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Carboniferous" in his legend arose from the fact that the Nation River formation is so similar to the Upper Cretaceous rocks that at many places it is difficult to differentiate between the two. From the distribution of the Lower Cretaceous rocks along the Yukon, the writer surmises that much of the rock along the international boundary from Sitdown Creek northward to the 66th parallel is most likely also of Lower Cretaceous age. It is believed that between Sitdown and Ettrain Creeks, however, Cairnes's "Cretaceous and Upper Carboniferous" group is more likely to be the Nation River formation. Hence, on the accompanying geologic map the country rock from Sitdown northward to the limits of the map is shown as Lower Cretaceous, and from the Nation River southward to Ettrain Creek the Nation River formation is indicated, with a blank unmapped area between the two formations. Another minor outcrop of Lower Cretaceous rocks is seen along the northeast bank of the Yukon about 10 miles below the mouth of Thanksgiving Creek. This narrow belt may be the northwestward continuation of the narrow belt of the same rocks that crosses Coal and Woodchopper Creeks.

Blackwelder 8 has used the term Kandik formation in referring to the Lower Cretaceous rocks of the upper Yukon, and inasmuch as these rocks are typically exposed in the valley of the Kandik River, from the Yukon northeastward probably to the boundary, this formation name seems particularly fitting and is here formally applied. To be sure, neither the top nor the bottom of the Lower Cretaceous rocks has been recognized in this area, but as Jurassic rocks are absent in interior Alaska this formation is known to lie everywhere unconformably upon rocks older than Lower Cretaceous; it is also believed to underlie unconformably the Upper Cretaceous series. The term Kandik formation may therefore be regarded as a formational name that includes all the Lower Cretaceous sedimentary rocks of this area, of whatsoever lithologic character, although it seems highly probable that only sandstone, slate, and conglomerate are represented.

LITHOLOGY

The Kandik formation consists in the main of a monotonous sequence of black slate and thin beds of sandstone. The slates are carbonaceous, argillaceous rocks, which in some of the thicker beds show little stratification. No calcareous shale or limestone was seen, and the slates appear not to be bituminous or oily, thus differing markedly from the Upper Triassic shales. Moreover, no chert or siliceous slate appears in the sequence, which thus differs from the lower Mississippian (?) rocks. These argillaceous members are ad

Blackwelder, Eliot, unpublished notes.

visedly for the most part termed slate rather than shale, for in most of them fracture cleavage is well developed. Plate 11, B, shows a typical exposure of the Lower Cretaceous quartzitic sandstone and slate. The sandstone beds show no sign of such cleavage, although some of them are much jointed.

The sandstones occur for the most part in thin beds, from a few inches to 1 or 2 feet thick, but some thick massive beds occur in the sequence. One very thick bed of this type is exposed along the south bank of the Yukon just below Glenn Creek. The thinner beds of sandstone are usually dark gray on a fresh break but weather to a dull-brown color, which is due probably to their content of ferrous iron. They are composed essentially of grains of quartz, with little chert but with a certain amount of altered feldspar and ferromagnesian minerals. The thicker beds are inclined to be more purely quartzose, and some of them by partial recrystallization closely approach quartzite. In the Rampart district, where these same Lower Cretaceous rocks occur, the heavy quartzose beds served the writer to a certain extent as horizon markers, inasmuch as they make prominent hogbacks; they will probably be useful in a similar way along the Yukon when more detailed mapping is attempted.

Along the banks of the Yukon no conglomerate was seen in the Lower Cretaceous sequence. A few miles upstream from the Kandik River, however, on the north side of the Yukon, a great thickness of these rocks is exposed at Kathul Mountain. The top of this mountain is 2,400 feet above the level of the Yukon, and in the course of a trip up the mountain the writer observed that the upper threefourths of the sequence was composed largely of sandstone and conglomerate, the conglomerate being particularly evident near the top. This conglomerate, however, is rather fine grained, and much of it might better be described as a grit, although some beds containing pebbles as large as 3 inches in diameter were seen. The component fragmental material is subangular to rounded and consists of quartz, chert, slate, and fragments of other dark-colored rocks, perhaps in part of volcanic origin. Except in the fineness of grain these conglomeratic beds do not differ materially from some of the conglomerates of the Upper Cretaceous and Eocene sequence. It is possible, indeed, that these conglomerates and grits may mark the base of the Upper Cretaceous, but the writer is inclined to believe that they form an integral part of the Lower Cretaceous series.

STRUCTURE AND THICKNESS

Little work was done by the writer on these Lower Cretaceous rocks. Such observations as were made, however, indicate that the rocks are considerably deformed, though the folds are for the most

part of the open type and of large amplitude, so that considerable stretches of river bluffs show what appears to be homoclinal structure. Some steep dips were noted at places along the river, and these, considered in relation to the nature of the foldings, suggest the presence of some large unrecognized faults. Large unexamined areas of such rocks occur between Glenn and Coal Creeks, and it is rather hazardous to extrapolate the visible structure into such unknown

areas.

The rocks at the west side of Kathul Mountain dip southeastward, and those at the east end dip northwestward, so that this mountain occupies approximately the center of a gentle syncline. As this mountain rises 2,400 feet above the Yukon, it is safe to say that at least 2,400 feet of Lower Cretaceous strata are present at this locality. Neither the top nor the base of this sequence has been recognized, however, and it is probable, therefore, that two or three times that thickness of strata may be present in this wide belt of rocks. No data are at hand for making any closer estimate of the stratigraphic thickness.

AGE AND CORRELATION

Fossils are rather scarce in the Kandik formation, but nevertheless a number of small collections have been made, which are adequate for determining the geologic age. This fauna, which has been determined by T. W. Stanton, of the United States Geological Survey, is listed below.

