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half an inch to 5 inches thick, interbedded with argillite, slate, and a few beds of limestone conglomerate. Much of the dolomite shows very fine alternating black and gray laminae, at some places as many as 50 to the inch. Some of the laminae are at an angle to the upper and lower surfaces of the beds and strongly suggest cross-bedding. All these beds are silicified to a greater or less degree, and some of the silicified argillite and slate is perhaps better designated chert. The fragmental limestone beds are of two types. The more common type consists of rounded limestone pebbles in a limestone matrix; some of these rocks show little or no silicification. The other type is an oolitic limestone in which the oolites are largely silicified to little balls of chert. Just above Glenn Creek this group of dolomitic rocks contains some altered intrusive rocks that were probably originally of basaltic or diabasic character but that now consist of secondary minerals, such as calcite and chloritic products, and are classifiable generically as greenstones. The rocks along the Yukon at the boundary, mapped by Cairnes 29 as the Yukon group and assigned to pre-Cambrian time, are in this report included with the undifferentiated Paleozoic rocks. Cairnes describes the Yukon group in general as composed of schistose amphibolites and quartzite and mica schists, with a few beds of limestone. He further states that these rocks are much folded, faulted, and distorted and are so highly metamorphosed in places that it is difficult or impossible to determine their origin or original character.
Along the ridge south of Fortymile the writer noted greenstone schist, quartzite schist, massive quartzite, graphitic schist, and several varieties of green schist, which correspond closely to the Yukon group as described. Such rocks continue downstream from Fortymile to the boundary, where they appear to be more consistently greenish and to contain a larger proportion of metamorphosed basic igneous rocks. In the river bluffs along the north side of the Yukon below Ogilvie's station are seen sheared, banded, and massive greenstones and some green and yellow rocks of sedimentary origin, within which are included several bands of marble, one of them 5 feet or more thick. Rocks of the same general character occur in the hills on the opposite side of the river, and in the interbanded marbles of this sequence, a short distance back from the Yukon, Paleozoic fossils were discovered in 1925. In rocks of the same type 13 miles south of Eagle, in the headwaters of Boundary Creek, Prindle in 1903 found Paleozoic fossils. These rocks, therefore, have in this report been included as a part of the undifferentiated Paleozoic sequence. This is not to be interpreted in any way as a reclassification of the age of the Yukon group as defined by Cairnes. It is merely a with
Cairnes, D. D., op. cit., pp. 39-44.
drawal of a certain portion of the rocks previously mapped as a part of the Yukon group, with a new assignment of them to the Paleozoic instead of to the pre-Cambrian.
To the north of these undifferentiated Paleozoic rocks, but separated from them by an overlapping band of Cretaceous and Eocene rocks, is a belt of limestone, which in turn is adjoined to the north by a group of rocks mapped by Cairnes as a part of his Tindir group, of Cambrian or pre-Cambrian age. This group of rocks consists essentially of drab shale and slate with some relatively thin beds of quartzite and conglomerate. Northwest of the mouth of Eagle Creek these rocks are well exposed along the north bank of the Yukon in a greatly disturbed fault zone and are there seen to contain also greenstone, probably of intrusive character. Except in this faulted zone the rocks of this formation are not so greatly metamorphosed as the undifferentiated Paleozoic rocks along the banks of the Yukon farther upstream, but this fact alone can not be regarded as decisive evidence of Paleozoic age. For reasons given later these rocks are not believed to be of Cambrian or pre-Cambrian age, as stated by Cairnes, and they are, therefore, here included as a part of the undifferentiated noncalcareous Paleozoic rocks.
Farther north along the international boundary, in several irregular areas from McCann Hill to Hard Luck Creek, Cairnes 30 maps a so-called "shale-chert " group, which ranges in age from Ordovician to Carboniferous. These rocks are mapped in this report as undifferentiated noncalcareous Paleozoic. With regard to their lithology, Cairnes makes the following statement:
This series consists dominantly or entirely of shales and cherts, which are prevailingly closely and finely interbedded. The cherts in places become really cherty shales or shaly cherts and occur in most places in beds ranging in thickness from 1 to 6 inches. Locally, however, they are more thinly bedded, and occasionally, on the other hand, they are in strata as much as 12 inches thick. They are also generally dark gray to black in color. The shales are also typically thinly bedded and in most places are soft and friable and gray to black or bluish black in color, the darker beds being in places decidedly calcareous in character. Occasional red shales also occur, however, locally intercalated with the darker strata, but these do not appear to be very persistent, or at least the color is not. Hard gray quartzitic shales are in addition somewhat extensively developed in places. These quartzitic beds contain locally sufficient iron to produce upon oxidation a bright-red to yellow coloration on weathered surfaces, but only rarely are these rocks red on a fresh fracture. 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.
