from the mouth of Coal Creek downstream for 15 miles, extending on the south bank just beyond the mouth of Thanksgiving Creek. 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: Section of Woodchopper volcanics on south bank of Yukon River below Coal Creek Feet Lower Cretaceous slate exposed at mouth of Coal Creek. 100 200 1,400 Greenstone, massive, basaltic. Shows numerous slickensided 700 Covered along beach but exposed on hillside. Probably 1,600 Limestone. Seen only from river, but believed to be a part of the Woodchopper volcanics. Strike N. 35° 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. 75° 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 54 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 5 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. 65° W. and veers more toward the north downstream 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. 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 21⁄2 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 DISTRIBUTION 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. LITHOLOGY 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 sedimentary 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. |