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Woodchopper volcanics. Both formations are regarded as late Middle Devonian, though Kirk feels that the paleontologic evidence is slightly in favor of placing these Middle Devonian beds above the Woodchopper volcanics.
Similarly the total thickness of these beds is not known, because the top of the sequence may have been removed by erosion. At the head of the North Fork of Shade Creek about 600 feet of strata are exposed.
The Devonian, next to the Carboniferous, is probably the most widespread of all the Paleozoic systems, not only in interior Alaska but in Alaska as a whole, and a tabulation of all the localities in Alaska where Devonian rocks are found would be quite beyond the scope of this paper. Three fairly definite horizons have been recognized, as follows:
Upper Devonian, characterized by Spirifer disjunctus and other Upper Devonian invertebrates. This is typically developed in the Brooks Range of Arctic Alaska and on Prince of Wales and Chichagof Islands in southeastern Alaska.
Late Middle Devonian, as seen in the sediments of the Woodchopper volcanics along the Yukon above and below Woodchopper Creek and in the argillite-chert beds of Shade and Eagle Creeks.
Middle Devonian proper, whose fauna is typically developed in the Salmontrout limestone on the Porcupine River. The Devonian fossils from the undifferentiated limestone along the boundary, collected by Cairnes, Harrington, and the writer, are a part of this Middle Devonian fauna. The same horizon is extensively represented elsewhere in interior and southeastern Alaska and has also been recognized on the Chandalar River in northern Alaska and in the Kuzitrin Valley on Seward Peninsula.
A horizon whose fossils have been variously referred to “Devonian or Silurian " and " Silurian or Devonian " is represented at numerous localities in interior and northern Alaska, but in this paper this horizon has been assigned to the Silurian system. No Lower Devonian sedimentary rocks are known in Alaska.
Six groups of rocks that are believed to be of Carboniferous age are shown on the accompanying map as follows:
A group of volcanic rocks, here designated the Circle volcanics, which are correlated with the upper part of the Rampart group and are considered to represent the base of the Carboniferous system in this district; a formation of chert and slate, which underlies the Calico Bluff formation; the Calico Bluff formation, of marine origin; a transitional formation, believed to overlie the Calico Bluff forma
tion and to underlie the Nation River formation; the nonmarine Nation River formation; and the Tahkandit limestone.
The rocks here classified as lower Mississippian and named Circle volcanics crop out for a distance of about 15 miles along the east bank of the Yukon upstream from Circle. This is the only known occurrence of these rocks in the area covered by this report, but rocks that are considered to be the same crop out at Fort Hamlin, at the lower end of the Yukon Flats, and continue downstream below Rampart. Plate 7, B, shows a typical occurrence of these volcanic rocks along the Yukon above Circle.
LITHOLOGY AND STRUCTURE
The rocks of the Circle volcanics are essentially basaltic lavas, of greenstone habit, not unlike the lavas of the Middle Devonian Woodchopper volcanics. They differ from the Woodchopper lavas in that they are cut by diabasic and gabbroic intrusive rocks, which, however, are not a part of the formation and on detailed work will be mapped separately. The formation contains also a certain proportion of interbedded sedimentary rocks, mainly chert and argillite, with some tuffs and flow breccias. Along the river above Circle the interbedded sedimentary rocks appear to constitute only a minor proportion of the formation, but farther down the river, below Fort Hamlin, they may constitute as much as half of the formation. It is particularly noteworthy that at neither of these two localities, nor southeast from Rampart, where these rocks have also been studied by the writer, have any limestones been found that are comparable with the limestones interbedded in the Woodchopper volcanics. Some thin beds of fossiliferous calcareous tuff have been found in this formation in the Rampart district, and one small lens of fossiliferous limestone was seen in 1923 along the north bank of the Yukon a short distance upstream from Rampart. Both the petrography and the lithology of this formation, therefore, distinguish it from the Woodchopper volcanics; and in addition the fossils found in these rocks in the Rampart area indicate that a part at least of this formation is younger than the Woodchopper volcanics. Little is known of the structure of these lavas and associated sediments above Circle. At their southernmost limit, 12 or 15 miles below Thanksgiving Creek, they appear to dip northwestward, thus apparently overlying other Carboniferous rocks which adjoin them on the southeast. From
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, A.) 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