Along Hard Luck Creek, where these shale-chert beds are well exposed, they have a red and black and in places even a decided ribboned appearance. The chert bands are generally from one-half to 2 inches in thickness and are finely interbedded with the shales. All the beds of the entire section are black or
20 Cairnes, D. D., op. cit., pp. 81-44.
nearly so, but certain alternate layers weather red in places. The chert beds also in places become shaly or even friable when exposed to the atmosphere and decrepitate somewhat readily.
Cairnes also describes a "shale group," which he believed to be of Carboniferous age but mapped with his Ordovician-Carboniferous sequence. With regard to the lithology of these rocks, he states:
The shale group within the mapped boundary belt here being considered is developed at a number of points between Jones Ridge and Yukon River and wherever identified overlies the members of the Devono-Ordovician shale-chert group. Possibly the most extensive individual development of these Carboniferous shale beds occurs just to the north of Hard Luck Creek, where they compose the greater part of a low northeasterly trending ridge to the east of the boundary line and to the south and east of Jones Ridge. These beds were also identified on McCann Hill and at other points but become rapidly much less prominent to the south of Hard Luck Creek.
On the geological map to accompany this memoir these Carboniferous shale beds have been mapped with the members of the Devono-Ordovician shalechert group, as except where fossils were obtainable it was in many places difficult or impossible in the field to distinguish certain members of these rock groups, as both contain beds that are lithologically practically identical.
This shale group consists dominantly of shales but includes also clays, cherts, calcareous sandstones, and thinly bedded limestones. The shales and clays are prevailingly soft and friable and range in color from light gray to black, but dark bluish-gray beds are possibly the most extensively developed. The cherts are dominantly dark gray to black in color but constitute a much smaller portion of this formation than the shale members. They occur prevailingly in beds having a thickness of less than 1 inch, but chert strata were noted which are as much as 3 inches or more in thickness. Calcareous sandstones and arenaceous clays are also developed in places and are typically grayish to brownish in color. The limestones generally occur in beds less than 12 inches thick and range in color from light gray to brown. This shale section has thus quite a decidedly striped or ribboned appearance, due to the frequently alternating shale, limestone, and sandstone beds.
The band of limestone south of Eagle Creek, which lies between the undifferentiated metamorphic Paleozoic rocks and the slatequartzite group referred by Cairnes to the Cambrian or pre-Cambrian, is composed of a number of heavy plates of limestone separated from one another by greater thicknesses of thin-bedded limestone. The general appearance of this limestone belt, when viewed from a distance, would lead to the belief that the limestone is a white crystalline variety, but close inspection shows it to be in large measure a dark-gray noncrystalline thin-bedded variety. Even the thicker plates show well-developed bedding planes and are by no means typically massive limestone. One peculiar phase of this limestone belt is the presence of beds of limestone conglomerate and breccia. The fragments consist usually of pieces of noncrystalline limestone, of varying shades of light gray, commonly well rounded but in part subangular, embedded in a matrix of dark-gray
noncrystalline limestone. The texture of the limestone and its numerous ripple markings lead to the inference that this formation is of near-shore and possibly estuarine origin. No dolomitic phases were seen. Some limonitic concretionary forms, simulating shells, were collected, but their organic origin is questionable, and no true fossils were seen, though two days was devoted to a search for them. The trend of this limestone, about N. 55° W., would carry it just north of Eagle and up the north side of Mission Creek, and there is little doubt that the undifferentiated limestone found at that locality is a continuation of this limestone, although it is more metamorphosed.
The two great masses of limestone, dolomite, and associated rocks along the international boundary from the Tatonduk River to Hard Luck Creek and between Cathedral and Ettrain Creeks, which were mapped by Cairnes as ranging in age from Cambrian to Devonian, are here mapped as a part of the undifferentiated calcareous Paleozoic sequence. With regard to the Cambrian-Silurian part of this sequence, Cairnes 1 gives the following lithologic data:
These rocks are prevailingly white to light gray in color, but occasional beds occur having a dark gray to nearly black or even a pink or reddish appearance. Nearly everywhere, however, on weathered surfaces the different members have the peculiar grayish to bluish-gray rough appearance characteristic of limestones. The rocks are dominantly crystalline, and in places beds of particularly beautiful marble occur, which prevailingly range in color from pure white through various shades of gray, occasional reddish beds being, however, noted in places.
