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71

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 To estimated the thickness of the Nation River formation at 3,700 feet. Cairnes " 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

TO 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.

71 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 72 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

73 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.

PERMIAN SERIES

TAHKANDIT LIMESTONE

DISTRIBUTION

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

place name would have been Nation River, but this has already been used for a formation of Pennsylvanian (?) age. The next best place name is Tahkandit, the old Indian name of the Nation River, and this Permian limestone is therefore designated the Tahkandit limestone.

LITHOLOGY

At the type locality along the Yukon River above the mouth of the Nation River the Tahkandit formation consists essentially of a cream-colored to white massive limestone, with some beds of conglomerate, sandstone, and shale in the lower half. The best section is seen along the southwest bank of the Yukon, where both the overlying and underlying rocks are exposed. (See fig. 6.) The section as measured by the writer along these bluffs, stated from the top downward, is as follows:

Section of Permian and Pennsylvanian(?) rocks on southwest bank of Yukon River above mouth of Nation River

Tahkandit limestone:

Feet

Massive cream-colored to light-gray limestone, containing
numerous highly fossiliferous beds. Collection 5839__ 133
Massive limestone grading downward into fine conglom-
erate composed of gray and green chert pebbles. Col-
lection 5839c

90

Conglomerate of gray and green chert pebbles. Highly
fossiliferous

11

Thin beds of conglomerate alternating with green and
brown shales. Lower half covered. Collection 5839b_
Massive cream-colored limestone, grading downward into
conglomerate. This part of the section shows a small
vertical fault striking N. 55° W., along which rocks
are downthrown 4 feet on southwest side____.
Fossiliferous conglomerate, which becomes coarser at
base. Collection 5839a

Beds of conglomerate 1 to 2 feet thick, alternating with
sandstone. The sandstone bed directly under the 20-
foot bed of conglomerate shows on its top curious ir-
regular imprints 4 to 12 inches long that resemble a
ropy lava that was cooled while being agitated; these
are believed to be of inorganic origin. This group of
beds includes several fossiliferous zones; a 1-foot bed
of green sandstone is particularly fossiliferous____
Nation River formation (Pennsylvanian?):

[blocks in formation]

Drab shale with occasional thin beds of quartzose sand-
stone. Rocks are exposed for 1,000 feet along beach,
but as they form an anticline, only about 300 feet of
strata are present------

300

Massive white limestone exposed in a syncline for 570
feet along beach

100

[merged small][merged small][graphic]

(PERMIAN)

A. ANTICLINE AND SYNCLINE IN TAHKANDIT LIMESTONE
ALONG THE NORTHEAST BANK OF THE YUKON RIVER JUST ABOVE THE
MOUTH OF THE NATION RIVER

[graphic]

B. CONGLOMERATE AND SANDSTONE OF THE NATION RIVER FORMATION ON THE EAST BANK OF THE YUKON RIVER A FEW MILES BELOW EAGLE

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