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Analysis of water from cold sulphur spring 11.4 miles N. 4° E. of Eagle,

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It seems rather probable that a part of the contained solids of this water has been leached from the overlying limestone.

STRUCTURE AND THICKNESS

This limestone is not continuously exposed along the banks of the Tatonduk River, but exposures seen indicate a rather consistent westward dip at 20° to 30°. Southward from the Tatonduk River this limestone continues across wooded hills to the Yukon, bends sharply eastward, and finally veers northeastward, forming a well-defined anticline that plunges rather steeply toward the Yukon just north of Calico Bluff. The evidence at present available indicates that the east end of the anticline is cut off by a fault.

From the structural data available on the Tatonduk River the thickness is estimated at 800 to 1,200 feet, say 1,000 feet; but if the structure is more intricate than is now supposed, the thickness may be less. The top of the Middle Cambrian limestone, however, has not been definitely recognized, and it is possible that a complete section at some other locality will show a thickness greater than 1,200 feet.

AGE AND CORRELATION

Fossils were found in this limestone, apparently in the upper half of the formation, as follows:

25AMt148 (2062). Northeast side of Yukon River, at an altitude of about 1,400 feet above the river, 3.1 miles N. 21⁄2° E. of Eagle:

Nisusia or Jamesella sp.

Dorypyge, 2 sp.

Albertella mertieii.

Stenotheca rugosa.
"Ptychoparia," 2 sp.

Ogygopsis sp.

These fossils were examined by C. E. Resser, of the United States National Museum, who determined them to be of lower Middle Cambrian age. Resser's report is as follows:

All of the foregoing species are quite similar or possibly identical with forms in the Langston limestone or the Ross Lake shale and hence belong to a horizon well down in the Middle Cambrian.

One part of this collection consists of a darker, more crystalline limestone, as compared with the whiter and finer-grained limestone which contains the more abundant fossils. Both apparently contain the same fauna, and in both the fossils silicify on weathering.

This material does not correlate with that secured by the Canadian Geological Survey along the international boundary. There are two distinct horizons represented by the Canadian material, the lower one possibly being a Cambrian horizon somewhat above Doctor Mertie's horizon; but the other is well up in the Ozarkian, containing a species or two of Symphysurina.

Of other collections made along the international boundary, Resser has seen only those of L. D. Burling, and it may well be that a reexamination of the Cambrian collections made by Cairnes will show that his horizons are more nearly equivalent to those of collection 2062, just described.

UPPER CAMBRIAN LIMESTONE

DISTRIBUTION

Limestone containing Upper Cambrian fossils has been recognized at two localities. The first is a thin plate of limestone that crops out along the north side of the Yukon north of Calico Bluff. The second occurrence is in the headwaters of the North Fork of Shade Creek at the top of the great mass of limestone that extends from Shade Creek northeastward to the Tatonduk River. The geographic position of these two limestones suggests that they may be continuous, but this relation has not yet been proved.

LITHOLOGY

The limestone along the north bank of the Yukon is a massive light-gray finely crystalline rock, with conglomeratic and oolitic phases, in general appearance not unlike the Middle Cambrian limestone farther up the hillside. (See pl. 4, A.) The conglomeratic phase of this limestone contains small pebbles of gray limestone and a few well-rounded pebbles of shiny black chert. The limestone at the head of Shade Creek consists of alternating thin layers of light and dark gray rock.

STRUCTURE

The limestone band north of Calico Bluff strikes about N. 30° W. and dips perhaps 60° SW., toward the Yukon. It was not measured instrumentally but appears not to exceed 300 feet in thickness. Evidently this limestone forms the uppermost bed of a pitching anticline, composed mainly of Cambrian strata and previously men

tioned in connection with the Middle Cambrian limestone. It is overlain by beds of lower Mississippian age, and this relation is believed to be due to faulting.

The Upper Cambrian limestone in the head of the North Fork of Shade Creek strikes northwest and dips gently southwest. It lies at the top of a mass of undifferentiated limestone. It is underlain presumably by older Cambrian strata. On the other hand, the sequence is not continuously downward all the way north to the Tatonduk River, for both Silurian and Middle Devonian fossils have been found in the undifferentiated limestone on the Tatonduk River. The Upper Cambrian limestone is directly overlain by graptolitebearing beds of Ordovician (Normanskill) age.

AGE AND CORRELATION

Two fossil collections have been made, as follows:

28AMt263. Limestone bluff along northeast bank of Yukon River, north of the north end of Calico Bluff:

Archaeocyathus? sp.

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28AMt262. North Fork of Shade Creek, 1.15 miles N. 291⁄2° W. from Hug" boundary triangulation station (McCann Hill):

Acrothele sp.

Kirk's report on this material is as follows:

No more definite age assignment of these lots of fossils is possible than to call them Cambrian. The fossil in lot 263 appears to be very like the Upper Cambrian forms referred to Archaeocyathus. Such scant evidence as there is would point to the Middle or Upper Cambrian age of the containing beds.

Archaeocyathus does not occur in any of Cairnes's Cambrian collections, but Acrothele is found in four of his eight collections, all four determined as Upper Cambrian. Lying apparently at the top of this mass of limestone at the head of Shade Creek, and being directly overlain by Ordovician beds, this Acrothele-bearing limestone seems therefore most logically regarded as Upper Cambrian. These two limestones, therefore, are believed to represent about the same stratigraphic horizon, and both of them are correlatable with the beds from which some of Cairnes's collections were obtained, notably with those on Jones Ridge.

ORDOVICIAN SYSTEM

38

Rocks of Ordovician age are known to be present along the international boundary but have not been recognized as such along the Yukon between the boundary and Circle. Cairnes 88 in his work along the boundary made several collections of Ordovician fossils, including one collection of graptolites. The invertebrates include forms from

Cairnes, D. D., The Yukon-Alaska international boundary between Porcupine and Yukon Rivers: Canada Geol. Survey Mem. 67, pp. 66-69, 1914.

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.

"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 4 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.

SILURIAN SYSTEM

DISTRIBUTION

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 22 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.

LITHOLOGY

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.

1926.

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.

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