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west. Figure 2 shows a thin bed of quartzite squeezed into an appressed fold whose limbs are practically parallel. Fragments of other beds of quartzite, surrounded by slate, are also visible, and on both sides of the quartzite and slate are relatively massive but nevertheless highly disturbed bodies of the recrystallized basic volcanic rocks of the Yukon group. This fault was traced 3 miles southeastward into the hills south of Eagle Creek. The small body of greenstone included in the limestone where the limestone formation reaches the Yukon lies within this fault zone and owes its peculiar environment to a dislocation attendant upon this thrust faulting. To the northwest the fault zone is apparent for 10 miles up the north side of Mission Creek, where its presence is inferred from the disturbed condition of the country rocks, as well as from fault breccias and an abnormal disposition of the geologic terranes.

FIGURE 2.-Diagrammatic sketch showing structure of rocks in
the fault zone on the north bank of the Yukon River 3 miles
above Eagle

The belt of undifferentiated limestone, which lies at the international boundary northeast of the Paleozoic metamorphic rocks, is well exposed in the mountains south of Eagle Creek, but its contact with the Paleozoic metamorphic group to the southwest is covered by rocks of Cretaceous and Eocene age. To the southeast, however, farther up the Yukon Valley, rocks similar to the slate-quartzite group were seen on the southwest as well as the northeast side of the limestone belt. At the boundary the northeastern contact of the limestone with the slate-quartzite group is not a simple one but, northwest from the limestone, consists of a narrow zone of the slate-quartzite group followed by another plate of limestone, followed in turn by the main belt of the slate-quartzite group. This relationship, in the absence of a recognizable fault zone at this locality, suggests a stratigraphic gradation from the limestone to the slate-quartzite group. The structure of the limestone in this belt is complex, but as a rule the bedding planes are apparent. The southwestern flanks consist dominantly of

thin-bedded limestone, which is much crumpled and shows numerous reversals of dip. The central and northern part of the limestone, however, contains numerous plates of more massive limestone, which dip consistently southwest at angles of 60° to 75°, though approaching verticality at some places. Appressed folds, in both the thin and the thick bedded parts of the limestone, are common, but these also dip usually to the southwest.

Relatively little is known of the structure of the slate-quartzite group of rocks that adjoins the limestone belt on the northeast and possibly on the southwest, because this formation along the boundary is not well exposed, even above timber line. At an altitude of 3,139 feet, at “Hog" station, and thence northwestward down the spur toward the mouth of Eagle Creek, these rocks crop out at intervals and show a cleavage plane which dips in general southwestward 30° to 60°, more commonly nearer 60°. Numerous wavy lines along this cleavage plane suggest deformed ripple markings, and these, together with sundry concretionary forms and marks suggestive of worm borings, lead to the belief that these cleavage planes also represent bedding planes. A northeastward dip was noted, however, at one or two localities on the ridge and also where the same rocks crop out in the fault zone along the river.

In weighing these lithologic, structural, and stratigraphic data, the greater degree of metamorphism of the rocks of the Yukon group should at the outset be discounted as absolute evidence bearing on the relative age of these three formations. Cumulative observations on the metamorphic rocks of Alaska have shown that degree of metamorphism alone, unsupported by contributory data, can lead to very erroneous conclusions; and in this area the greater degree of metamorphism of the Paleozoic metamorphic rocks may be discounted almost completely because of the known presence of a great granitic batholith to the south, from the direction of which tangential forces of great magnitude have been applied against the adjoining rocks to the north. The localized nature of this metamorphism is also emphasized by the occurrence of fossils of Silurian or Devonian age in these metamorphic rocks, whereas within a distance of 25 miles rocks of definite Cambrian age are found in an almost entirely unmetamorphosed state.

As has previously been stated, 80 per cent of the cleavage planes and axial planes of the appressed folds in the Yukon group dip southwestward, and the dominant dip of the bedding planes, where visible, in the more metamorphosed rocks south of the Yukon is also southwestward. In other words, cleavage and bedding in these rocks are more commonly parallel than otherwise. It seems almost evident, therefore, that tangential forces applied from the southwest have

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thrust the Paleozoic metamorphic rocks and to a less degree the limestone formation northeastward and have caused a general overturning of the whole sequence. Although both the limestone and the Paleozoic metamorphic rocks dip southwestward, the dip of the limestone is in general considerably higher than that of the metamorphic rocks, approximating verticality at the northeastern limit of the limestone. The hypothesis favored by the writer, therefore, postulates a great overthrusting movement of the rocks of this region from the southwest, the effects of which are naturally most evident near the northeastern border of the granitic batholith, where these thrusting stresses originated. As a result, the Paleozoic metamorphic rocks have been more or less completely overturned and metamorphosed and have acquired a southward-dipping cleavage. The limestone and the slate-quartzite formations have also been affected, but to a smaller degree, by the same forces, with partial overturning and the development of a similar cleavage.

This hypothesis explains the observed differences in metamorphism in these three formations but does not settle their relative ages. A number of different structural interpretations might be made. The limestone belt and the slate-quartzite formation may be part of either an overturned anticline or a syncline. The beds of the three kinds of rock may all lie on one side of such a fold, thus forming a continuous sequence; or any one of the three may lie in the axial plane of such a fold, thus changing the sequence; or any previously existing structure may have been materially modified by thrust faulting concurrent with the folding. It seems best, therefore, not to assign a definite sequence to these three formations.

