« PrejšnjaNaprej »
this point to a point 15 miles downstream, where the exposures on the east side of the river end, numerous reversals of dip were observed, and it is obvious that these lavas are much folded. The folds, however, are of the open type, probably owing to the competency of the beds. This formation is bounded on the north and also on the west by the alluvial deposits of the Yukon Flats, and its relation to adjoining hard-rock formations in these directions is therefore indeterminate.
AGE AND CORRELATION
These lavas and associated pyroclastic and sedimentary beds were originally grouped by Spurr 55 with those here separated as the Woodchopper volcanics, under the name Rampart "series," and were assigned to the Devonian on the basis of some obscure plant remains found in these rocks below Rampart; but fossils found in the calcareous tuffs interbedded with the lavas in the Rampart area by Overbeck in 1918 and by the writer in 1923 show that this part of the Rampart "series" is of Mississippian age. No fossils have yet been found in the typical Circle volcanics upstream from Circle, and the stratigraphic relation of these volcanic rocks to the rocks of the Devonian and Carboniferous systems is obscure. Hence their correlation and age assignment must be based mainly upon lithologic data and upon their relations to adjoining formations in neighboring areas where the stratigraphy and structure are more clearly revealed. Lithologically the Circle volcanics correspond exactly with the Rampart group farther down the Yukon; and as the Rampart group seems definitely to be of Mississippian age these rocks also may be assumed to be Mississippian. But if the analysis of the Carboniferous stratigraphy in the Eagle-Circle district, as subsequently given, is correct, the Circle volcanics can not be high in the Mississippian sequence. The Calico Bluff formation is of upper Mississippian age, and the stratigraphic units both above and below it have been recognized and mapped. Above the Calico Bluff formation lie successively two younger formations followed by the Permian limestone, without a trace of volcanic activity; and conformably below the Calico Bluff formation lies another Mississippian formation, which at least in its upper part is likewise devoid of volcanic rocks. It would seem, therefore, that if the Circle volcanics belong in fact in the Carboniferous system they must lie well toward its base.
A formation in the type locality of the Rampart group in the Tolovana district was described by the writer 56 in 1916 as consisting
55 Spurr, J. E., and Goodrich, H. B., Geology of the Yukon gold district, Alaska: U. S. Geol. Survey Eighteenth Ann. Rept., pt. 3, pp. 155-169, 1898.
50 Mertie, J. B., jr., The gold placers of the Tolovana district: U. S. Geol. Survey Bull. 662, pp. 239-244, 1918.
essentially of chert with a minor proportion of limestone and argillaceous rocks. This formation contains at the base a chert conglomerate. It rests upon rocks of Middle Devonian age and is in turn overlain by slates then supposed to be Pennsylvanian, but later the fossils of these slates were redetermined as Mississippian. In 1916 it was not determined whether the chert formation lay conformably or unconformably above the Middle Devonian rocks. The Rampart group was found to the north of the slates, but its structural and stratigraphic relations to the adjoining Carboniferous rocks were also adjudged to be indeterminate, although it was thought to underlie them. In 1918 and 1923 Mississippian fossils were found also in the Rampart group, but the available paleontologic data were still insufficient to place the Rampart in its proper stratigraphic relation to the Mississippian slates and the underlying chert formation. Thus it appears that neither in the Tolovana district alone nor in the Eagle-Circle district alone are the data available for assigning the Rampart group to its true stratigraphic position. But on considering jointly the data from both these districts a complete sedimentary sequence from the base of the chert formation to the Perinian may be said to exist, in which there appears to be no place for a volcanic formation. Yet as shown by its fossils the Rampart group is of Mississippian age. Where, therefore, can it be placed stratigraphically except at the base of the Carboniferous system? Even under this interpretation a degree of uncertainty exists as to the exact relation between the Rampart group and the chert formation of the Tolovana district, for the Rampart group itself comprises a considerable proportion of interbedded chert, as seen along the Yukon below Fort Hamlin and in the North Fork of the Hess River, nor is the chert formation of the Tolovana district free of basic igneous material. It is not possible, therefore, to say definitely that in the Tolovana district the Rampart group in its entirety is older than the chert formation. Perhaps the two groups of rocks are in part contemporaneous. But in the Eagle-Circle district we are dealing only with the upper part of the chert formation, which underlies conformably the Calico Bluff formation and appears to be free of volcanic material. Without final commitment regarding the geologic history of the Rampart group and all of the chert formation, it seems safe to regard the Circle volcanics as older than the cherty rocks that lie directly below the Calico Bluff formation.
