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9321. Trout Creek about 3 miles from confluence with Yukon River. Collector, P. J. Hilliard, of Eagle, Alaska.
13423, 13425, 13426, and 13427. Yukon River, southwest bank 24 miles upstream from Nation. The relative positions of these four collections are given in the stratigraphic section on page 131. Collector, J. B. Mertie, jr.
13424. Southeast side of ley of Nation River Yukon River. Collector, J. B. Mertie, jr.
The Triassic rocks of Alaska have been described in detail by Martin,84 and it is unnecessary to include here a faunal and lithologic correlation of the Upper Triassic rocks of the Yukon with the other Triassic rocks. Upper Triassic rocks are known at many localities in Alaska, of which Martin lists the Nizina Valley, Kotsina and Kuskulana Valleys, Cooper Pass, upper Susitna Valley, Kenai Peninsula, west coast of Cook Inlet, Iliamna Lake, Alaska Peninsula, Kodiak Island, Admiralty Island, Kupreanof Island, Gravina Island, Firth River, Canning River, Noatak Valley, Cape Lisburne, and Cape Thompson. To these should be added the numerous areas of Upper Triassic rocks recently discovered in northwestern Alaska,85 which, together with those at Cape Lisburne and on the Canning and Firth Rivers, in northeastern Alaska, indicate that a continuous Upper Triassic belt crosses northern Alaska from the Arctic Ocean to the international boundary. These Arctic Upper Triassic rocks differ lithologically, however, from those found along the Yukon in that they include a considerable amount of chert.
The rocks at the base of the Upper Triassic sequence at the Nation River contain certain genera of ammonites, such as Placites, Popanoceras, Trachyceras, Clionites, Monophyllites, and Nathorstites, which do not occur in the higher beds. These are accepted by Stanton as an integral part of the Upper Triassic fauna but are believed to represent a lower faunal horizon than the fossils found in the higher strata.
No Lower Triassic rocks have ever been found in Alaska; nor is the Middle Triassic represented, except possibly at one questionable locality at Brooks Mountain, in Seward Peninsula. In this connection, it should again be emphasized that the Tahkandit limestone, as well as the other Permian rocks in Alaska, represent only the lower part of the Permian sequence. Nevertheless, Upper Triassic rocks, wherever found in Alaska, appear to lie upon Permian, Pennsylvanian, or Mississippian rocks, without any angular discordance of bedding. It is believed that a gradual uplift of Alaska began in late Carboni ferous time and culminated perhaps in the early Triassic, followed by submergence again in Upper Triassic time. The lack of any apparent angular unconformity between the Permian and Upper
S4 Martin, G. C., Triassic rocks of Alaska : Geol. Soc. America Bull., vol. 27, 1916.
& Smith, P. S., and Mertie, J. B., jr., Geology and geography of northwestern Alaska : U. S. Geol, Survey Bull. 815, pp. 185–194, 1930.
Triassic rocks leads to the belief that this uplift was of the plateauforming type and was accompanied in Alaska by a minimum of warping and rock deformation; if any deformation took place it affected only the older, pre-Carboniferous rocks, which were more deeply buried. As this plateau-forming uplift in Carboniferous to Triassic time was not worldwide in extent, it follows that in areas adjacent to Alaska there must be a hinge zone, where this time interval will be found to be represented by deformed strata. Hence the postulate of an unconformity involving little or no rock deformation is intended to apply only to Alaska. It is believed that the deformation of the Carboniferous, Permian, and Triassic rocks, which resulted in the folding and faulting of these rocks, took place in one or more stages in post-Triassic time.
CRETACEOUS AND TERTIARY SYSTEMS
Jurassic rocks, though present in great thickness in southern Alaska and on the Alaska Peninsula, are unknown in the interior of Alaska and are very sparingly developed, if at all, in Arctic Alaska. The system next above the Upper Triassic in the Yukon region, therefore, is the Cretaceous. Two series of Cretaceous rocks have been differentiated in the upper Yukon Valley, of which the older is a well-developed Lower Cretaceous series. The younger is an Upper Cretaceous series, but as these rocks apparently grade upward lithologically into Eocene rocks in such a manner that it has not been possible to draw a line between the two series, they are herein mapped together and described under the heading Upper Cretaceous and Eocene series.
LOWER CRETACEOUS SERIES
The Lower Cretaceous rocks in this area occur mainly at two localities which will probably later be found to form parts of one continuous belt. Along the Yukon these rocks crop out about 10 miles below the Nation River and continue on both sides of the Yukon downstream to Coal Creek, thence thinning to a narrow band which continues northwestward to Woodchopper Creek and for some undetermined distance farther. (See pl. 11, B.) The other area is along the international boundary from the neighborhood of Sitdown Creek northward for at least 25 miles. This area, which was mapped by Cairnes, was not differentiated by him as Lower Cretaceous nor in fact as exclusively Cretaceous; his cartographic designation was
Cretaceous and Upper Carboniferous.” The inclusion of “ Upper Carboniferous” in his legend arose from the fact that the Nation River formation is so similar to the Upper Cretaceous rocks that at many places it is difficult to differentiate between the two. From the distribution of the Lower Cretaceous rocks along the Yukon, the writer surmises that much of the rock along the international boundary from Sitdown Creek northward to the 66th parallel is most likely also of Lower Cretaceous age. It is believed that between Sitdown and Ettrain Creeks, however, Cairnes's “ Cretaceous and Upper Carboniferous” group is more likely to be the Nation River formation. Hence, on the accompanying geologic map the country rock from Sitdown northward to the limits of the map is shown as Lower Cretaceous, and from the Nation River southward to Ettrain Creek the Nation River formation is indicated, with a blank unmapped area between the two formations. Another minor outcrop of Lower Cretaceous rocks is seen along the northeast bank of the Yukon about 10 miles below the mouth of Thanksgiving Creek. This narrow belt may be the northwestward continuation of the narrow belt of the same rocks that crosses Coal and Woodchopper Creeks.