The influence of thrust faulting with its attendant metamorphism can not be said to extend for any considerable distance north of the Yukon. Numerous faults are present, some of which were recognized in the field; but they seem to be of a different type. Cairnes shows one fault plane of low dip between Cathedral and Tindir Creeks at which the rocks of his Tindir group are brought into contact with younger rocks and which therefore indicates a thrusting movement of rocks from north to south, a relation diametrically opposed to that seen along the Yukon. This locality, however, lies on the north flank of the Ogilvie Mountains, where a different type of structure may prevail. This fault may also be either an earlier or a later structural feature than the dislocations attendant upon the Mesozoic granitic intrusion.

The obvious duplication of beds resulting from close folding, overthrusting, and faulting in the Paleozoic metamorphic rocks along the Yukon at the boundary and in the limestone belt and slate-quartzite group to the northeast precludes any accurate estimate of the thickness involved, even if the top and base of each of these three units

were positively recognized. The upper limit of the metamorphic Paleozoic rocks along the Yukon is a fault zone, and the base is undetermined, so that it seems utterly useless to hazard even a guess as to the thickness of this part of the sequence. If the limestone band and the slate-quartzite group have about the same degree of complexity, the width across the strike indicates that these two units have thicknesses of about the same order. The limestone is better exposed than the slate-quartzite group and therefore affords a better basis for an estimate of thickness. Though not accurately measured, the total thickness of this limestone certainly can not exceed 3,000 feet, and when allowance is made for the probable duplication of beds due to folded structure, an estimate of half that thickness seems all that is warranted.

The following structural data are given by Cairnes with regard to the Silurian-Cambrian limestone and dolomite, here mapped as undifferentiated Paleozoic limestone:

*

The rocks are all so much folded and faulted that only in a few places could the positions of the different beds within the series be even approximately determined stratigraphically; and unless fossils could be found it was impossible, even in these places, to draw the geological age boundaries, as no distinctive persistent lithological horizon markers could be distinguished. These beds in the northern portion of the belt have an aggregate thickness of 4,000 feet and possibly very much more than this amount, but no section of them was at all closely measured at any one point, it being found very difficult to do so on account of folding and faulting. * * To the south these beds do not appear to be so thick, but even there they have an aggregate thickness of at least 3,000 feet.

Cairnes's statement with regard to the thickness of the Devonian limestone, also mapped in this report as undifferentiated Paleozoic limestone, is as follows:

These Devonian limestones appear to have an aggregate thickness of from 300 to 500 feet, and wherever a contact was observed with the underlying Silurian beds they overlie these unconformably.

AGE AND CORRELATION

Rocks of very diverse character and age are grouped on the map into two lithologic units, under the general assignment of undifferentiated Paleozoic. The evidence concerning the fauna and age of these rocks may conveniently be presented under three general headings, as follows: Undifferentiated metamorphic Paleozoic rocks along the international boundary and similar and associated rocks south of the Yukon; undifferentiated nonmetamorphic and essentially noncalcareous Paleozoic rocks along the international boundary north of the Yukon; undifferentiated Paleozoic limestone along the international boundary north of the Yukon.

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Metamorphic rocks.-Only the three fossil collections noted below have so far been made from the undifferentiated metamorphic Paleozoic rocks within the area covered by this report.

3AP77. Valley of a headwater tributary of Boundary Creek, 13 miles south of Eagle; from a thin bed of crystalline limestone; collected by L. M. Prindle, 1903. These fossils were identified by G. H. Girty as crinoid columns and showed only that the rocks are of Paleozoic age.

Two other collections, made by the writer during the season of 1925, were found in the lower valley of a stream which empties into the Yukon on the same side of the river and about a mile upstream from Boundary Creek. The exact localities are:

25AMt52 (2059). West slope of Loop Mountain, in Yukon Territory, Canada, 2 miles S. 40° E. of international boundary topographic station 112. 25AMt53 (2060). Boulder in creek, directly down hill from 25AMt52 and about 1.9 miles S. 40° E. of international boundary topographic station 112. Both these collections consist of crinoid columnals. With regard to them, Edwin Kirk states: "These two lots are apparently of the same age. They are Paleozoic and not earlier than Silurian. They probably are Devonian." These crinoids are especially significant because they occur in rocks previously mapped as a part of the Yukon group. A part of the rock from which collection 25AMt53 (2060) was taken is shown in Plate 4, B.

To the south, however, beyond the area covered by this map but within rocks that are grouped with the undifferentiated Paleozoic rocks, L. M. Prindle made three collections in 1904, 1905, and 1907, all of which offer corroborative evidence of the Paleozoic age of this group of rocks. The numbers and localities of these collections are as follows:

4AP46. 40 miles west of Eagle.

5AP319. Dennison Fork, about 20 miles south of Chicken Creek.

7AP82. Fortymile River, one-fourth mile below mouth of Napoleon Creek. Collection 4AP46 consisted of a coral doubtfully referred to Zaphrentis? sp., which suggested to E. M. Kindle a Devonian or Silurian age. Collection 5AP319 showed some striated plant stems which were referred by F. H. Knowlton to Calamites radiatus and indicated only a Paleozoic age. Collection 7AP82, from a somewhat schistose limestone, was identified by Kindle as crinoid stems of little diagnostic value.

In addition to such fragmentary paleontologic evidence, however, considerable is known of the age of this group of rocks from other fossil collections made in this same district and near-by districts, and from formations which, though belonging to the same group, have been in part differentiated into mappable units. The other fossil lists will be presented under the appropriate headings. The

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