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States National Museum, who has made most of these paleontologic determinations, regards this fauna as late Middle Devonian and very closely related to the Upper Devonian faunas.
Late Middle Devonian fossils from the Woodchopper volcanics along Yukon
River between Eagle and Circle
Collier 4, 5, 6. Locality not recorded. Collector, A. J. Collier.
2AC60. Yukon River, east bank, 3 miles below mouth of Tatonduk River. Pebble from river gravel. Collector, A. J. Collier.
2AC62. Yukon River, east bank, 7 miles below mouth of Tatonduk River. Pebble from river gravel. Collector, A. J. Collier.
2AC90. Yukon River, north bank, opposite Woodchopper; upper end of a series of bluffs. Collector, A. J. Collier.
2AC96. Yukon River, southwest bank, 3 miles below Woodchopper Creek, Collector, A. J. Collier.
2AC97. Yukon River, southwest bank, 4 miles below Woodchopper Creek. Collector, A. J. Collier.
2AC131. Locality not recorded. Collector, A. J. Collier.
841. Yukon River, north bank, opposite Woodchopper road house. Collector, E. M. Kindle.
842. Yukon River, north bank, 2 miles above Woodchopper Creek. Collector, E. M. Kindle.
846. Yukon River, southwest bank, 2 miles below Woodchopper Creek. Collector, E. M. Kindle.
156, 168, 169, 170. Yukon River, north bank, 2 to 3 miles above Woodchopper Creek. Collector, Eliot Blackwelder.
171. Yukon River, southwest bank, about 2142 miles below Woodchopper Creek. Collector, Eliot Blackwelder.
172. Yukon River, north bank, opposite Woodchopper road house. Collector, Eliot Blackwelder.
2065. Yukon River, north bank, opposite Woodchopper road house. Collector, J B. Mertie, jr.
The rocks along the Yukon (Woodchopper volcanics) containing the late Middle Devonian fauna differ from most of the other Middle Devonian rocks of Alaska in that they are mainly volcanic, though they include also some interbedded fossiliferous sedimentary rocks. The fauna, also, appears to be distinguishably different from the typical Middle Devonian fauna, such as that of the Salmontrout limestone. Therefore, although the stratigraphic limits of this formation can not be exactly given, the rocks and fauna as a whole appear to be sufficiently distinctive to warrant a name, and the term “ Woodchopper volcanics” is proposed for this assemblage of rocks.
ARGILLITE, CHERT, AND CHERTY GRIT
The Middle Devonian rocks mapped as argillite, chert, and cherty grit are found at the head of the North Fork of Shade Creek and extend in a narrow belt both eastward and westward from the type locality. Rocks of the same lithologic character are also found in the valleys of Eagle and Last Chance Creeks, south of the type locality.
The Middle Devonian rocks at the head of the North Fork of Shade Creek consist of argillite, slate, chert, and cherty grit. The argillaceous varieties are dark colored and everywhere more or less siliceous, at some places so much so that although cleaving like slate, they appear to be as hard as flint and might better be designated silica slate and silica argillite. Chert seems to be more abundantly developed in the upper part of the sequence. Probably the most interesting and certainly the most diagnostic lithologic type is a cherty grit that contains peculiar involute fossil forms by means of which it has been possible to correlate this formation at its type locality with similar rocks on Eagle Creek. This grit is in reality a sedi- . mentary chert breccia, composed of angular to subangular fragments of chert, mainly black but subordinately gray, in a matrix of chalcedonic silica and some sandy material. The chert grains range from a quarter of an inch in diameter down to microscopic dimensions, and the contained involute fossil forms have a similar range in size.
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. 75o 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 ai 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. 75o 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:
28AMt260. North Fork of Shade Creek, 0.72 mile N. 13° W. from “Hug" boundary triangulation station (McCann Hill):
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
orizon 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