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A. VERTICAL BEDS OF CHERT AT NORTH END OF BLUFF ON THE WEST SIDE OF THE YUKON RIVER BELOW TAKOMA CREEK

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B. EAST SIDE OF CALICO BLUFF, ON THE YUKON RIVER BELOW EAGLE Shows Calico Bluff formation underlain (at right) by lower Mississippian chert formation. A and B indicate two of the prominent beds. (See also fig. 5.)

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Note transverse septa in the longitudinal sections. Enlarged about 15 diameters.

in interior and northern Alaska is inferred rather than proved. The pebbles are usually subangular. The matrix is also chert, and in interior Alaska pebbles and matrix are identical in composition. The rocks appear to have been originally siliceous, to judge from certain fossils found in them near Livengood. These fragmental cherty rocks occur at the base, not at the top, of a thick formation composed mainly of homogeneous gray to black chert, with some siliceous slate and limestone.

Cherts, to be sure, are found in the pre-Mississippian Paleozoic rocks, and one might assume that the fragmental material in the chert breccias and conglomerates originated in long residual accumulations of chert débris derived from these cherty beds in the lower Paleozoic. But an enormous amount of vein quartz, much of which was unquestionably formed in pre-Mississippian time, is also present in the early Paleozoic and pre-Cambrian rocks of interior Alaska, and as this vein quartz is at least as resistant to weathering as chert, it would be natural to expect to find vein quartz as well as chert among the fragmental material, if it is assumed that long-continued surface weathering destroyed the other less resistant type of country rock. But such is not the case. It is therefore necessary to conclude that the matrix and pebbles are syngenetic-that is, they are essentially contemporaneous, in terms of geologic rather than secular time units. The origin of the chert conglomerate and breccia is therefore tied directly to the question of the origin of the overlying chert formation.

The writer has stated in previous publications the belief that this chert is of primary and probably of marine origin, though no evidence of its marine character was definitely known when it was first described by the writer 57 in 1916. Subsequently, in 1918, Overbeck collected some coralline and crinoidal material from the chert in its type locality, some 12 miles west of Livengood, thus establishing its marine character. What, then, is the mechanism of the formation of such rocks? Some limestone conglomerate and primary limestone breccia have been proved to form more or less simultaneously with the massive limestones about them, by the eroding and sorting action of oceanic currents upon marine limestone reefs. Such reefs, it seems, could be either of coralline or inorganic origin. But the sharp angularity of many of these chert pebbles shows clearly that the original fragments must have already attained a considerable degree of cohesive strength at the time of their formation, or else they suffered practically no transportation or sorting by water. The

57 Mertie, J. B., jr., Gold placers of the Tolovana district: U. S. Geol. Survey Bull. 662, pp. 239-244, 1918.

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subangular character of most of this fragmental débris and the rounded character of some, however, favors the idea that the fragments were well indurated at the time of origin of the breccia but suffered some sorting or transportation by water.

The writer confesses his inability at present to formulate a satisfactory genetic hypothesis for these cherty fragmental rocks. The thesis here proposed, however, is that ordinary terrestrial processes of erosion and stream transportation had little if any direct part in the formation of either the cherts or the fragmental cherty rocks; that both the fragmental cherty rocks and the overlying massive chert are of primary marine origin; and that the fragments and matrix of the breccias and conglomerates are essentially syngenetic. It is hoped that at some time a comparative study may be made of thin and polished sections of the cherts and chert breccias of this horizon from a number of localities in interior and northern Alaska. Specimens recently collected by the writer in northern Alaska show much vein quartz as well as chert among the pebbles of rocks that appear otherwise to be essentially the same as the rocks of this chert formation of interior Alaska. These variations may give rise to new ideas regarding the genesis of such rocks. In the meanwhile, it is hoped that field studies will continue to contribute stratigraphic and paleontologic data bearing on these questions.

STRUCTURE AND THICKNESS

The rocks of this formation exposed at the north end of Calico Bluff are relatively little deformed and constitute the lower beds of a synclinal basin that plunges gently about N. 30° W. The highest of these rocks underlying the Calico Bluff formation at the north end of the bluff are shown in Plate 8, B. The lowermost beds crop out downstream, north of Calico Bluff, for 6,000 feet in a low bench along the west bank of the river. The base of this sequence is nowhere exposed in this vicinity, and the assignment of the chert breccia and conglomerate farther downstream as the base of this formation can not be substantiated from any data obtainable along the river. This correlation is based on comparative stratigraphic studies made by the writer 58 at other localities in the Yukon-Tanana region, particularly in the Livengood district of the Fairbanks quadrangle.

Few structural observations of the chert breccia and conglomerate in the Woodchopper and Coal Creek areas are available, but the areal distribution of the two belts there present, one north and one south of the Middle Devonian Woodchopper volcanic rocks, suggests strongly the existence at this locality of an anticlinal fold whose

58 Mertie, J. B., jr., The gold placers of the Tolovana district: U. S. Geol. Survey Bull 662, pp. 239-244, 1917.

major axis of elongation trends northwestward. Unfortunately a band of Lower Cretaceous rocks, overlying unconformably these basal Mississippian rocks, lies between the Middle Devonian and the lower Mississippian rocks on both Coal and Woodchopper Creeks, completely concealing the contact between the two formations. The hills, however, are low and the exposures poor in the Woodchopper area, and it is doubtful if much information regarding the stratigraphic relations could be gained even if the Lower Cretaceous rocks were absent. The great lithologic differences between the Middle Devonian Woodchopper and the Mississippian rocks, and the evident kinship of the chert formation with Mississippian rocks, as exhibited at Calico Bluff, favor the belief that a stratigraphic and perhaps a structural break separates the base of this chert formation from the underlying Middle Devonian rocks.

The rocks of this sequence 10 or 12 miles below Thanksgiving Creek appear to dip in a general northwesterly direction and are believed to underlie the volcanic series adjoining them downstream. It is possible that these rocks form the northwest end of the Woodchopper anticline, plunging northwestward at this point under younger rocks.

Only at the north end of Calico Bluff has it been possible so far to measure the thickness of these rocks, and there, of course, only a part of the sequence is exposed. Here about 1,700 feet of strata crop out below the arbitrary line which is assumed as the base of the Calico Bluff formation, but the base of the section is not exposed. Considering the amount of chert breccia exposed in Coal and Woodchopper Creeks, particularly in Coal Creek, it is probably safe to assume that the total thickness of this formation is several thousand feet, but any more exact statement is not warranted.

AGE AND CORRELATION

Formations consisting dominantly of chert and siliceous slate and argillite have been found at several localities in interior and northern Alaska and have in general been difficult to place stratigraphically, for the following reasons: They resemble one another lithologically; they are usually incompetent, so that critical structural data are lacking; they rarely yield diagnostic fossils; and they are so far separated from the standard sections of the United States that the ordinary stratigraphic nomenclature can at best be applied only in the most general way. The formation of chert and related rocks appears to have been more than ordinarily prevalent in Devonian and Mississippian time, and where such rocks occur at two or more horizons in the same general area, as in the Eagle-Circle district,. their correct differentiation becomes difficult. The chert formation

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