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formation in the lower Kuskokwim Valley of southwestern Alaska, and Brown 77 found it represented by a limestone formation in the upper Kuskokwim Valley southwest of Lake Minchumina.
In the Alaska Range region Permian rocks are extensively developed. Rocks 6,000 to 7,000 feet thick in the headwater region of the Chistochina River were described by Mendenhall 78 under the name Mankomen formation. This formation consists of limestone, sandstone, and shale. Farther south, in southern Alaska, Artinskian fossils have been found by Capps 70 in a limestone on Skolai Creek, at the head of the White River, and it is possible that the Permian rocks continue on around the northeast side of the Wrangell Mountains to join with the Mankomen formation farther northwest. In southeastern Alaska Permian rocks were found first on Kuiu Island and are now known to be extensively developed on Kupreanof, Admiralty, and Suemez Islands, as well as at other localities.
UPPER TRIASSIC SERIES
In the Yukon Valley, as practically everywhere else in Alaska, the only part of the Triassic system present is the Upper Triassic, and in the Eagle-Circle district Upper Triassic rocks are known only at two localities. At the Nation River Upper Triassic rocks adjoin the Permian limestone on the northwest. In the valley of Trout Creek, about 12 miles farther up the Yukon, the Upper Triassic rocks lie southeast of the Permian limestone.
At the Nation River locality Upper Triassic rocks crop out on both sides of the Yukon, but are somewhat better exposed, particularly at low stages of the river, on the southwest bank. A tape traverse from the farthest point downstream where such rocks are exposed upstream to the Permian limestone showed the following section. The distances given are horizontal distances along the river beach, but the general structure and thickness are shown graphically in Figure 6.
77 Brown, J. S., The Nixon Fork country : U. S. Geol. Survey Bull. 783, pp. 105-106, 1926.
78 Mendenhall, W. C., Geology of the central Copper River region, Alaska : U. S. Geol. Survey Prof. Paper 41, pp. 40–51, 1905.
78 Capps, s. R., Mineral resources of the Nabesna-White River district, Alaska : U. S. Geol. Survey Bull. 417, pp. 18–24, 1910.
Seotion of Upper Triassic rocks on Yukon River at the mouth of Nation River
Fossiliferous black shale. Strike is N. 70° E. Dip is 70° S.
in downstream part of section but becomes gradually less
argillaceous limestone about 1 foot thick. Collection 13423. 150
so many reversals of dip due to numerous small folds that
joining to the southeast are the uppermost beds of the
100 This section, imperfect as it is, shows that these Upper Triassic rocks consist essentially of black bituminous shale, interstratified with thin beds of gray to black limestone. Much of the black shale exposed at this locality is oil shale, but none of these rocks were tested quantitatively for oil.
The oil shale at Trout Creek was noted by P. J. Hillard, of Eagle, as early as 1901, and in October, 1915, he sent a sample to the United States Geological Survey. No distillation test of the shale was made at that time, however, the specimen being apparently of more interest on account of its contained fossils than on account of its bituminous nature. The shale from Trout Creek consists of a mat of closely compressed shells of Pseudomonotis and Halobia in a matrix of black shale. The fossil shells are so numerous and so closely compressed that not even the thinnest sliver of shale can be discovered without the impression of a shell upon it. A recent microscopic examination by Miss Taisia Stadnichenko, of the United States Geological Survey, indicates that the bituminous material originated from spores. A sample of this oil shale collected by the writer in 1925 was distilled in the chemical laboratory of the Geological Survey by E. T. Erickson, whose report is given below:
The sample as received appeared to have been exposed to weathering, which is likely to influence the character as well as the yield of the oil. For further chemical test the shale deposit should be resampled at a position unaffected by weathering.
A distillation test was made according to the oil-shale distillation method used by the Bureau of Mines so for determining the yield of crude shale oil. The total time required for the distillation was one and one-half hours; rate of oil distillation, 0.5 cubic centimeter a minute.
Upon comparison with other crude oils that were obtained from typical oil shales by the Bureau of Mines distillation apparatus, using a similar distillation rate, the crude oil obtained from the sample of shale from Trout Creek may be described as high in gravity and low in setting point. In these respects it is more nearly like the crude oil obtained from the Kentucky oil shale.
