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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 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 intra formational.
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:
ee 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
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, the axes of the minor folds pitch gently N. 75° 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. 75° 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. 75° 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
Blackwelder, Eliot, unpublished notes.
Shade Creek and is bounded by a fault zone on the southwest. On the northeast side of this area the rocks of the Nation River formation lie without apparent structural discordance upon Middle Devonian argillite and chert. This observation makes it necessary to postulate a discontinuity in sedimentation, if not an unconformity, at the base of the Nation River formation. If a structural unconformity does in fact exist, however, the evidence here indicates no great discordance in dip between the Nation River formation and the underlying beds.
The rocks of the Nation River formation south and east of the Nation River are also folded, but the larger structural relations are more evident. The distribution of the Permian limestone and the Nation River beds there suggests strongly the presence of a large anticline plunging southwest by west. The repeated minor folding of the Permian limestone at the Nation River, however, shows clearly that this is not a simple arch but an assemblage of minor folds welded into the larger anticline. Further evidence of this structure is present along the southwest bank of the Yukon between Montauk Cabins and the Permian limestone at the Nation River; for the rocks of the Nation River formation, which dip dominantly southeastward just below Montauk Cabins, are reversed farther downstream, as the formation plunges below the Permian limestone. (See fig. 6.)
At the Nation River locality the most striking structural feature of the Nation River formation and also of the Permian rocks is their regional strike or trend, which is about northeast and therefore nearly at right angles to the trend of the other Paleozoic formations farther up the Yukon. Such a structural trend might suggest that the Nation River and Permian sequence may not have been affected by the dynamic movements to which some of the older rocks were subjected, and this in turn suggests that the Nation River and Permian rocks may rest unconformably upon the Mississippian and older rocks. Brooks was firmly of the opinion that such an unconformity existed. But 20 years ago the presence of the Nation
Brooks, A. H., and Kindle, E. M., Paleozoic and associated rocks of the upper Yukon, Alaska: Geol. Soc. America Bull., vol. 19. p. 294, 1908.
sh, Shale; ls, limestone; cong, conglomerate; ss, sandstone FIGURE 6.-Section along southwest bank of Yukon River opposite mouth of Nation River, showing structural relations of Upper Triassic rocks, Tahkandit limestone (Permian), and Nation River formation (Pennsylvanian?).
River formation and the underlying transitional formation in the Eagle area had not been recognized. As these two formations in the Eagle area partake of the structure possessed by the neighboring Paleozoic rocks, the variance in regional structure in the Nation River area loses the significance that it was formerly given. Insufficient work has yet been done to explain this variant structure, but at least it certainly does not necessarily indicate the presence of a structural unconformity at the top of the Calico Bluff formation.
The Nation River rocks are again exposed on the northwest side of the Nation River and continue up that strea:n in a northeasterly direction to the international boundary, but the field evidence indicates that this belt of Nation River beds is separated from the Permian and Upper Triassic beds to the southeast by a great fault, or perhaps a zone of faulting. Nothing whatever is known of the attitude of this fault plane or fault zone, but the assumption of its existence is indispensable to explain the occurrence of Nation River beds on both sides of the northwestward-dipping Permian limestone. Nor is the extent of the throw known, but the northwest is apparently the upthrown side, for Nation River beds now occur at the surface where Upper Triassic or younger beds should normally be exposed.
In general, therefore, from data at present available, it may be said that the Nation River formation at its top appears to grade upward without any marked stratigraphic break into the Permian rocks. At its base a discontinuity of sedimentation has been definitely recognized, and a structural unconformity may exist, but no marked discordance in dip between the Nation River formation and underlying beds has been recognized. If the Nation River beds represent the beginning of terrestrial sedimentation following the elevation of this area above sea level in post-Mississippian time, there would probably be a gradual transition from marine to terrestrial sedimentation, with or without a structural unconformity, depending on whether or not the underlying Mississippian rocks were deformed during the regional elevation. As a matter of fact, discontinuities in sedimentation have been recognized in the Carboniferous-Triassic sequence at many places in Alaska, but so far as the writer is aware no structural unconformity in this sequence has yet been recognized elsewhere in Alaska. This generalization indicates that the regional elevation in Alaska, which began in Carboniferous time and culminated in the Triassic, was accompanied by a minimum of rock deformation, and it constitutes evidence that must be given some weight in the consideration of this problem. Such evidence, so far as it goes, is opposed to the idea that the Nation River formation rests unconformably upon Mississippian rocks.