Mississippian rocks is known on Chichagof, Kuiu, and Prince of Wales Islands. MISSISSIPPIAN OR PENNSYLVANIAN SERIES TRANSITIONAL FORMATION DISTRIBUTION The rocks of a transitional formation of Mississippian or Pennsylvanian age are exposed on the north and south sides of the Seventymile River southwest of Calico Bluff, in a narrow zone north of the mouth of the Tatonduk River, and along the northeast side of the Yukon River northwest of Nation. Rocks at other localities along the boundary, which have been mapped in this report as undifferentiated, may later be shown to belong to this horizon. LITHOLOGY The rocks of this transitional formation consist essentially of sandy shale, argillite, slate, and some chert. On exposed hilltops and slopes, especially where old burns have bared the surface to view, these rocks are considerably weathered, so that the bedding and joint planes, particularly in the argillaceous varieties, are commonly covered with a very thin red film that is limonitic or perhaps in part hematitic, and the effect of this weathered fragmental débris is to give to the hillsides occupied by such rocks a bright-red appearance when viewed from a distance upon a sunny day. These brightly colored hillsides have been noted by many geologists going down the Yukon but apparently were not examined by anyone until the season of 1925. The vivid coloring has been noticed particularly in the hills northwest of Eagle and northwest of Nation. At the base of these hills near Nation, in a swamp, in the alluvium of the river valley, there is a deposit of bright-red clay, evidently derived from the red rocks farther up the slope. It is said that the Indians used to visit this locality to obtain red pigment for tattooing and other tribal uses. The red-weathering beds northwest of Eagle, along the south side of the Seventymile River, consist of thin-bedded black carbonaceous shale, weathering to shades of gray and brown, calcareous shale, siliceous slate, siliceous limestone, chert, and some beds of conglomerate. The pebbles of the conglomerate are mainly chert. Some of these beds weather to a light reddish brown, which under the sun's rays appears bright red at a distance. Along the west bank of the Yukon about 12 miles below Eagle this same formation, as seen in landslides, consists of beds of shale and chert. The same red-weathering, hematite-covered siliceous slate and chert were identified on the north side of the Seventymile River, in contact with the Calico Bluff formation. The red-weathering beds northwest of Nation consist of cherty grit, quartzitic sandstone, and sandy shale and are in contact with the Nation River formation. The exposures along this zone are not good, as the area is mostly timber covered, but evidently the transitional formation exists here, although the area mapped as such may in fact include part of the Nation River formation. At the locality north of the mouth of the Tatonduk River the rocks, as seen from a distance, are cream-colored rather than reddish. They appear to be at or near the base of the formation. This conclusion is drawn from the fact that the formation here contains both upper Mississippian marine fossils and obscure plant remains, a combination suggesting its transitional nature, and that it lies next or very close to the upper Mississippian rocks that are exposed a little farther east. These rocks are in part calcareous. No really good exposures of this formation were seen here, the outcrops being mostly weathered rubble on the hill slopes, and therefore nothing whatever is known regarding its structure or thickness at this locality. STRUCTURE AND THICKNESS There is no one locality at which the formation is completely developed from its base, as at present recognized, at the top of the Calico Bluff formation, to its top, just below the Nation River formation. Moreover, being a relatively nonresistant group of rocks, like the Upper Triassic series, it crops out on low, inconspicuous ridges, many of which are timber covered, and the exposures are both poor and discontinuous. Hence it is particularly difficult to formulate any ideas of the general structure or to evaluate the structural data in order to arrive at any estimate of thickness. Along the north side of the Seventymile River near its mouth the rocks of this formation strike N. 40° W. and dip 70° NE., thus apparently plunging under the strip of Calico Bluff formation that adjoins them to the northeast. The rocks of both formations at this locality, however, constitute part of a faulted sequence, and the contact between the two formations is interpreted as a fault contact, probably one in the series of fault contacts that are exposed to the northeast, on the opposite side of the Yukon. South of the Seventymile River this formation adjoins the Nation River formation. The beds are greatly disturbed and therefore very irregular in strike and dip but seem on the average to strike about northeast. The dips are high, ranging from 40° to 75°, both to the southwest and to the northeast, thus yielding little information regarding the structural relation of this formation to the adjoining Nation River beds. It is believed that the rocks of this formation overlie the Calico Bluff formation and underlie the Nation River formation, but no data are available for stating definitely the structural relationships to either the underlying or the overlying rocks. The basal beds of the Nation River formation, however, as seen at the head of the North Fork of Shade Creek differ lithologically from the formation here described, and it is therefore inferred that these beds are more likely to grade downward into the Calico Bluff formation than they are to grade upward into the Nation River formation. If in fact these rocks are thus related stratigraphically to the Calico Bluff formation, it is likely that the Nation River formation may overlie them unconformably. No reliable estimate can at present be made of the thickness of this formation, for neither its base nor its top is accurately known. It is believed, however, that its thickness is probably materially less than that of the Nation River formation, which is believed to be from 4,000 to 6,000 feet. Perhaps, from what is now known of this formation, 1,000 feet or at most 2,000 feet may be taken as a working estimate. AGE AND CORRELATION The stratigraphic evidence for the placement of this formation within the Carboniferous sequence has been sketched in the discussion of the lithology and structure. These rocks are evidently related to the Nation River formation, for at the two best-known localitiesnorthwest of Eagle and northwest