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sulphur staining. It occurs in beds 2 to 6 inches thick, with thin partings of black shale. At the most northerly exposures it has been compressed into numerous sharp folds, though in general all these beds are fairly regular in their attitude. No distinct line can be drawn between the beds of the chert formation and those of the overlying Calico Bluff sequence, but three prominent bands of yellowweathering limestone are seen at the north end of Calico Bluff, and the cherty rocks appear to become dominant just stratigraphically below these bands. This division between the two formations is a purely arbitrary one, but in the absence of any better one it is utilized for their cartographic separation.
Sontheast of Calico Bluff, at the mouth of Shade Creek, are other good exposures of the cherty rocks, which at this place consist essentially of siliceous slates. These slates are black and in part graphitic, as may be seen along some of the slickensided planes. They are, however, hard as flint and might better be called silica slates. Much yellow-brown iron staining is visible in the bluffs, as well as some white and yellow incrusted sublimates resembling gypsum and sulphur. The beds are from a fraction of an inch to 2 feet thick. A few rocks of other character were noted in these exposures, including true conchoidally fracturing chert, an exceedingly fine-grained quartzite, and a 2-foot bed of dark-gray politic limestone. The limestone has a strong petroliferous odor when fractured, but when tested in the chemical laboratory of the Geological Survey it yielded only a trace of oil.
South of the Yukon River, in the valleys of Coal and Woodchopper Creeks, is a formation that appears from the exposures along the creeks to be essentially a chert conglomerate or, more accurately, a chert breccia. This rock is only imperfectly exposed along Woodchopper Creek, but in the bedrock of Mineral Creek, a tributary of Woodchopper Creek, where placer mining is in progress, it has been exposed by the mining operations. The chert conglomerate may there be seen in unconformable contact with the much younger conglomerate of Upper Cretaceous or Eocene age. On Coal Creek chert breccia, chert conglomerate, and chert grit are intermittently exposed in bluffs along the west wall of the valley some 6 or 8 miles from the Yukon. Another good exposure occurs on the ridge about 900 feet above the Yukon River and about 2 miles S. 60° W. from the mouth of Coal Creek. The rock here is mainly a dark-gray chert, with a subordinate proportion of chert breccia. The mountain 3 or 4 miles north of the Woodchopper road house, at the head of Eureka Creek, is also composed of chert conglomerate and breccia.
The northermost exposures of this formation of siliceous slate and chert are seen along the northeast bank of the Yukon about 10 miles below Thanksgiving Creek. Here the river swings in, at the lower end of some islands, to rock bluffs of Lower Cretaceous slate and quartzite. Upstream (southeast), along a slough behind these islands, the Lower Cretaceous rocks continue but a quarter of a mile and give place to a formation of siliceous black slate and chert which, still farther upstream, changes to shaly sandstone and black and green slate, with a few beds of chert. Farther up on the hillsides these rocks weather to tones of yellow and brown, giving a characteristic appearance which readily differentiates these hills from the hills composed of the Lower Cretaceous rocks. Downstream from the Lower Cretaceous bluffs intermittent outcrops of chert and siliceous slate may be seen at low stages of the river, and still farther down, just above the junction of the north and south channels, there are good exposures of vitreous pellucid chert and siliceous shale in shades of red, green, and gray. These rocks terminate downstream at a creek entering from the north, where the two channels of the river unite, and below this point is a thin unmappable zone of Lower Cretaceous rocks, followed by a great series of volcanic rocks. These two small areas of chert and siliceous slate should, with little doubt, be correlated with the other rocks of this formation.
The rocks of the chert formation near Woodchopper and Coal Creeks range in color from pearl-gray and cream color into darker shades of gray and jet-black. The constituent fragments range in size from particles too small to be distinguished with the naked eye up to pebbles 1 inch or more in diameter and are usually either angular or subangular. In granularity, therefore, these rocks might be classified as ranging from a chert sandstone or chert grit to a sedimentary chert breccia. The unusual characteristic is a matrix composed also of chert of the same general character as the pebbles. So well indurated are these rocks that they will always fracture directly across the pebbles when broken. Samples of this chert breccia may be seen in which the pebbles are so firmly welded to the matrix that the fragmental character is scarcely apparent; other samples resemble a breccia that might be produced by fracturing in a fault zone; but still others may be seen in which the pebbles are fairly well rounded from the action of water and the sedimentary character is unquestionable.
The writer has seen rock of this peculiar type at many localities in interior and northern Alaska, mainly at or near the top of the Devonian sequence or the base of the Mississippian, and the lithologic description given above would fit almost equally well any of the observed occurrences. The problem of their origin is a most interesting but very difficult one, in the consideration of which a number of facts must be evaluated. At the known localities in interior Alaska the pebbles are exclusively chert; in northern Alaska they include both quartz and chert; but the exact correlation of the two formations
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.)
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 67 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
67 Mertie, J. B., Jr., Gold placers of the Tolovana district: U. S. Geol. Survey Bull. 662, Pp. 239-244, 1918.