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A. UNDIFFERENTIATED PALEOZOIC GREENSTONE IN BLUFF ON THE WEST

SIDE OF THE YUKON RIVER AT THE MOUTH OF MISSION CREEK

[graphic]

B. LOWER MISSISSIPPIAN VOLCANIC ROCKS ALONG THE EAST BANK OF THE

YUKON RIVER ABOVE CIRCLE

by Smith and the writer 52 into the valleys of the Alatna and Noatak Rivers. The Skajit limestone, which is believed to extend nearly or quite all the way across northern Alaska, is now believed to be mainly of late middle Silurian or early upper Silurian age. Above the Skajit limestone but below the Devonian sequence in northern Alaska lies another group of rocks which is composed mainly of noncalcareous beds but contains one or more prominent beds of limestone; this is believed to be of upper Silurian age. Similarly, along the Porcupine River, in northern Alaska, Kindle 53 found both middle and upper Silurian rocks, the middle Silurian consisting of a massive magnesian limestone and the upper Silurian of graptolite-bearing shale.

In general, the upper Silurian is more widespread in Alaska than the middle Silurian, though the latter is more conspicuous on account of its massive limestones. In addition to their occurrence in northern Alaska, upper Silurian rocks are found in Seward Peninsula, in the Nixon Fork country southwest of Lake Minchumina, at several places in the Yukon-Tanana region, along the international boundary north of the Yukon, and at a large number of localities in southeastern Alaska. The rocks along the Yukon that are assigned to the upper Silurian (?) are believed to be correlative with this general upper Silurian horizon or horizons.

DEVONIAN SYSTEM

The Devonian system is represented along the Yukon between the international boundary and Circle by a group of rocks of late Middle Devonian age, herein named the Woodchopper volcanics, and by another group of rocks, also called late Middle Devonian, to which no formational name has been applied. The typical Middle Devonian rocks and fauna of interior Alaska are found along the international boundary north of the Yukon but have not been separately mapped. Lower Devonian rocks have nowhere been found in Alaska.

MIDDLE DEVONIAN SERIES

WOODCHOPPER VOLCANICS

DISTRIBUTION

The late Middle Devonian rocks to which the name Woodchopper volcanics is here applied crop out along both banks of the Yukon from the mouth of Coal Creek downstream for 15 miles, extending on the south bank just beyond the mouth of Thanksgiving Creek.

Smith, P. S., and Mertie, J. B., Jr., Geology and geography of northwestern Alaska : U. S. Geol. Survey Bull. 815, pp. 124-132, 1930.

* Kindle, E. M., Geologic reconnaissance of the Porcupine Valley, Alaska : Geol. Soc. America Bull., vol. 19, pp. 325-327, 1908.

62744306

7

LITHOLOGY

The Woodchopper volcanics between Coal and Thanksgiving Creeks consist essentially of basaltic lavas of greenstone habit and associated pyroclastic material, interbedded with massive limestone and more or less shale, slate, and chert. It is possible that some of the included argillaceous rocks may be infolded or infaulted parts of the Lower Cretaceous sequence, which borders this formation on the east, south, and west; the limestone, however, carries Middle Devonian fossils and is an integral part of the formation. This group of rocks, although essentially igneous in origin, is treated here among the sedimentary succession because of the bedded character of the lavas and because of the fossils found in the interbedded sedimentary rocks. Other basaltic rocks that crop out along the east bank of the Yukon above Circle were formerly included with this series but are now believed to be of later origin. The sedimentary members of the Woodchopper volcanics constitute a minor part of the total and are too small in areal extent to be separately mapped.

Along the south side of the Yukon the section of the Woodchopper volcanics begins somewhat indefinitely about half a mile below Coal Creek. The sequence of rocks as seen along the beach, going downstream, is as follows, the measurements given being distances along the river bank:

Seotion of Woodchopper volcanics on south bank of Yukon River belov Coal

Creek

Feet
Lower Cretaceous slate exposed at mouth of Coal Creek.
Covered. Probably top of Woodchopper volcanics---------- 2,500+
Limestone, dark gray, rather coarsely crystalline, fossilif-
erous. Permian. Strike N. 35° E.; dip 50° SE..

100
Greenstone and greenstone tuff, with some black chert and
siliceous slate directly under the limestone -

200 Covered

1,400
Greenstone, massive, basaltic. Shows numerous slickensided

fault planes, marked usually by thin seams of calcite and
epidote. These fault planes are low lying and dip very
gently eastward.--

700
Covered along beach but exposed on hillside. Probably
greenstone and tuff_

1,600 Limestone. Seen only from river, but believed to be a part of

the Woodchopper volcanics. Strike N. 35o E.; dip 60° SE. 1, 200 From this point down to Woodchopper Creek and for some distance below on the south side of the river there is an uninterrupted series of lavas and pyroclastic rocks. Plate 1, C, shows the bold

bluffs in which these rocks are exposed, and Plate 5, B, shows a closer view just below the mouth of the creek.

