738. Hard Luck Creek, 5.93 miles north and 0.33 mile west from intersection of one hundred and forty-first meridian and sixty-fifth parallel: Fenestella sp. Eatonia? sp. Pugnax sp. Atrypa reticularis. Martinia cf. M. maia. 739. 6.75 miles north and 0.93 mile east from intersection of one hundred and forty-first meridian and sixty-fifth parallel : Cyathophyllum sp. Leptaena rhomboidalis. Atrypa reticularis. 740. Hard Luck Creek, 5.93 miles north and 0.47 mile west from intersection of one hundred and forty-first meridian and sixty-fifth parallel: Fenestella sp. 741. Hard Luck Creek, 5.93 miles north and 0.33 mile west from intersection of one hundred and forty-first meridian and sixty-fifth parallel: Chonetes sp. 742. 0.55 mile north and 2.1 miles west from intersection of one hundred and forty-first meridian and sixty-fifth parallel : Stropheodonta sp. The locality of the collection from the Middle Devonian limestone of the Tatonduk River made by the writer and the determination of its contained fossils by Edwin Kirk, of the United States National Museum, are given below: 25AMt161 (2063). North bank of Tatonduk River, 0.38 mile N. 41%1⁄2° W. of international boundary topographic station 104: Alveolites sp. Striatopora sp. Acervularia cf. A. arctica (Meek). Heliophyllum sp. Favosites cf. F. polymorpha Goldfuss. Gypidula comis Owen. Atrypa reticularis (Linnaeus). Atrypa near A. hystrix Hall. Chonetes cf. C. pusilla Hall. Camarotoechia sp. Finally, it should be remembered that this wealth of paleontologic material comes mainly from a narrow zone along the international boundary, from the Yukon River northward to the Nation River, a distance of about 60 miles. Numerous stratigraphic horizons in the Paleozoic have already been recognized, and others are doubtless represented. When a topographic map shall have been made of the 600 square miles lying in the triangle between the boundary and the Yukon and Nation Rivers, the geologist will have before him probably the most interesting piece of geologic mapping of Paleozoic rocks that exists in Alaska. Cambrian, Ordovician, Silurian, Devonian, and Carboniferous rocks are all represented, and the recognition and mapping of all the formations in these five systems should lead to a vast amount of information regarding the history of the Paleozoic era in the Yukon Valley. As no formational and little group mapping has been done along the boundary, not a great deal can be said as yet regarding the correlation of the rocks there with Paleozoic groups and formations elsewhere in Alaska. Paleontologically, however, certain correlations stand out strikingly, of which the Silurian, Devonian, and Mississippian faunas afford the best examples. One of the prominent horizon markers of central and northern Alaska is the great middle Silurian limestone developed in the White Mountains north of Fairbanks and from Kotzebue Sound eastward for 600 or 700 miles to the Chandalar River. To judge from the middle Silurian fossils collected by Cairnes and from the fact that Kindle 3 also collected middle Silurian fossils from the dolomite just above the lower ramparts of the Porcupine River, it would seem that this middle Silurian sequence is probably continuous with beds at the same horizon in northern Alaska. Similarly, the upper Silurian fossils collected along the boundary by Cairnes suggest the presence there of the upper Silurian (?) formation mapped by Smith and Mertie 34 in northern Alaska, correlative with a similar formation in central Alaska that overlies the middle Silurian limestone of the White Mountains. The Middle Devonian fauna along the boundary may also be correlated with a similar fauna widely known in interior and northern Alaska. The type locality for this fauna in northern Alaska is on the Porcupine River at the mouth of the Salmontrout River in a limestone called by Kindle 35 the Salmontrout limestone. No fossils that were distinctly Upper or Lower Devonian were collected by Cairnes along the boundary. The absence of Lower Devonian fossils is to be expected, as such fossils have not yet been found anywhere in Alaska. But an Upper Devonian fauna, characterized principally by Spirifer disjunctus, is well developed in northern Alaska, and it is possible that some of the other Devonian collections made by Cairnes, which were believed by Kindle not to belong to the Middle Devonian sequence, may in fact belong in the Upper Devonian. The Carboniferous fossils found by Cairnes along the international boundary may be correlated closely with Carboniferous faunas found elsewhere in interior and northern Alaska in that they can be split into two groups, one of late Mississippian and one of Permian age. The type upper Mississippian formation of interior Alaska is Kindle, E. M., Geologic reconnaissance of the Porcupine Valley, Alaska: Geol. Soc. America Bull., vol. 19, p. 324, 1908. Smith, P. S., and Mertie, J. B., jr., Geology and geography of northwestern Alaska: U. S. Geol. Survey Bull. 815, pp. 132-139, 1930. Kindle, E. M., op. cit., p. 229. the Calico Bluff formation, on the Yukon, which is described on pages 101-106, and that of northern Alaska is the Lisburne limestone, most recently described by Smith and Mertie.36 The type Permian formation for central Alaska is the Tahkandit limestone, at the mouth of the Nation River, described on pages 125-127 of this report; and for northern Alaska, the Sadlerochit sandstone, originally described by Leffingwell as Pennsylvanian but later assigned by Girty to the Permian. 