Slike strani
PDF
ePub

In the hills north of Eagle Creek these rocks are imperfectly exposed along burned ridges and spurs and are overlain by rocks of the Nation River formation, which forms cappings on the higher hilltops. The exposures are too much separated to be pieced together into a continuous stratigraphic section, but the lithology shows the general character of this part of the sequence. Among the rocks seen were chert and siliceous slate, veined with secondary quartz; quartzitic graywacke; sandstone that weathers yellow-brown; finegrained light-gray quartzite, probably a finely recrystallized chert; and fossiliferous cherty grit, which occurs near the Nation River beds and therefore high in the sequence of beds in the transitional formation. In going downhill toward Eagle Creek from the ridge that separates Eagle and Last Chance Creeks, much siliceous slate with beds of black chert is found cropping out along the spurs. The chert here as elsewhere in this formation weathers white, owing to the formation of a covering of opalescent material, which ranges from a thin veneer to a layer half an inch thick. The chert is interbedded with siliceous slate in beds 6 inches to 2 feet thick, and in places the bedding planes are emphasized by banding. The rocks exposed in the bluffs along Eagle Creek, which probably represent a lower horizon in the formation than the rocks above enumerated, include black shale that weathers brown, with numerous flattened ellipsoidal and reniform chert concretions along the bedding planes; nodular black sandy shale in beds from a few inches to a foot thick, containing pyrite nodules ranging from disklike forms to those approaching the frustum of a cone; soft thin-bedded dark-gray nodular sandy shale; bluish-gray hackly argillite, perhaps somewhat calcareous; siliceous slate in beds 1 to 4 inches thick; and chert like that seen farther up on the hill slopes. Apparently, therefore, these lower beds, as seen along Eagle Creek, are somewhat less siliceous than the higher beds seen on the hill slopes to the north and at the head of Shade Creek.

STRUCTURE AND THICKNESS

At the head of the North Fork of Shade Creek these rocks strike northwest and dip gently southwest. They overlie a wedge of Ordovician slates, too thin to be shown on the accompanying map, and underlie the rocks of the Nation River formation. No structural discordance between these rocks and either the overlying or the underlying rocks is apparent. About 3 miles northwest of the Shade Creek croppings, near the Middle Cambrian limestone, the contact between these Middle Devonian rocks and the overlying Nation River formation is exceptionally well exposed in the head of a gulch which drains southwestward to the Yukon. Here both these beds and the beds of the Nation River formation dip southward and, as elsewhere, no

structural discordance is visible. Nevertheless discontinuities in sedimentation must be represented, both at the base and at the top of these Middle Devonian beds, because considerable parts of the stratigraphic sequence are absent at both horizons.

The rocks of this formation, as exposed in the valley of Eagle Creek, are highly disturbed, and numerous observations of the attitude of the beds yields little more than a general idea of the regional trend, which appears to be about N. 75° W. The beds are closely folded but not welded into flattened or appressed folds of the type seen in the older rocks to the south. The dip of the beds reverses at short intervals, in places within a few feet, so that little idea of the general dip can be obtained. The whole sequence consists of incompetent beds, and the idea naturally suggests itself that these rocks have been a place of readjustment for some of the tangential forces that produced the thrust faulting and overturning of the more competent older rocks to the south. In other words, the more massive rocks to the south have been a buttress against which most of the tangential forces from the south have been expended, but some of the compressional stresses have been transmitted northward into these weaker beds, producing the observed close folding. The rocks of this series that crop out along the northwest bank of the Yukon below Eagle continue N. 75° W. into the hills north of Mission Creek but are nowhere very well exposed. This belt of rocks is believed to be overlain unconformably to the north by the Nation River formation, of Pennsylvanian (?) age, but its contact with the older rocks to the south is probably determined by faulting.

AGE AND CORRELATION

The assignment of these rocks to the Middle Devonian is based on a collection of fossils made in 1928. The location and determination of those fossils are given herewith:

[ocr errors]

28AMt260. North Fork of Shade Creek, 0.72 mile N. 13° W. from Hug" boundary triangulation station (McCann Hill):

Favosites sp.

Acervularia (?) sp.

Zaphrentis sp.

Leiorhynchus sp.

Spirifer sp.

Stropheodonta sp.

Bactrites (?) sp.

Orthoceras sp.

With regard to this collection, Doctor Kirk states: "All of this chert material is fragmentary and does not permit of closer identification. It is almost certain, however, that a high Middle Devonian horizon is represented."

In the valley of Eagle Creek no determinable invertebrates were collected but certain peculiar involute fossil forms were found, which occur also with the invertebrates above listed. The rocks of Eagle Creek and of the North Fork of Shade Creek are considered to represent the same stratigraphic horizon. The best collections of these obscure fossils, however, were made in the Eagle Creek Valley at locality 25AMt85 (2061), on the south flank of the ridge north of the creek, in Yukon Territory, three-eighths of a mile S. 23° E. of international boundary topographic station 108. The fossils occur in a cherty grit, which is really a fine-grained cherty breccia. In the hand specimens they appear in longitudinal section as elongated chalcedonic rods with a maximum length of a quarter of an inch, and in cross section as involute structures, some of which are so small that they can not be seen without the aid of a hand lens. They are shown in thin section in Plate 9. These fossils have been referred to a number of Paleozoic paleontologists, none of whom have been able to give any absolute determination of their character or age. Edwin Kirk, of the United States National Museum, to whom they were originally referred and subsequently referred again, has given the writer the following statement, which sums up concisely all that is known at present regarding them:

The zoological affinities of these curious fossils are doubtful. From a study of their gross structure as well as their structure in thin section under the microscope it appears that the shells were originally chitinous, though now silicified. This, in connection with their elongate subconical form, suggests that the fossils are referable to the Pteropoda. The structure as indicated by available material is peculiar and problematical. Originally there appears to have been a sort of "cone-in-cone" structure, with the adjacent walls connected by irregular septa. In some of the specimens it appears that subsequent to the death of the organisms the horny sheaths were split longitudinally, and one free margin or both margins rolled inward, resulting in a secondary involute structure.

The assignment of these fossils to the Pteropoda has in reality been made mainly by a process of elimination, although exactly comparable structures are not at present known in that group. Such types of pteropods indicate that the containing rocks are upper Paleozoic in age, but no correlation closer than this can be made.

This statement by Kirk, though it does not add materially to the stratigraphic placement of the containing beds, is in harmony with the other paleontologic evidence. Conversely, however, the invertebrates collected in 1928 make it possible now to state that these involute forms are characteristic of the Middle Devonian of this region, and this fact may be used to advantage at other localities where perhaps more diagnostic invertebrates are absent.

It is not possible to state definitely the stratigraphic position of these Middle Devonian argillite and chert beds with regard to the

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.

CARBONIFEROUS SYSTEM

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.

MISSISSIPPIAN SERIES

CIRCLE VOLCANICS

DISTRIBUTION

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

« PrejšnjaNaprej »