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5867. West bank of Yukon River 6 miles above mouth of Seventymile River (Calico Bluff). Collector, A. J. Collier.

2AC56. South bank of Yukon River 2 miles above mouth of Seventymile River. Pebble in conglomerate. Collector, A. J. Collier.

2645. North bank of Yukon River about 5 miles above Seventymile River. Collector, Arthur Hollick.

2646. West bank of Yukon River just above mouth of Sheep Creek (Tatonduk River). Collector, Arthur Hollick.

2647. Calico Bluff, Yukon River. Collector, Arthur Hollick. 2644, 2644A, and 2644B. Calico Bluff, Yukon River. Collector, A. H. Brooks.

2651 and 2651A. North bank of Yukon River above island near Star. (Star is an abandoned settlement at the mouth of Seventymile River.) Collector, A. H. Brooks.

843. Calico Bluff, Yukon River. Collector, E. M. Kindle.

845. North bank of Yukon River 1 mile above Seventymile River. Collector, E, M. Kindle.

1796, 1796A, and 1796B. North end of Calico Bluff, Yukon River. Collector, Eliot Blackweider.

1797, 1797A, 1797B, 1797C, 1797D, 1797E, and 1797F. North bank of Yukon River at big bend north of Calico Bluff. Collector, Eliot Blackwelder.

1798 and 1798A. Calico Bluff formation, southwest of mouth of Sheep Creek, (Tatonduk River). Collector, Eliot Blackwelder.

5279. North bank of Yukon River nearly opposite Seventymile River. Collector, G. H. Girty.

5302. South end of Calico Bluff, Yukon River. Collector, G. H. Girty.

5302A. Base of Calico Bluff, Yukon River. Float specimens. Collector, G. H. Girty.

5303. North end of Calico Bluff, Yukon River. Collector, G. H. Girty.

5304. West bank of Yukon River opposite mouth of Sheep Creek (Tatonduk River). Collector, G. H. Girty.

5843 and 5843A. Calico Bluff, Yukon River. Just below base of zone B. Collector, J, B. Mertie, jr.

5843B. Calico Bluff, Yukon River. Zone C. Collector, J. B. Mertie, jr.

5843C. Calico Bluff, Yukon River. From 4-foot bed of limestone that lies 41 feet stratigraphically above top of zone C. Collector, J. B. Mertie, jr.

5843D. Calico Bluff, Yukon River. In the 215-foot zone (see section, p. 97) about 450 feet vertically above river. Collector, J. B. Mertie, jr.

5843E. Calico Bluff, Yukon River. Float specimens. Collector, J. B. Mertie, jr.

It will be seen from the faunal list above given that 117 genera have been identified, and it is likely that 250 or more species are represented. Girty's work on these collections has been of a general nature, with the purpose of determining the age of the several collections sent to him, rather than an intensive study involving complete specific identifications and the description of new species. Many new species are present in these collections, and a thorough and detailed paleontologic study of this material would probably reveal some interesting biologic data. This fauna is related more closely to the marine Asiatic faunas described by Tschernyschew than to the Mississippian faunas of the Rocky Mountain region, and this fact,

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together with the presence of many new species, has deterred Girty from correlating it very closely with the sections of the States. Girty believes the Calico Bluff formation to be of upper Mississippian age, though not necessarily correlative with the latest Mississippian rocks. Roughly the Calico Bluff formation can be correlated with the Chester group of the United States.

Mississippian rocks, like those of the Middle Devonian series, are widespread in Alaska. One of the most persistent zones of such rocks is found along the north slope of the Brooks Range, in Arctic Alaska, where an upper Mississippian group of rocks, known as the Lisburne limestone, extends from Cape Lisburne on the Arctic Ocean eastward almost if not quite continuously for a distance of 600 miles to the international boundary. This formation is composed of limestone and chert, and although lithologically it differs from the Calico Bluff formation, paleontologically it is very closely correlative. Stratigraphically below the Lisburne limestone in northern Alaska lies the Noatak formation, which is also known to be of Mississippian age, though not necessarily upper Mississippian. The Noatak formation is related more closely in age to the chert formation of the Yukon and to the Circle volcanics and Rampart group than to the Calico Bluff formation.

North of the Yukon, along the international boundary, Cairnes 63 collected a fauna similar to that of the Calico Bluff formation but did not differentiate the containing rocks as a separate formation. This fauna is presented in this paper in the discussion of the undifferentiated Paleozoic rocks. By plotting the localities of Cairnes's fossils, however, it appears that many of the upper Mississippian localities fall in his “shale-chert "group, of Ordovician to Carboniferous age, here mapped as undifferentiated Paleozoic, which lies just east of the boundary, between the Tatonduk River and Hard Luck Creek, and especially near to Hard Luck Creek. Hence it seems certain that the Calico Bluff formation reappears along the boundary some 15 miles northeast of its type locality at Calico Bluff.

Mississippian rocks are also known at the head of the White River and in the York district of Seward Peninsula. South of the Alaska Range another great series of basic volcanic rocks of greenstone habit, known as the Strelna formation, of Mississippian age, has an extensive development in the valley of the Chitina River. This formation, however, contains a larger proportion of interbedded sediments than the Rampart group. The Strelna formation is believed by the writer to continue southeastward into southeastern Alaska, and certain of the volcanic rocks of Lynn Canal are believed to represent this horizon. Farther southeast, in southeastern Alaska, a large representation of

63 Cairnes, D. D., The Yukon-Alaska international boundary between Porcupine and Yukon Rivers : Canada Geol. Survey Mem, 67, pp. 93–103, 1914.

Mississippian rocks is known on Chichagof, Kuiu, and Prince of Wales Islands.

MISSISSIPPIAN OR PENNSYLVANIAN SERIES

TRANSITIONAL FORMATION

DISTRIBUTION

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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 142 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.

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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.

ACE 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 localities northwest 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

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