37 No great masses of limestone such as those that occur along the international boundary north of the Yukon are known along the Yukon between Eagle and Circle, but two small belts of undifferentiated limestone are found in this stretch of the river at Bull Creek and Woodchopper Creek. No fossils have been found in either of these, and their exact age is therefore unknown. The typical middle Silurian limestone as developed elsewhere in central and northern Alaska is so thick and so prominent that it is not believed that these small limestone belts can belong to the same horizon. The Middle Devonian limestone along the Yukon is associated with volcanic rocks, a condition which does not appear to exist with the belts of limestone under consideration. The Permian and Mississippian limestones are so extremely fossiliferous that it seems impossible that these undifferentiated limestones could belong to either of those horizons. There remain, so far as our present stratigraphic knowledge goes, the Cambrian, Ordovician, and upper Silurian limestone horizons, and of these the surrounding stratigraphy suggests more strongly the upper Silurian. The limestone bands crossing Bull Creek and Woodchopper Creek are therefore believed to be of possible upper Silurian age, but in the absence of conclusive proof they are mapped as undifferentiated limestone. CAMBRIAN SYSTEM For cartographic purposes the differentiated Cambrian and underlying rocks are shown in five units, as follows: An Upper Cambrian limestone; a Middle Cambrian limestone; a Middle Cambrian or older formation that underlies the Middle Cambrian limestone; a Lower (?) Cambrian or pre-Cambrian red-bed formation that forms the base of the visible stratigraphic sequence on the Tatonduk River; the Tindir group, as mapped by Cairnes, which is here interpreted as Cambrian or pre-Cambrian. The last two of these groups have already been described under the heading "Cambrian or pre-Cambrian rocks." 30 Smith, P. S., and Mertie, J. B., Jr., Geology and geography of northwestern Alaska: U. S. Geol. Survey Bull. 815, pp. 168-185, 1930. 7 Leffingwell, E. de K., The Canning River region, northern Alaska: U. S. Geol. Survey Prof. Paper 109, pp. 113-115, 1919. MIDDLE CAMBRIAN OR OLDER ROCKS DISTRIBUTION The rocks here described as Middle Cambrian or older constitute a mappable unit that lies geographically and stratigraphically between the red beds and the Middle Cambrian limestone. The known exposures are restricted to the Tatonduk River. LITHOLOGY This formation consists of beds of dark-gray limestone and dolomite from 6 inches to 3 feet thick, interbedded with shale and argillite, the latter perhaps somewhat calcareous. Much of the clay shale is nodular. There are also a few beds of limestone grit, a finegrained phase of the limestone conglomerate described on page 66. Two such beds, the upper 25 feet thick and the lower 5 feet thick, separated by red shale, occur at the very base of this formation and show that the formation grades downward without a stratigraphic break into the red beds. The apparently concordant structure suggests that this formation also grades upward into the massive beds at the base of the Middle Cambrian limestone, but this relation can not be regarded as proved. STRUCTURE AND THICKNESS This formation is not continuously exposed in the bluffs along the Tatonduk River, but the visible exposures indicate a rather uniform westerly dip, ranging from 20° to 30°. Geographically, this formation occupies a belt along the Tatonduk about half a mile wide, measured across the strike, and the maximum thickness can not, therefore, much exceed 1,000 feet. This estimate, however, may be materially lessened if the structure should be found to be more intricate. AGE AND CORRELATION No fossils have yet been found in these rocks, and their age can therefore not be given on paleontologic grounds. They underlie the Middle Cambrian limestone and are thus Middle Cambrian or older. Much uncertainty is involved in a more definite age assignment, because of the possibility that an unconformity may exist at the base of the Middle Cambrian limestone. No positive evidence of such an unconformity was seen on the Tatonduk River, but Cairnes's data concerning conditions farther north along the boundary, particularly at Jones Ridge, suggest strongly that such an unconformity exists. Cairnes describes the rocks of his Tindir group along Tindir Creek and between Ettrain and Hard Luck Creeks as consisting of dolomite, limestone, quartzite, slate, shale, and greenstone. The lithology suggests that the formation here described as Middle Cambrian or older may be correlated with a part of Cairnes's Tindir group. MIDDLE CAMBRIAN LIMESTONE DISTRIBUTION The Middle Cambrian limestone crops out in the hills on the north side of the Yukon north of Calico Bluff and continues northward to the Tatonduk River and for an unknown distance beyond. LITHOLOGY The Middle Cambrian limestone is rather varied in character and appearance. Some of it seems to be pure limestone, but much of it is silicified to a greater or less degree. No dolomite was noted, but parts of the formation may have an appreciable content of magnesium. Textural varieties are also conspicuous, such as oolitic limestone and limestone conglomerate. The pure limestone is usually white and finely crystalline. The silicified varieties have the same general appearance but contain varying amounts of quartz or of chert. Where much secondary quartz is present the rocks simulate a granular white quartzite. At one locality the partly silicified limestone contains disseminated through the rock a shiny black conchoidally fracturing mineral that suggests a hydrocarbon similar to gilsonite. This limestone formation is thin bedded at the top but very thick bedded in its lower half, making conspicuous cream-colored bluffs along the north side of the Tatonduk River. Viewed from the south bank of the river the limestone in these bluffs appears to grade upward into thin beds of gray chert, but the closer inspection afforded by swimming along the bluffs on the south side shows that on that side there is a sharp change from thin-bedded limestone to thinbedded chert, with a covered zone a few feet wide that may conceal a dip fault parallel to the westward-dipping beds. Where the Middle Cambrian limestone plunges southwestward toward the Yukon, north of Calico Bluff, a cold sulphur spring issues beneath the base of the limestone and discharges into a little gulch that drains southward to the river. This spring, which is about 100 yards from the Yukon, discharges from two vents some 20 or 30 feet apart, which are conspicuously colored with a white sulphurous deposit. The odor of sulphureted hydrogen is strong and may be detected 100 yards or more from the spring. A sample of the sulphur water was analyzed in the chemical laboratory of the United States Geological Survey by Margaret D. Foster, who reports the following composition: |