The question might be raised, however, as to whether the Circle volcanics could not properly be correlated with the Woodchopper volcanics and be regarded as Devonian. This, it must be admitted, is a possible interpretation, if the correlation of the typical Circle volcanics with those exposed along the Yukon below Fort Hamlin
is denied, particularly as no fossils have been found either in the typical Circle volcanic rocks above Circle or in the cherty rocks underlying the Calico Bluff formation. This interpretation has been rejected for two reasons: (1) The lithology of the Woodchopper volcanics differs from that of the typical Circle volcanics above Circle; (2) the Woodchopper volcanics are Middle Devonian, and no Upper Devonian rocks have yet been recognized in central Alaska; hence a transition from the marine Woodchopper volcanics into the volcanic rocks farther downstream is not regarded as probable. Volcanism probably occurred in Upper Devonian time, but where igneous rocks are recognized, they are believed to be mainly intrusive and at least in part ultrabasic. Certainly no evidence exists for the belief that Upper Devonian volcanism was associated with the deposition of marine sediments, such as are found in the Woodchopper volcanics and the Rampart group.
From these considerations the Circle volcanics are correlated with the volcanic rocks of the Rampart group and are regarded as lower Mississippian.
The chert formation, which is tentatively referred to the lower Mississippian, occurs definitely at three localities and questionably at certain others. The type locality is at the north end of Calico Bluff, where these rocks are particularly well expressed. A second locality is in the bluff on the north bank of the Yukon, just below the mouth of Shade Creek. (See pls. 6, A, and 8, B.) Another small area is seen on the west side of the Yukon opposite the mouth of the Tatonduk River. Rocks correlated with this lower Mississippian formation, though probably equivalent to its base, are found in a belt crossing Coal and Woodchopper Creeks a few miles south of the Yukon, and a similar belt is seen north of the Yukon opposite Woodchopper Creek. The north end of the bluffs below Takoma Creek may also belong to the same sequence. (See pl. 8, 4.) Two other areas of similar rocks which are correlated with this formation occur along the north side of the Yukon 10 or 12 miles below Thanksgiving Creek.
At the north end of Calico Bluff the upper Mississippian Calico Bluff formation, which consists essentially of thin-bedded limestone and shale, grades downward stratigraphically into shale and chert of similar appearance, and finally, at the north end of the exposure, into nearly pure thin-bedded chert. This lower chert is mostly black but weathers to a yellowish-brown color that may be due in part to
sulphur staining. It occurs in beds 2 to 6 inches thick, with thin partings of black shale. At the most northerly exposures it has been compressed into numerous sharp folds, though in general all these beds are fairly regular in their attitude. No distinct line can be drawn between the beds of the chert formation and those of the overlying Calico Bluff sequence, but three prominent bands of yellowweathering limestone are seen at the north end of Calico Bluff, and the cherty rocks appear to become dominant just stratigraphically below these bands. This division between the two formations is a purely arbitrary one, but in the absence of any better one it is utilized for their cartographic separation.