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2674. Washington Creek 6 miles above mouth. Collector, A. J. Collier. 3783. Yukon River, southwest bank about 400 yards below Glenn Creek. Collector, E. M. Kindle.

3784. Yukon River, south bank about 11⁄2 miles below Sam Creek. Collector, E. M. Kindle.

3785. Yukon River, north bank about 6 miles above Charlie Village. Collector, E. M. Kindle.

9389. Yukon River 81⁄2 miles above Washington Creek. Collector, Eliot Blackwelder.

62744-30- -10

13428. Yukon River, southwest bank just below Glenn Creek. Collector, J. B. Mertie, jr.

13429. East side of Woodchopper Creek about 1 mile in a straight line from Yukon River. Collector, J. B. Mertie, jr.

In addition to the invertebrate collections above enumerated, Blackwelder in 1915 found remains of a plant which was identified by F. H. Knowlton as Chondrites heerii Eichwald. This was found along the southwest bank of the Yukon about 11⁄2 miles above the mouth of the Kandik River.

Of the six genera of invertebrates that have so far been found in these rocks, Aucella crassicollis Keyserling may be said to be the type fossil. This fossil is particularly abundant at the locality on Woodchopper Creek, in beds of slate. Other undetermined species of Aucella are probably also present at this locality. Along the international boundary Cairness also made a number of collections of Lower Cretaceous fossils, from Sitdown Creek northward to the sixtysixth parallel. These were determined originally by T. W. Stanton, of the United States Geological Survey. (For explanation of numbers, see p. 48.)

XIV k 25:

Nucula sp.

Astarte? sp.

Panopaea? sp.

Undetermined pelecypod casts.

XIV q 31:

Aucella crassicollis Keyserling. XV a 31, 32:

Aucella crassicollis Keyserling.

Astarte sp.

XV h 30, 31, and 1 30:

Aucella crassicollis Keyserling. XV j 30:

Aucella crassicollis Keyserling.

This Lower Cretaceous horizon is represented at many places in Alaska. The locality nearest to the Yukon is in the Rampart district, where a great thickness of Aucella-bearing slate and quartzite is found. In northern Alaska Schrader 88 found Lower Cretaceous rocks on both the south and the north sides of the Brooks Range, and he gave to these two groups of rocks the designations Koyukuk and Anaktuvuk "series." In the recent work in northwestern Alaska 9 the Lower Cretaceous rocks north of the Brooks Range,

87 Cairnes, D. D., The Yukon-Alaska international boundary between Porcupine and Yukon Rivers: Canada Geol. Survey Mem. 67, pp. 105-107, 1914.

88 Schrader, F. C., A reconnaissance in northern Alaska: U. S. Geol. Survey Prof. Paper 20, 1904, pp. 74-77.

Smith, P. S., and Mertie, J. B., jr., Geology and geography of northwestern Alaska: U. S. Geol. Survey Bull. 815, pp. 196-207, 1930.

roughly equivalent to the Anaktuvuk group, were shown to continue westward in a more or less continuous zone from the Anaktuvuk River to the Arctic Ocean. This group is probably closely correlative with the Kandik formation of the Eagle-Circle district.

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91

In the Chitina Valley of southern Alaska two formations, the Kotsina conglomerate and the Kennicott formation, both believed to be in part at least of Lower Cretaceous age, were described originally by Rohn.90 Aucella-bearing shales were found by Capps o1 at the head of the Chisana River and are also known to exist to the east at the head of the White River. They were also found by Mendenhall 22 in the valley of the Nelchina River, a stream which heads against the Matanuska River and flows eastward to the Copper River.

In southwestern Alaska, east of Kuskokwim Bay, rocks of Lower Cretaceous age are included in the group differentiated by Spurr 93 as the "Oklune series." The Herendeen limestone of the Alaska Peninsula, as mapped by Atwood, is also of Lower Cretaceous age. Lower Cretaceous rocks are known on Admiralty and Etolin Islands, in southeastern Alaska.

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UPPER CRETACEOUS AND EOCENE SERIES

DISTRIBUTION

Rocks of Upper Cretaceous and Eocene age crop out in a continuous belt 1 to 15 miles wide for a distance of 85 miles from the international boundary northwestward to Woodchopper Creek and beyond. This belt extends southeastward from the boundary for an unknown distance into Yukon Territory. Along the Yukon these rocks crop out only at few places north and northwest of Calico Bluff. A small area of rocks at the head of the Charley River mapped by Prindle in 1911 as Upper Cretaceous (?) is also included here.

LITHOLOGY

Little additional work was done on this group of rocks during the season of 1925; they were examined at several localities on the north side of the Yukon, in Yukon Territory, and the northern boundary

so Rohn, Oscar, A reconnaissance of the Chitina Valley and Skolai Mountains, Alaska: U. S. Geol. Survey Twenty-first Ann. Rept., pt. 2, p. 431, 1900.

01 Capps, S. R., The Chisana-White River district, Alaska: U. S. Geol. Survey Bull. 630, p. 52, 1916.

* Mendenhall, W. C., A reconnaissance from Resurrection Bay to the Tanana River, Alaska U. S. Geol. Survey Twentieth Ann. Rept., pt. 7, p. 309, 1900.

Spurr, J. E., A reconnaissance in southwestern Alaska: U. S. Geol. Survey Twentieth Ann. Rept., pt. 7, p. 167, 1900.

4 Atwood, W. W., Geology and mineral resources of parts of the Alaska Peninsula : U. S. Geol. Survey Bull. 467, pp. 39-41, 1911.

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