Calico Bluff than to the Nation River formation. Brooks also believed that an unconformity existed at the base of the Nation River formation, and no data have so far been acquired that controvert that conclusion. If such an unconformity does in fact exist, the evidence is even stronger for relating this transitional formation with the Calico Bluff formation. Nevertheless, all available evidence of the age of these beds is still inconclusive, and it has therefore seemed best on the geologic map to designate them simply Mississippian or Pennsylvanian.
Another possible assignment of at least a part of this transitional formation should not be overlooked. From the lithologic descriptions it will be noticed that some of these rocks, particularly those south of the Seventymile River, resemble very much the Middle Devonian sequence at the head of the North Fork of Shade Creek. It is thus possible, in the absence of fossil evidence, to correlate these rocks with the Middle Devonian argillite-chert sequence, and this possibility will need to be considered in future work in this area.
No similar rocks occurring in this portion of the stratigraphic column are known elsewhere in interior or northern Alaska, so that no stratigraphic correlations can be made at present. The most striking lithologic similarity is apparent in reading Cairnes's description 65 of his “shale-chert group,” which he mapped as Carboniferous to Ordovician, and which in this report is assigned to the undifferentiated Paleozoic. Cairnes states in part:
Hard gray quartzitic shales are in addition somewhat extensively developed in places. These quartzitic beds contain locally sufficient iron to produce upon oxidation a bright-red to yellow coloration on weathered surfaces, but only rarely are these rocks red on a fresh surface. These reddish beds decompose readily to form a red or yellowish sand or mud, which is a very noticeable feature of many of the hillsides on which vegetation is lacking.
64 Brooks, A. H., and Kindle, E. M., Paleozoic and associated rocks of the upper Yukon, Alaska : Geol. Soc. America Bull., vol. 19, p. 292, 1908.
85 Cairnes, D. D., The Yukon-Alaska international boundary between Porcupine and Yukon Rivers : Canada Geol. Survey Mem. 67, p. 82, 1914,
This description fits very well the description of this transitional formation primarily given by the writer and suggests strongly that the rocks of this formation are also represented along the boundary north of the Yukon.
NATION RIVER FORMATION
The Nation River formation is exposed at two general localities along the Yukon. It crops out along both sides of the Yukon a short distance below Eagle and extends eastward up the valley of Shade Creek for an unknown distance, forming the bedrock at McCann Hill, a prominent dome-shaped hill along the international boundary. The other area consists really of two belts of the Nation River formation, separated by a zone of Permian limestone, which extend from the Yukon northeastward up the valley of the Nation River to and beyond the international boundary, although the distribution of this formation between the Yukon and the boundary is not exactly known. The belt south of the Nation River is believed to extend southwestward as far as the valley of Michigan Creek, but its limit has not been ascertained. A small anticlinal flexure brings the upper part of the formation to the surface in the middle of the Permian limestone just above the Nation River.
The rocks of this formation consist of gray clay shale, sandstone, and conglomerate and resemble very much the Upper Cretaceous and Eocene rocks, from which, indeed, they are at some localities hard to distinguish. Where this formation crops out about 3 miles below Eagle it consists essentially of a drab sandy clay shale and sandstone, the sandstone in beds from a few inches to 10 or 20 feet thick. This sandstone consists of grains of chert, decomposed feldspar, and more or less quartz and carbonaceous material. It weathers to a darkbrown color. The beds are commonly ripple marked but only slightly cross-bedded and show numerous mud lumps and concretions, especially along the bedding planes. Conglomerate beds with a thickness as great as 10 feet are intercalated with the shale and sandstone. This conglomerate is dark colored. It is composed of chert pebbles, mainly light gray and dark gray but with some green, set in a sandy matrix. Below Boulder Creek the beds are coarser grained and consist dominantly of sandstone in beds from 3 inches to 6 feet thick, with some thin partings of sandy shale and numerous beds of a conglomerate that contains pebbles as much as half an inch in diameter. Above Boulder Creek much comminuted plant débris occurs in these rocks, and below Boulder Creek large carbonized stems, some of which are 2 inches in diameter and 2 feet long, were seen in the sandstone, but no plant remains of diagnostic value in determining the age of the rocks were seen. Plate 10, B, shows a typical exposure of the conglomerate and sandstone of the Nation River formation.
To the east of the Boulder Creek area, particularly on McCann Hill, Cairnes 68 observed a massive conglomerate 60 feet thick, which he believed to represent the base of the Nation River formation. Overlying this conglomerate he reported 230 feet of brownish to nearly black grit, overlain in turn by another bed of conglomerate 25 to 30 feet thick, above which came sandstone, shale, and intercalated beds of conglomerate, whose combined thickness here was not known. The mere presence of the conglomerate beds does not necessarily imply that this sequence represents the base of the formation, for beds of conglomerate are clearly intraformational throughout the sequence. But the structure of the Nation River formation in the McCann Hill area is relatively simple, and the writer is inclined to believe that the sequence given by Cairnes does in reality represent the base of the formation.
Much the same assemblage of rocks, including shale, sandstone, and conglomerate, is exposed along the west bank of the Yukon from Trout Creek downstream to the Permian limestone. At and below the Nation River, on the northeast bank of the Yukon, is the type locality of the Nation River formation. The rocks exposed in the bluff just below the mouth of the Nation River are essentially gray clay shale, interbedded with one thick bed and several thinner beds of massive conglomerate. The thick bed of conglomerate, which apparently forms the top of the hill, consists of gray, red, and green chert pebbles and a few pebbles of quartzite, some of which are as much as 4 inches in diameter, in a matrix that is composed of fine fragments of cherty and sandy material with a white, possibly calcareous cement. These conglomerates appear to be intraformational.
