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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.
As no complete section of the Nation River beds has yet been found, the thickness of this formation is somewhat in doubt. In the Boulder Creek area the structure is complex, and the Nation River beds are bounded at least on one side by a fault zone. On the northwest side of the Nation River the structure is somewhat simpler, but the exposures are intermittent, and here also a fault zone lies at one edge of the formation. From Montauk Cabins down to the Nation River the sequence is perhaps the least disturbed, but everywhere the major structure is modified by minor folds; moreover, the base of the formation is not definitely exposed. Under these conditions no exact estimate of the thickness can be given. The areal distribution of the formation, however, interpreted in the light of its general structure, suggests a great thickness of rocks. Brooks 70 estimated the thickness of the Nation River formation at 3,700 feet. Cairnes 71 estimated its thickness, as seen along the international boundary, at 4,000 feet, but he evidently included with the Nation River formation some limestone beds of Permian age. Blackwelder 72 suggested the possibility that about 5,000 or 6,000 feet of strata are represented by this formation, but he was properly cautious with regard to the reliability of such figures. The writer has no data sufficient to favor any one of these estimates as against the others. All three are of the correct order of magnitude-that is, from about 4,000 to 6,000 feet. Brooks apparently did not recognize the presence of a fault zone up the Nation River, and his estimate might accordingly be rated as a little low. On the whole, Blackwelder's estimate of 5,000 to 6,000 feet is probably as nearly correct as can be given from the data so far collected.
AGE AND CORRELATION
The Nation River formation, so far as it has been studied along the Yukon by the writer and by American geologists who preceded him, appears to be nonmarine in origin. Many fragmentary plant remains are present in these rocks, though unfortunately no material of specially diagnostic character has so far been collected. On the other hand, no marine fossils have been found. This condition, considered in connection with the ripple marks, cross-bedding, and muddy concretionary forms, suggests that these rocks are of fluviatile origin. At the Nation River locality, however, they grade upward into marine deposits, and the conclusion is therefore reached that the place of their formation may have been at so short a distance from marine waters that a relatively slight shifting of the strand line
70 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.
Cairnes, D. D., The Yukon-Alaska international boundary between Porcupine and Yukon Rivers: Canada Geol. Survey Mem. 67, p. 90, 1914.
72 Blackwelder, Eliot, unpublished notes.
sufficed in early Permian time to change the conditions of deposition from nonmarine to marine.
Several collections of these fragmentary plant remains have been made by geologists, a brief summary of which is here given:
2970. Yukon River, east bank 2 miles below Tatonduk River. Collector, A. J. Collier. Lepidodendron? sp.
3AH7. Yukon River, west bank 3 miles above Nation River. Collector, Arthur Hollick. Spirophyton sp.
1655. Yukon River, northwest bank 5 miles north of Eagle. Collector, E. M. Kindle. Lepidophyte group.
Martin 81. Yukon River, east bank 2 miles below Tatonduk River. Collector, G. C. Martin. Specimens not identified.
Martin 89. Southeast bank of Nation River half a mile above mouth, at coal mine. Collector, G. C. Martin. Specimens not identified.
1501/19. Yukon River, 4 to 5 miles below Eagle.
Collector, Eliot Black
welder. Protolepidodendroid group. A variety of decorticated stems. 1507/I. Yukon River, north bank about 2 miles above Calico Bluff. Collector, Eliot Blackwelder. Indeterminate vegetal material.
1507/X. Yukon River, west bank 51⁄2 miles above Nation River. Collector, Eliot Blackwelder. Indeterminate vegetal material.
1507/63. Yukon River, north bank 11⁄2 miles below Nation River. Collector, Eliot Blackwelder. Bothrodendroid? group.
25AMt127. Yukon River, northeast bank about 71⁄2 miles N. 33° E. of Eagle. Collector, J. B. Mertie, jr. Specimens not identified.
David White, of the United States Geological Survey, who made the identifications noted above, is inclined, on the whole, to assign these plant remains to the lower Carboniferous, or Mississippian, but the material is so poor that little confidence can be placed in any age assignment that depends alone on the character of this flora. The stratigraphic relations therefore become of much more importance. It is fairly sure that the Nation River formation overlies the Calico Bluff formation and underlies the Permian limestone. The formation appears to grade upward into the Permian limestone on the west side of the Yukon just above the Nation River, but its relation to underlying formations is as yet obscure. Therefore the formation is apparently related more closely to the overlying Permian limestone than to the underlying upper Mississippian rocks. This Permian limestone, however, is so low in the Permian sequence that its contained fossils were originally identified by G. H. Girty as Artinskian and correlated with the Pennsylvanian rather than the Permian. The Nation River formation, therefore, may be in its upper part of earliest Permian age, but a better assignment for this thick sequence of rocks as a whole is believed to be Pennsylvanian. It is therefore here classified as Pennsylvanian(?).
No formation similar in character to the Nation River formation and of the same age is known anywhere else in Alaska. Collier 73
78 Collier, A. J., Geology and coal resources of the Cape Lisburne region, Alaska: U. S. Geol. Survey Bull. 278, pp. 18-19, 1906.
has described a coal-bearing fresh-water formation which is said to lie at the base of the Mississippian sequence of rocks at Cape Lisburne, on the Arctic coast, about 700 miles northwest of the type locality of the Nation River formation, but these two formations do not seem to be specially comparable, either in lithology or in age. It is probable that the Nation River beds represent a phase of sedimentation related to some great drainage system similar to but not necessarily coextensive with the present Yukon Basin, and as such they would probably not have any exact counterpart elsewhere in Alaska.
The Permian rocks herein designated Tahkandit limestone are restricted to four general localities. The type locality is along the Yukon just above the mouth of the Nation River (the old Indian name for which is Tahkandit), where a belt of such rocks crosses the Yukon, trending northeast. (See pl. 10, A.) Another belt crosses the valley of Trout Creek, a stream that enters the Yukon from the southwest about 8 miles above the mouth of the Nation River. A block of Permian limestone infaulted in the Middle Devonian volcanic rocks is exposed on the south bank of the Yukon a short distance below the mouth of Coal Creek. (See pl. 11, 4.) Permian rocks are found also along the international boundary near Ettrain Creek and 25 or 30 miles northeast of the mouth of the Nation River. The rocks near Ettrain Creek were mapped by Cairnes as undifferentiated Carboniferous, but a reexamination of Cairnes's Carboniferous fossil collections has shown the presence of several collections of Permian age, the localities of which, when plotted on the map, fall in or near these areas of undifferentiated Carboniferous. Moreover, the rocks there are limestone similar to the Permian limestone. The writer has therefore inferred the presence of Permian rocks on Ettrain Creek and has so shown these areas on the accompanying map. It is probable that the Permian limestone continues intermittently up the southeast side of the Nation River and connects with the Permian limestone at Ettrain Creek.
No formational name has previously been assigned to this Permian formation, but its lithology and fauna are so distinctive that a formational name is certainly warranted, notwithstanding the fact that the upper limit of the Permian sequence is not definitely known. This difficulty may be lessened by using the term "limestone " instead of "formation," in applying a formational name. The most fitting