welder. These rocks include massive and thin-bedded limestone, in part silicified, argillaceous, or calcareous, also siliceous shale, slate, and chert. About a mile below Thanksgiving Creek, on the west bank of the river, a little gulch empties into the Yukon through a timber-covered flat. On the south side of this gulch are the northernmost exposures of the Middle Devonian greenstone-limestone sequence. On the north side of the gulch the rock is a siliceous black slate, at places markedly graphitic, which continues downstream for perhaps 1,500 feet. This gulch is evidently in a fault zone, for the slate and a few associated calcareous beds are greatly sheared and show numerous fault striae. From this point downstream to the mouth of Takoma Creek, a distance of about half a mile, the rocks are well exposed in bluffs along the river bank and consist of massive and thin-bedded limestone with several beds of siliceous shale and slate similar to that just described. The massive limestone forms picturesque pinnacles and bluffs with numerous caves in the lower half. (See pl. 2, B.) It has the general aspect of a considerable sequence of thick limestone beds, but it contains several thin zones of interbedded shale and slate. The limestone for the most part is a fine-grained light-gray to black noncrystalline or cryptocrystalline rock, apparently without any dolomitic beds. It is much fractured and veined with calcite, and many of the calcite veins are closely folded, indicating the degree of deformation of these rocks, which is not otherwise so apparent in the massive beds. From the mouth of Takoma Creek downstream these rocks continue to crop out on the west side of the river for a distance of about 8,900 feet. Some observations on the lithology are noted below. The measurements given are horizontal distances as paced along the beach. Section of Silurian rocks on west bank of Yukon River, as traced northward from mouth of Takoma Creek Covered. Hard black limestone, quartzose at south end, in beds a few Interbedded black limestone and drab clay shale, with a few Feet 200 250 450 500 1,450 Thin-bedded black limestone. At south end strike is N. 80° Folded thin-bedded limestone and shale. About 150 feet be- Feet 220 230 350 350 Thin-bedded black limestone with beds of drab slate at 500 350 Thin-bedded black limestone with beds of drab slate at 500 Covered.. 200 Black slate, with some beds of limestone about 1 foot thick. Dip northerly‒‒‒‒‒ 100 Covered__ 100 Thin-bedded limestone and slate. Strike N. 75° E.; dip 625 Covered____ 100 Limestone and slate at south end; followed by thin-bedded 1,075 Covered_‒‒‒ 180 Limestone and slate, with southerly dip at south end; the 350 Same material, with northerly dip at south end; the north- 325 Slate with beds of the same limestone and three zones of 650 500 100 Thin-bedded limestone, with little or no slate. Much crumpled▬▬▬▬▬ 100 STRUCTURE AND THICKNESS The structure north and south of Takoma Creek is difficult to interpret. It is evident from the folding and faulting (see fig. 4) that there is much duplication of strata. The drag folds seen in the limestone south of Takoma Creek and in the thin-bedded rocks north of Takoma Creek give the impression that the sequence has suffered shale; ch, chert; sl, slate 500 1,500 FEET FIGURE 4.-Geologic section from Takoma Creek northward along west bank of Yukon River, showing structure of upper Silurian (?) rocks. 1s, Limestone; sh, compressional folding induced by pressure applied from the north. The faults do not appear to be particularly related to this tangential deformation but suggest rather a block faulting of later date. It may even be that the whole sequence, which apparently dips dominantly northward, is overturned, although of this there is no direct evidence. With these considerations in mind, it is somewhat hazardous to make an estimate of the thickness of strata here present. Blackwelder's estimate 4 is 7,500 feet. The writer's estimate is considerably lower. Only one zone in this sequence appears to be relatively free of faults and folds. In this zone, which starts 4,000 feet north of Takoma Creek and extends for some 3,000 feet north along the beach, the rocks appear to have a monoclinal dip, and from this sequence it would appear that at least 2,500 feet of strata are present. Between this zone and Takoma Creek the rocks may consist of the same beds duplicated by folding and faulting. From this zone north to the end of the bluff exposures the rocks, though folded and faulted, are obviously of different lithologic character. If they belong to the same sequence, the thickness may be amplified by perhaps another 1,000 feet of strata, but it is by no means certain that the cherty rocks at the north end are even of the same geologic age. From their lithologic character, the writer is inclined to regard them as decidedly younger, perhaps correlative with the cherty rocks that directly underlie the upper Mississippian sequence at Calico Bluff. The beds south of Takoma Creek, including the massive limestone shown in Plate 2, B, should doubtless be correlated with this sequence, but their thickness also is subject to much doubt, on account of