A feature of special interest is the occurrence of a bed of bituminous coal in this formation on the southeast side of the Nation River, about three-quarters of a mile from its mouth. With regard to this coal Collier, 67 in 1902, wrote as follows:
Be Cairnes, D. D., op. cit., p. 89.
67 Collier, A. J., The coal resources of the Yukon, Alaska : U. S. Geol. Survey Bull. 218, pp. 35–36, 1903.
Only one bed of coal has been found at Nation River. The mine workings are abandoned and, having caved in, could not be examined. The face of the bluff on which they are located is subject to local slides by which the outcrops of coal are covered. A recent prospect hole near the top of the bluff showed about 2 feet of crushed coal and shale standing nearly vertical,
W. E. Williams, who was superintendent of the mine, informed the writer that the coal here was never well defined. The coal was found in pockets and kidneys often as large as 8 feet thick and 13 feet long. When the mine was abandoned a large body of this kind that had been located was left unmined. Large pieces of this coal were found in the creek bed before the coal body was located.
The coal mined at Nation River is distinct in character from any other coal mined on the Yukon. It is a bituminous coal containing a low percentage of water and showing no traces of woody structure. If these coals prove to be of Kenai [Eocene] age, the differences in their composition may be accounted for by the greater degree of metamorphism which they have suffered. An analysis was made of a sample taken from a large pile, probably 100 tons, mined in 1898. It had been exposed to the weather on the river bank since that time, but apparently was not greatly altered. It had the following composition :
Analysis of coal (sample 68) from Nation River mine
[Analyst, E. T. Allen, U. S. Geological Survey]
1. 39 40. 02 55. 55 3. 04
2. 98 Fuel ratio
1. 39 This coal makes a good coke in the laboratory. A large part of the coal was dry and unfrozen and of good quality, while a smaller part was frozen and almost worthless as fuel. The distribution of the frost was probably due to the circulation of water through the bodies of coal.
In 1897 the Alaska Commercial Co. attempted to open a coal mine at this place, and about 2,000 tons of coal were mined and sledded to the Yukon, to be burned on river steamers or transported to the Dawson market. Owing to the irregularity of the coal deposit and the consequent uncertainty of the supply and to the expense of mining it was abandoned several years ago. During the summer of 1902 one man was prospecting and attempting to relocate the coal bed. This coal is of better quality than that of any other mine of the region, except for the large percentage of sulphur, but the disturbed condition of the seam makes it doubtful whether it can be worked at a profit.
The coal-bearing bed at the Nation River is now completely concealed, but if no fault is present it appears to be overlain by a body of
gray clay slate with thin interbedded sandstones, which in turn is overlain by massive beds of conglomerate similar to that seen on top of the hill northwest of the mouth of the Nation River. Still farther up the hill slope to the east are exposed other beds of shale and sandstone.
On the opposite (southwest) side of the Yukon an anticlinal flexure within the Permian limestone brings to the surface the uppermost part of the Nation River formation, which here consists dominantly of a drab shale with some thin beds of quartzose sandstone. At this locality the evidence favors strongly the idea of a continuous gradation from the Nation River formation upward into the overlying Permian limestone.
STRUCTURE AND THICKNESS
Between Eagle and Calico Bluff the rocks of the Nation River formation are folded, in places rather closely. In the exposures below Boulder Creek one recumbent fold was noted. In general, according to Blackwelder,88 the axes of the minor folds pitch gently N. 75o W., and this structure corresponds roughly, though not exactly, to the structure at Calico Bluff, where a well-defined synclinal basin pitches gently N. 30° W. Although the structure at Calico Bluff is unusually simple, so far as that general area is concerned, it would not be justifiable to extrapolate that synclinal structure for any great distance in any direction from Calico Bluff. The fault zone at and above Eagle, which might well be called the Eagle overthrust, lies only a few miles south of the Boulder Creek locality, and there is every reason to believe that other parallel faults are present trending in this same general direction. Unfortunately there is an alluvium-filled basin between the Boulder Creek locality and Calico Bluff as well as west of Calico Bluff, and in this zone important structural evidence is doubtless buried. On the east side of the Yukon, however, at low water the Nation River formation may be seen along the banks lying unconformably upon older rocks of undetermined age.
At the mouth of the Seventymile River and again on the west bank of the Yukon opposite the mouth of the Tatonduk River there is good evidence of faults trending in the same general directionthat is, N. 75o W. Both the regional structure and the distribution of formations northwest, west, and southwest of Calico Bluff therefore indicate the presence of a fault or perhaps a zone of faulting west of Calico Bluff which trends N. 75o W., and under this interpretation the Nation River rocks in the Boulder Creek area would be bounded on the northeast by such a fault zone.
The Nation River rocks in the valley of Shade Creek, including the McCann Hill area, seem to have a structure that is somewhat simpler and more nearly comparable with the type of folding at Calico Bluff. They may constitute a block of relatively slightly deformed rocks that extends from Calico Bluff southeastward up
63 Blackwelder, Eliot, unpublished notes.