The chemical composition of oil shales has not yet been sufficiently investi. gated to compare them with typical petroleums, such as paraffin-base petro leum, which is low in specific gravity and high in content of paraffin series hydrocarbons; or a naphthene-base petroleum, which is largely composed of naphthene series and other cyclic hydrocarbons; or a mixed-base petroleum, which is intermediate in gravity and composition between paraffin-base and naphthene-base petroleums. The high gravity of the crude oil obtained from the shale of Trout Creek favors its commercial use for the production of lubricating oils. The low setting point indicates the absence of commercial quantities of paraffin.
Oil shale is also known at other places in Alaska. A shale phenomenally high in its content of oil has recently been found in the valley of the Christian River, about 75 miles north of Fort Yukon. This shale contains 122 gallons of oil to the ton of rock and is therefore of higher grade than any oil shale so far found in the United States. Another bituminous deposit, first noted by Ensign (now Rear Admiral, retired) W. L. Howard in his traverse down the Etivluk River, northwestern Alaska, in 1886, has recently been revisited by P. S. Smith 81 This material has now been determined by David White, of the United States Geological Survey, as tasmanite. Similar material was also found by Smith on the Kivalina and Meade Rivers, in northwestern Alaska.
STRUCTURE AND THICKNESS
On account of the softness of this Upper Triassic shale and limestone the rocks of this formation crumble when exposed to the atmosphere and are quickly eroded away. In addition the incompetency of these soft beds has rendered them particularly subject to deformation. The resulting poor exposures and accentuated folding make it difficult to obtain any satisfactory idea of the structure. The generalized section shown in Figure 6 conveys the writer's impression that these rocks are welded into a mass of small folds with a general northwesterly dip. A narrow covered zone separates the lowest of the Upper Triassic rocks from the uppermost Permian beds, but the two formations appear at least to agree in strike and dip. Little doubt can exist, however, that the contact between the two formations represents a great hiatus in sedimentation that corresponds geologically to Middle and Lower Triassic time and probably also to upper Permian time.
80 Karrick, L. C., A convenient and reliable retort for assaying oil shales for oil yield: Bur. Mines Rept. 2229, 1921.
81 Smith, P. S., and Mertle, J. B., Jr., Geology and geography of northwestern Alaska: U. S. Geol. Survey Bull. 815, p. 283, 1930.
The top of the Upper Triassic sequence is not exposed, and therefore the total thickness of the formation can not be given. Martin, 82 who examined these rocks in 1914, estimated that at least 400 feet of strata are exposed along the beach opposite the mouth of the Nation River. Blackwelder,83 in 1915, noted that the visible exposures indicated a thickness of 575 feet and possibly several times as much. The section given in Figure 6, though in part idealized, is plotted by the writer from the observed strike and dip and indicates that Martin's estimate is about correct, as nearly as may be judged from the available exposures.
AGE AND CORRELATION
Twenty fossil collections have been made from these Upper Triassic rocks, mostly from the Nation River locality. This fauna has been assembled by the writer in the table given below. T. W. Stanton, of the United States National Museum, has identified all this material.
& Martin, G. C., Triassic rocks of Alaska : Geol. Soc. America Bull., vol 27, pp. 701-702, 1916.
88 Blackwelder, Eliot, unpublished notes.
Fossils from Upper Triassic rooks along. Yukon River near mouth of Nation
Camarophoria? ct. C. crumena.
XX: Xi X
4054. Yukon River a quarter of a mile northeast of mouth of Nation River. Collector, E. M. Kindle.
8895. Yukon River, southwest bank about 1 mile above Nation River. From a 10-foot bed of dark noncrystalline limestone, which is probably not more than 50 feet above the crystalline Permian limestone. Collector, G. C. Martin.
8896, 8897, 8898, and 8899. Yukon River, southwest bank about 1 mile above Nation River. 8896 is about 31 feet stratigraphically above 8895. 8897 is about 10 feet stratigraphically above 8896. 8898 is from float along river bank between localities 8897 and 8899. 8899 is about 300 feet stratigraphically above 8897. Collector, G. C. Martin.
10266 and 10267. Trout Creek about 3 miles from confluence with Yukon River. Collector, G. C. Martin.
9382. Yukon River, southwest bank southwest of Nation River. Collector, Eliot Blackwelder.
9383. Yukon River, southwest bank about 2 miles above Nation. Collector, Eliot Blackwelder.
9384. Yukon River, southwest bank opposite Nation River. Collector, Eliot Blackwelder.
9385, 9387, and 9388. Hillside one-third of a mile northeast of mouth of Nation River. Collector, Eliot Blackwelder.