In texture these limestone-dolomite rocks vary from firm, dense dolomites to coarsely crystalline, almost pure limestones. They are also characteristically somewhat massive in appearance, due largely to the degree of metamorphism which they have suffered; but where the bedding planes are discernible the strata are dominantly from 1 to 6 feet in thickness, although much thinner beds from 1 to 6 inches thick are locally characteristic of the series. Beds of limestone having an oolitic structure also occur to the south of Tatonduk River and elsewhere, the oolitic grains being generally about one-tenth of an inch or less in diameter.
In composition these beds range from limestones to dolomites but appear to be all dominantly more or less magnesian. They are frequently tested in the field with cold acid, and only rarely was a member of this group found that effervesced freely, but nevertheless nearly all were more or less attacked. It would thus seem that these rocks are prevailingly transitional in composition between pure limestones and true dolomites, either of these forms being of somewhat exceptional occurrence. The more dolomitic beds are prevailingly harder and finer-textured than the limestones and are dominantly white to light gray in color, none of the very dark colors occurring, such as characterize the limestones in places. Further, the dolomites in places, as on Mount Marlow and elsewhere in Ogilvie Mountains, are more or less porous and contain numerous cavities, which are genera ly quite small but range in size from microscopic to several inches in diameter. These cause the containing rocks 31 Cairnes, D. D., op. cit., pp. 58-61.
to be very rough on weathered surfaces. The cavities are dominantly lined with well-defined crystals, mainly of quartz and calcite, and are considered to indicate rather conclusively that the dolomites are of secondary origin and are derived from limestones, the amount of pore space representing the decrease in volume during the replacement process. Also, as fossils were very rarely, if ever, found in the dolomites and are quite plentiful in places in the adjoining limestones, this would seem to indicate that some change had occurred in the dolomite beds since originally deposited which destroyed any contained organic forms.
In a few places grayish, yellowish, to nearly black shales are intercalated with these limestone-dolomite beds, and at one point, on the western side of Mount Slipper, over 200 feet of thinly bedded shales occur, with dolomites above and below them. Shales, however, are of very minor importance quantitively in this Silurian-Cambrian terrane.
The entire series is prevailingly siliceous, and toward the south the beds contain a great amount of translucent to semitranslucent chalcedonic quartz or chert, which in places considerably exceeds the limestones and dolomites in amount. This chert has in places been deposited largely along the bedding planes of the containing rocks in seams ranging from microscopic up to 8 or 10 inches in thickness and thus gives the rocks in general a decidedly banded appearance. When somewhat regularly deposited along the bedding planes in this way the chert has in places the appearance of being contemporaneous with the containing beds, but when more closely examined it may be seen to intersect the strata; in fact, seams or masses of chert occur cutting the limestone and dolomite beds at all angles, and the smaller seams are frequently distinctly traceable back to larger seams or irregular bodies.
The Devonian part of this limestone sequence is described as follows:
The Devonian limestones resemble very closely the limestone beds of the Silurian-Cambrian group, and except where fossils can be found it is difficult or impossible in many places to distinguish these rocks from the older limestones. They are, however, as a rule somewhat more homogeneous and darker in appearance, being typically dark bluish gray in color. They are also in most places characteristically coarsely crystalline, and when broken they generally emit a strong oily odor, which was seldom noted in connection with the underlying formations. In places a heavy bed or series of beds of white to light-gray sugar-grained quartzite occurs at the base of this limestone series, as in the vicinity of Tindir Creek, but this quartzite appears to be only locally developed.
Another belt of limestone of unknown age begins in the hills west of Nation at Spring Creek and extends in a northwesterly direction 8 or 10 miles, as far as Glenn Creek. This limestone shows several variations. Some of it is dark gray to black and noncrystalline, some is light gray and finely crystalline, and some beds of silicified cherty oolite and also beds of limestone breccia were seen. The dip appears to be dominantly northward, but as the nearest good exposures of other rocks are some miles away, this apparent structure has little significance. As no fossils were found the age of the limestone is indeterminate. It resembles perhaps more nearly than any other rock the plate of limestone that crops out on the north side of the