Southeast of Calico Bluff, at the mouth of Shade Creek, are other good exposures of the cherty rocks, which at this place consist essentially of siliceous slates. These slates are black and in part graphitic, as may be seen along some of the slickensided planes. They are, however, hard as flint and might better be called silica slates. Much yellow-brown iron staining is visible in the bluffs, as well as some white and yellow incrusted sublimates resembling gypsum and sulphur. The beds are from a fraction of an inch to 2 feet thick. A few rocks of other character were noted in these exposures, including true conchoidally fracturing chert, an exceedingly fine-grained quartzite, and a 2-foot bed of dark-gray oolitic limestone. The limestone has a strong petroliferous odor when fractured, but when tested in the chemical laboratory of the Geological Survey it yielded only a trace of oil.
South of the Yukon River, in the valleys of Coal and Woodchopper Creeks, is a formation that appears from the exposures along the creeks to be essentially a chert conglomerate or, more accurately, a chert breccia. This rock is only imperfectly exposed along Woodchopper Creek, but in the bedrock of Mineral Creek, a tributary of Woodchopper Creek, where placer mining is in progress, it has been exposed by the mining operations. The chert conglomerate may there be seen in unconformable contact with the much younger conglomerate of Upper Cretaceous or Eocene age. On Coal Creek chert breccia, chert conglomerate, and chert grit are intermittently exposed in bluffs along the west wall of the valley some 6 or 8 miles from the Yukon. Another good exposure occurs on the ridge about 900 feet above the Yukon River and about 2 miles S. 60° W. from the mouth of Coal Creek. The rock here is mainly a dark-gray chert, with a subordinate proportion of chert breccia. The mountain 3 or 4 miles north of the Woodchopper road house, at the head of Eureka Creek, is also composed of chert conglomerate and breccia.
The northermost exposures of this formation of siliceous slate and chert are seen along the northeast bank of the Yukon about 10 miles below Thanksgiving Creek. Here the river swings in, at the lower
end of some islands, to rock bluffs of Lower Cretaceous slate and quartzite. Upstream (southeast), along a slough behind these islands, the Lower Cretaceous rocks continue but a quarter of a mile and give place to a formation of siliceous black slate and chert which, still farther upstream, changes to shaly sandstone and black and green slate, with a few beds of chert. Farther up on the hillsides these rocks weather to tones of yellow and brown, giving a characteristic appearance which readily differentiates these hills from the hills composed of the Lower Cretaceous rocks. Downstream from the Lower Cretaceous bluffs intermittent outcrops of chert and siliceous slate may be seen at low stages of the river, and still farther down, just above the junction of the north and south channels, there are good exposures of vitreous pellucid chert and siliceous shale in shades of red, green, and gray. These rocks terminate downstream at a creek entering from the north, where the two channels of the river unite, and below this point is a thin unmappable zone of Lower Cretaceous rocks, followed by a great series of volcanic rocks. These two small areas of chert and siliceous slate should, with little doubt, be correlated with the other rocks of this formation.
The rocks of the chert formation near Woodchopper and Coal Creeks range in color from pearl-gray and cream color into darker shades of gray and jet-black. The constituent fragments range in size from particles too small to be distinguished with the naked eye up to pebbles 1 inch or more in diameter and are usually either angular or subangular. In granularity, therefore, these rocks might be classified as ranging from a chert sandstone or chert grit to a sedimentary chert breccia. The unusual characteristic is a matrix composed also of chert of the same general character as the pebbles. So well indurated are these rocks that they will always fracture directly across the pebbles when broken. Samples of this chert breccia may be seen in which the pebbles are so firmly welded to the matrix that the fragmental character is scarcely apparent; other samples resemble a breccia that might be produced by fracturing in a fault zone; but still others may be seen in which the pebbles are fairly well rounded from the action of water and the sedimentary character is unquestionable.
The writer has seen rock of this peculiar type at many localities in interior and northern Alaska, mainly at or near the top of the Devonian sequence or the base of the Mississippian, and the lithologic description given above would fit almost equally well any of the observed occurrences. The problem of their origin is a most interesting but very difficult one, in the consideration of which a number of facts must be evaluated. At the known localities in interior Alaska the pebbles are exclusively chert; in northern Alaska they include both quartz and chert; but the exact correlation of the two formations