Below Coal Creek, on the north bank of the river, the lavas were examined with some care for a distance of about 2 miles. The bedding of the flows is clearly apparent at many places, and numerous ellipsoidal flows were seen. Ellipsoids as large as 6 feet in diameter were noted. Much volcanic agglomerate or flow breccia and more or less tuff and tuffaceous sediments are also present interbedded with the lavas. Plate 6, B, shows typical ellipsoidal lava which crops out farther downstream. The lava itself is clearly basaltic in character, and some of it is amygdaloidal, with vesicular fillings of calcite. The original rock minerals, essentially plagioclase and augite, are now altered to chloritic products, and the accessory iron oxides are completely oxidized. More or less secondary pyrite and pyrrhotite are also distributed in these lavas.

From Woodchopper downstream massive beds of limestone, interbedded with the lavas, crop out at intervals in the bluffs along the river. Plate 5, A, shows two such masses of limestone on the north bank of the river opposite the Woodchopper road house. Numerous fossil collections have been made at this locality. Another prominent limestone occurs along the southwest bank of the river about 3 miles below Woodchopper. This limestone strikes N. 75o W. and dips 60° S. It is underlain by greenstone and overlain downstream by greenstone and tuff, mostly tuff. Limestone forms two bluffs about 6 miles below Woodchopper, on the same side of the river.

These limestones vary somewhat in appearance, the differences apparently depending more on the degree of metamorphism to which they have been subjected than on original differences in character. Some are light to dark gray dense noncrystalline or cryptocrystalline limestone, and others are partly recrystallized. An oily odor was detected when some of the limestone was fractured with the hammer, but this has no particular economic significance.

Beds of dark-gray to black slate and chert are found to the east of the limestone opposite Woodchopper road house and at some other places. Some of these rocks are doubtless an integral part of the sequence, but some of the slate, in the writer's opinion, is infolded or infaulted Lower Cretaceous slate.

STRUCTURE AND THICKNESS

The Woodchopper volcanics are complexly folded and faulted, and the resulting structure is most enigmatic. The assemblage of Lower Cretaceous, Permian, and late Middle Devonian (Woodchopper) rocks along the south bank of the Yukon 'below Coal Creek is certainly due principally to faulting. Similarly, along the north bank of the river above Woodchopper there appears to be a duplication of the limestone strata due to a distributed type of block faulting. The two bodies of limestone opposite Woodchopper road house are probably parts of the same band of limestone separated by faulting. Little is known of the true character of this faulting. Blackwelder 54 was inclined to regard the whole formation in the vicinity of Woodchopper as an assemblage of jostled wedges caused by folding under no great cover. This is as good an explanation as can be given at present and fits in with the writer's conception that the Middle Devonian Woodchopper volcanics at Woodchopper are folded into a major flexure of general anticlinal character. This hypothesis is based primarily on lithologic and areal relationships. A chert conglomerate, which crops out conspicuously, is believed to be the base of the next formation overlying the Woodchopper volcanics. This chert conglomerate is found up Woodchopper and Coal Creeks to the south of the Woodchopper rocks, and it also crops out in the hills about 3 miles north of Woodchopper. This symmetrical distribution of the chert conglomerate suggests an anticlinal arching of the Woodchopper rocks at Woodchopper, with an axial plane that strikes about N. 65o W. and veers more toward the north down. stream from Woodchopper. Such a fold, though of greater amplitude, would be correlated rather closely with the observed anticlinal fold in the Cambrian rocks north of Calico Bluff. If this folding had been consummated when the Woodchopper rocks were fairly close to the surface, the wedgelike faulting postulated by Blackwelder would surely have ensued, and this interpretation seems at present the most logical one available.

In view of the observed complexity and the uncertain interpretation of the structure, it is useless to venture any exact estimate of the thickness of the Woodchopper volcanics. The belt is 3 or 4 miles wide from north to south, and it is rather likely that the thickness amounts to several thousand feet.

AGE AND CORRELATION

Seventeen collections of late Middle Devonian fossils have been made at various times by workers in this area, all of which have come from the sedimentary rocks interbedded with the volcanic flows near Woodchopper on the Yukon. A number of other collections have been made from the typical Middle Devonia rocks along the international boundary, but these have been noted in the discussion of the undifferentiated Paleozoic limestones of the boundary. The table below shows that 27 genera in the Woodchopper rocks are represented along the Yukon River. Edwin Kirk, of the United

54 Blackwelder, Eliot, unpublished manuscript.

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