of Nation-they appear to adjoin areally the Nation River formation. It might, in fact, have been better to include. them as a part of that formation, but their distinctive lithology and appearance have led the writer to treat them as a separate unit. This will at least focus attention upon them when this area is worked in greater detail at some subsequent time. G. C. Martin, in 1914, made a collection (Martin 81) of some obscure plant remains from a locality given as "east bank of Yukon River 2 miles below Tatonduk River," which may be the same as the locality noted by the writer north of the mouth of the Tatonduk. These plant remains have not been identified. At this locality the writer noted some imperfect marine fossils, which were not collected. The inference is that both marine and terrigenous conditions are represented at this locality. The rocks at this locality, because they appear to adjoin the Calico Bluff formation, are interpreted as the basal part of the transitional formation, but the evidence for this is not complete and unequivocal. The importance of this locality, if the writer's stratigraphic assign 64 ment is correct, is that it may show a gradual stratigraphic transition downward into the marine Calico Bluff formation, of upper Mississippian age. Brooks, in an earlier paper, without reference to any particular group of rocks, felt that an intermediate formation might well exist between the Calico Bluff and Nation River formations. states: He It is not improbable that detailed mapping may reveal a considerable thickness of strata lying between the Calico Bluff and Nation River [formations] as here described. Whether such strata, if found, should be included in one or the other of these formations or be mapped as a distinct stratigraphic unit must be left to the future to be determined. It is conceived by the writer that this so-called transitional formation does, in fact, represent such an intermediate stratigraphic horizon; and the evidence at hand indicates that this transitional formation is more closely related lithologically and stratigraphically to the Calico Bluff than to the Nation River formation. Brooks also believed that an unconformity existed at the base of the Nation River formation, and no data have so far been acquired that controvert that conclusion. If such an unconformity does in fact exist, the evidence is even stronger for relating this transitional formation with the Calico Bluff formation. Nevertheless, all available evidence of the age of these beds is still inconclusive, and it has therefore seemed best on the geologic map to designate them simply Mississippian or Pennsylvanian. Another possible assignment of at least a part of this transitional formation should not be overlooked. From the lithologic descriptions it will be noticed that some of these rocks, particularly those south of the Seventymile River, resemble very much the Middle Devonian sequence at the head of the North Fork of Shade Creek. It is thus possible, in the absence of fossil evidence, to correlate these rocks with the Middle Devonian argillite-chert sequence, and this possibility will need to be considered in future work in this area. No similar rocks occurring in this portion of the stratigraphic column are known elsewhere in interior or northern Alaska, so that no stratigraphic correlations can be made at present. The most striking lithologic similarity is apparent in reading Cairnes's description 65 of his "shale-chert group," which he mapped as Carboniferous to Ordovician, and which in this report is assigned to the undifferentiated Paleozoic. Cairnes states in part: Hard gray quartzitic shales are in addition somewhat extensively developed in places. These quartzitic beds contain locally sufficient iron to produce upon 4 Brooks, A. H., and Kindle, E. M., Paleozoic and associated rocks of the upper Yukon, Alaska Geol. Soc. America Bull., vol. 19, p. 292, 1908. 65 Cairnes, D. D., The Yukon-Alaska international boundary between Porcupine and Yukon Rivers: Canada Geol. Survey Mem. 67, p. 82, 1914. oxidation a bright-red to yellow coloration on weathered surfaces, but only rarely are these rocks red on a fresh surface. These reddish beds decompose readily to form a red or yellowish sand or mud, which is a very noticeable feature of many of the hillsides on which vegetation is lacking. This description fits very well the description of this transitional formation primarily given by the writer and suggests strongly that the rocks of this formation are also represented along the boundary north of the Yukon. NATION RIVER FORMATION DISTRIBUTION The Nation River formation is exposed at two general localities along the Yukon. It crops out along both sides of the Yukon a short distance below Eagle and extends eastward up the valley of Shade Creek for an unknown distance, forming the bedrock at McCann Hill, a prominent dome-shaped hill along the international boundary. The other area consists really of two belts of the Nation River formation, separated by a zone of Permian limestone, which extend from the Yukon northeastward up the valley of the Nation River to and beyond the international boundary, although the distribution of this formation between the Yukon and the boundary is not exactly known. The belt south of the Nation River is believed to extend southwestward as far as the valley of Michigan Creek, but its limit has not been ascertained. A small anticlinal flexure brings the upper part of the formation to the surface in the middle of the Permian limestone just above the Nation River. LITHOLOGY The rocks of this formation consist of gray clay shale, sandstone, and conglomerate and resemble very much the Upper Cretaceous and Eocene rocks, from which, indeed, they are at some localities hard to distinguish. Where this formation crops out about 3 miles below Eagle it consists essentially of a drab sandy clay shale and sandstone, the sandstone in beds from a few inches to 10 or 20 feet thick. This sandstone consists of grains of chert, decomposed feldspar, and more or less quartz and carbonaceous material. It weathers to a darkbrown color. The beds are commonly ripple marked but only slightly cross-bedded and show numerous mud lumps and concretions, especially along the bedding planes. Conglomerate beds with a thickness as great as 10 feet are intercalated with the shale and sandstone. This conglomerate is dark colored. It is composed of chert pebbles, mainly light gray and dark gray but with some green, set in a sandy matrix. Below Boulder Creek the beds are coarser grained and consist dominantly of sandstone in beds from 3 inches to 6 feet thick, |