Blackwelder 86 has used the term Kandik formation in referring to the Lower Cretaceous rocks of the upper Yukon, and inasmuch as these rocks are typically exposed in the valley of the Kandik River, from the Yukon northeastward probably to the boundary, this formation name seems particularly fitting and is here formally applied. To be sure, neither the top nor the bottom of the Lower Cretaceous rocks has been recognized in this area, but as Jurassic rocks are absent in interior Alaska this formation is known to lie everywhere unconformably upon rocks older than Lower Cretaceous; it is also believed to underlie unconformably the Upper Cretaceous series. The term Kandik formation may therefore be regarded as a formational name that includes all the Lower Cretaceous sedimentary rocks of this area, of whatsoever lithologic character, although it seems highly probable that only sandstone, slate, and conglomerate are represented.
The Kandik formation consists in the main of a monotonous sequence of black slate and thin beds of sandstone. The slates are carbonaceous, argillaceous rocks, which in some of the thicker beds show little stratification. No calcareous shale or limestone was seen, and the slates appear not to be bituminous or oily, thus differing markedly from the Upper Triassic shales. Moreover, no chert or siliceous slate appears in the sequence, which thus differs from the lower Mississippian (?) rocks. These argillaceous members are ad
» Blackwelder, Eliot, unpublished notes.
visedly for the most part termed slate rather than shale, for in most of them fracture cleavage is well developed. Plate 11, B, shows a typical exposure of the Lower Cretaceous quartzitic sandstone and slate. The sandstone beds show no sign of such cleavage, although some of them are much jointed.
The sandstones occur for the most part in thin beds, from a few inches to 1 or 2 feet thick, but some thick massive beds occur in the sequence. One very thick bed of this type is exposed along the south bank of the Yukon just below Glenn Creek. The thinner beds of sandstone are usually dark gray on a fresh break but weather to a dull-brown color, which is due probably to their content of ferrous iron. They are composed essentially of grains of quartz, with little chert but with a certain amount of altered feldspar and ferromagnesian minerals. The thicker beds are inclined to be more purely quartzose, and some of them by partial recrystallization closely approach quartzite. In the Rampart district, where these same Lower Cretaceous rocks occur, the heavy quartzose beds served the writer to a certain extent as horizon markers, inasmuch as they make prominent hogbacks; they will probably be useful in a similar way along the Yukon when more detailed mapping is attempted.
Along the banks of the Yukon no conglomerate was seen in the Lower Cretaceous sequence. A few miles upstream from the Kandik River, however, on the north side of the Yukon, a great thickness of these rocks is exposed at Kathul Mountain. The top of this mountain is 2,400 feet above the level of the Yukon, and in the course of a trip up the mountain the writer observed that the upper threefourths of the sequence was composed largely of sandstone and conglomerate, the conglomerate being particularly evident near the top. This conglomerate, however, is rather fine grained, and much of it might better be described as a grit, although some beds containing pebbles as large as 3 inches in diameter were seen.
The component fragmental material is subangular to rounded and consists of quartz, chert, slate, and fragments of other dark-colored rocks, perhaps in part of volcanic origin. Except in the fineness of grain these conglomeratic beds do not differ materially from some of the conglomerates of the Upper Cretaceous and Eocene sequence. It is possible, indeed, that these conglomerates and grits may mark the base of the Upper Cretaceous, but the writer is inclined to believe that they form an integral part of the Lower Cretaceous series.
STRUCTURE AND THICKNESS
Little work was done by the writer on these Lower Cretaceous rocks. Such observations as were made, however, indicate that the rocks are considerably deformed, though the folds are for the most part of the open type and of large amplitude, so that considerable stretches of river bluffs show what appears to be homoclinal structure. Some steep dips were noted at places along the river, and these, considered in relation to the nature of the foldings, suggest the presence of some large unrecognized faults. Large unexamined areas of such rocks occur between Glenn and Coal Creeks, and it is rather hazardous to extrapolate the visible structure into such unknown areas.
The rocks at the west side of Kathul Mountain dip southeastward, and those at the east end dip northwestward, so that this mountain occupies approximately the center of a gentle syncline. As this mountain rises 2,400 feet above the Yukon, it is safe to say that at least 2,400 feet of Lower Cretaceous strata are present at this locality. Neither the top nor the base of this sequence has been recognized, however, and it is probable, therefore, that two or three times that thickness of strata may be present in this wide belt of rocks. No data are at hand for making any closer estimate of the stratigraphic thickness.
AGE AND CORRELATION
Fossils are rather scarce in the Kandik formation, but nevertheless a number of small collections have been made, which are adequate for determining the geologic age. This fauna, which has been determined by T. W. Stanton, of the United States Geological Survey, is listed below.
2674. Washington Creek 6 miles above mouth. Collector, A. J. Collier.
3783. Yukon River, southwest bank about 400 yards below Glenn Creek. Collector, E. M. Kindle.
3784. Yukon River, south bank about 142 miles below Sam Creek. Collector, E. M. Kindle.
3785. Yukon River, north bank about 6 miles above Charlie Village. Collector, E. M. Kindle.
9389. Yukon River 842 miles above Washington Creek. Collector, Eliot Blackwelder.