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the Upper and the Lower Ordovician, and it is likely that Middle Ordovician horizons are also represented. The graptolites were determined as high Lower Ordovician, or more specifically as correlative with the Normanskill. Cairnes, however, was unable to differentiate the Ordovician rocks from the Devonian, Silurian, or Cambrian, so that no separate mapping of the Ordovician system is at present possible. His collections of Ordovician fossils found within the area covered by this report are therefore listed under the undifferentiated Paleozoic rocks.
Another collection of Ordovician graptolites was made in 1928, but as with Cairnes's the fossils occurred in such a manner as to preclude the separate mapping of the containing beds. These fossils have likewise been listed with those from the undifferentiated Paleozoic rocks. A single fossil that resembles Obolus sp., according to Edwin Kirk, was found by Blackwelder on the north bank of the Yukon River about half a mile upstream from the mouth of Woodchopper Creek. The rocks near this locality, both upstream and downstream, are well known from numerous collections to be of Middle Devonian age. As the fossil was not found in place but in a talus pile, it may have been transported to the site where it was discovered by the action of river ice during the spring break-up. Faulting, of course, must be considered in any interpretation of the structure of the rocks above Woodchopper Creek, but if this fossil came originally from the cliffs above the talus pile where it was found, then faulting of far greater intensity and amplitude than have hitherto been suspected has probably taken place in this particular area.
Ordovician rocks, though not yet recognized as such along the Yukon, are known at a number of other localities in Alaska. Northwest of the area here considered, at a number of different places in the White Mountains of the Yukon-Tanana region, Prindle,39 Blackwelder, and the writer have collected fossils that were first determined as Upper Ordovician but were later referred to a horizon high in the Middle Ordovician. Middle Ordovician (Mohawkian) fossils have been found by Kindle 1 at two localities in the lower ramparts of the Porcupine River, some 25 miles below the Coleen River. Upper Ordovician and possibly also Middle Ordovician fossils, chiefly graptolites, were collected by Brooks and Prindle 42 in the Alaska Range region at the headwaters of the Kuskokwim River.
39 Prindle, L. M., A geologic reconnaissance of the Fairbanks quadrangle, Alaska: U. S. Geol. Survey Bull. 525, p. 42, 1913.
40 Blackwelder, Eliot, unpublished manuscript.
41 Kindle, E. M., Geologic reconnaissance of the Porcupine Valley, Alaska: Geol. Soc. America Bull., vol. 19, p. 323, 1908.
42 Brooks, A. H., The Mount McKinley region, Alaska, with a description of the igneous rocks and of the Bonnifield and Kantishna districts, by L. M. Prindle: U. S. Geol. Survey Prof. Paper 70, p. 72, 1911.
Upper Ordovician (Richmond) fossils were found in the valley of the Sulukna River, some 50 miles west of Lake Minchumina, by Eakin 43 in 1915, and in the same general region, though somewhat farther southwest, by Brown ** in 1924. In Seward Peninsula both Upper and Lower Ordovician fossils were collected in 1901 and later years by Collier, Washburne, Knopf, and Kindle, and most recently by Steidtmann and Cathcart. Fossils of Ordovician age are known also in southeastern Alaska.
In the interior of Alaska, therefore, Ordovician rocks appear to be widespread, differing markedly in this particular from the Cambrian rocks, which are localized in one basin; and as Lower, Middle, and Upper Ordovician horizons are all represented it would seem that the Ordovician system is well developed, both areally and stratigraphically. In view of the conditions elsewhere and the great variety of geologic formations already recognized along the Yukon, it is altogether likely that subsequent detailed work will identify several Ordovician horizons among the undifferentiated Paleozoic rocks in the upper Yukon Basin along the international boundary and possibly also along the Yukon between the boundary and Circle.
No rocks that can be definitely referred to the Silurian are known in this area, but a considerable portion of the undifferentiated Paleozoic rocks along the Yukon, including the undifferentiated limestones, may prove to be of this age. A belt of rocks believed to be of Silurian age lies along the west bank of the Yukon, beginning at a point about halfway between Thanksgiving and Takoma Creeks and extending north about 3 miles. It is not unlikely that the limestone that crosses Woodchopper Creek about 211⁄2 miles from its mouth is also of Silurian age, but of this there is no direct proof. Silurian rocks are extensively developed along the international boundary but have not been differentiated there from the other preMississippian Paleozoic rocks.
The sequence below Thanksgiving Creek has been seen and described by several geologists, notably by Brooks and later by Black
43 Eakin, H. M., The Cosna-Nowitna region, Alaska: U. S. Geol. Survey Bull. 667, p. 25, 1918.
Brown, J. S., The Nixon Fork country: U. S. Geol. Survey Bull. 783, pp. 103-105,
Steidtmann, Edward, and Cathcart, S. H., Geology of the York tin deposits, Alaska: U. S. Geol. Survey Bull. 733, pp. 23-26, 1922.
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
Hard black limestone, quartzose at south end, in beds a few
Interbedded black limestone and drab clay shale, with a few
Thin-bedded black limestone.
At south end strike is N. 80°
E., dip 40° S.; dip changes to north at north end_.
Folded thin-bedded limestone and shale. About 150 feet be-
Thin-bedded black limestone with beds of drab slate at
Thin-bedded black limestone with beds of drab slate at
Black slate, with some beds of limestone about 1 foot thick.
Thin-bedded limestone and slate. Strike N. 75° E.; dip
Limestone and slate at south end; followed by thin-bedded
Limestone and slate, with southerly dip at south end; the
Same material, with northerly dip at south end; the north-
Slate with beds of the same limestone and three zones of
Thin-bedded limestone, with little or no slate. Much
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;
FIGURE 4.-Geologic section from Takoma Creek northward along west bank of Yukon River, showing structure of upper
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 46 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
46 Blackwelder, Eliot, unpublished manuscript.