the close folding observed in the limestone and in the slate south of the limestone and on account of the faulted condition of the black slate. Perhaps another 1,000 feet of strata, mostly limestone, might be added to the sequence for the rocks exposed south of Takoma 40 Blackwelder, Eliot, unpublished manuscript. Faults Is, sh Is, sh IS shaly Is Is, sh SI Creek. The total thickness, as estimated by the writer, is therefore probably about 4,500 feet, but if the cherty rocks at the north end of these exposures belong in part to a later horizon, the thickness may be as little as 3,500 feet. AGE AND CORRELATION A considerable Silurian fauna has been collected along the boundary from rocks which are not separately mapped, and this fauna has been listed in the section on the undifferentiated Paleozoic rocks. In addition, five collections are tabulated below, of which three, Nos. 847, 173, and 174, came from the rocks near Takoma Creek. The other two, Nos. 848 and 849, were collected years ago and appear from the labels now with them to have come from the limestone on the northeast bank of the Yukon, north of the north end of Calico Bluff, which is now classified as Upper Cambrian. It is possible that this material may not originally have been in place, and it is also possible that the fossils may have become separated from their originally correct labels. As collections 848 and 849 are regarded by Kirk as Silurian, however, they are here included with the Silurian fauna. Fossils from Silurian rocks along Yukon River between Eagle and Circle 847. 12 miles below Woodchopper Creek, south bank of Yukon River. Collector, E. M. Kindle. 848. 2 miles below Calico Bluff, Yukon River, in ravine below bend. Collector, E. M. Kindle. 849. 2 miles below Calico Bluff, east bank of Yukon River. Collector, E. M. Kindle. 173 and 174. About 3 miles below Thanksgiving Creek, southwest bank of Yukon River. Collector, Eliot Blackwelder. Collection 847 evidently came from the limestone horizon just south of Takoma Creek and is doubtless the collection referred to by Brooks." This collection was regarded by Kindle as indecisive but indicative possibly of the Devonian. It has recently been redetermined by Edwin Kirk, who refers it questionably to the Silurian. 47 Brooks, A. H., and Kindle, E. M., Paleozoic and associated rocks of the upper Yukon, Alaska: Geol. Soc. America Bull., vol. 19, p. 279, 1908. Collection 173 was referred definitely to the Silurian. Collection 174 was referred questionably to the Devonian but is here included with the Silurian. It is characteristic of many of the rather poor collections from this general horizon, both here and elsewhere in Alaska, that the determinations given are invariably "Devonian or Silurian," "Silurian or Devonian," "questionably Devonian," or "questionably Silurian." This uncertainty, of course, is due in part to the paucity and poor quality of much of the material collected, but there seems to be a horizon in interior Alaska which can not readily be assigned definitely either to the Devonian or the Silurian. This is all the more difficult to understand, inasmuch as the Lower Devonian appears to be absent in Alaska. Therefore, when fossils are determined as "Silurian or Devonian" or "Devonian or Silurian," one must place them, if further assignment is attempted, either in the lower part of the Middle Devonian or the upper part of the Silurian. It can not be said that any definite evidence is available along the Yukon to favor either assignment, but from the general experience of the writer in interior and northern Alaska it seems advisable at present to favor a Silurian age for these questionable fossils. The sequence of beds on the south bank of the Yukon below Takoma Creek is therefore here referred to the upper Silurian (?). The Silurian rocks along the boundary include beds of both upper and middle Silurian age. The Silurian system is widespread in Alaska. In interior Alaska it is especially well developed in the White Mountains, north of Fairbanks, where middle Silurian fossils have been collected from a heavy limestone formation by Prindle,48 Blackwelder,49 and the writer. The same horizon is represented along the international boundary between the Yukon and Nation Rivers by the middle Silurian fossils collected by Cairnes, but this horizon has not yet been recognized as such along the Yukon between the boundary and Circle. 50 Schrader, in 1901, described a great Silurian limestone in northern Alaska which he called the Skajit limestone; this formation has subsequently been traced eastward by the writer 51 from its type locality on the John River into the Chandalar Basin and westward 48 Prindle, L. M., A geologic reconnaissance of the Fairbanks quadrangle, Alaska: U. S. Geol. Survey Bull. 525, pp. 39-45, 1913. 49 Blackwelder, Eliot, unpublished manuscript. 50 Schrader, F. C., A reconnaissance in northern Alaska: U. S. Geol. Survey Prof. Paper 20, pp. 56-58, 1904. 51 Mertie, J. B., jr., Geology and gold placers of the Chandalar district, Alaska: U. S. Geol. Survey Bull. 773, pp. 229-233, 1925; Geology and geography of the ChandalarSheenjek region, Alaska: U. S. Geol. Survey Bull. (in preparation). |