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Yukon about 2 miles below Calico Bluff, which is believed to be of Upper Cambrian age.
A similar belt of limestone crops out on Woodchopper Creek about 3 miles from the Yukon. This limestone also has a general northwesterly trend, but its extent along the strike northwest and southeast of Woodchopper Creek is not known. It is also varied in aspect. Some of it is black, noncrystalline, and carbonaceous and resembles the limestone south of Eagle Creek, and some of it resembles more closely the limestone west of Nation and that below Calico Bluff.
STRUCTURE AND THICKNESS
Structural observations on the undifferentiated Paleozoic rocks were made by the writer chiefly in the rock bluffs along the Yukon in the vicinity of the international boundary. The rocks in this zone, like most of the other undifferentiated Paleozoic rocks, lie northeast of a great mass of granitic rocks which are believed to have determined the degree of metamorphism and in large measure the dominant structure of the adjacent rocks. Although the observations are admittedly fragmental, they are nevertheless believed to represent conditions rather typical of the structure of all the undifferentiated metamorphic Paleozoic rocks lying northeast of the great Mesozoic batholith.
These rocks as exposed along the river and in the near-by hills north and south of the river are, as previously stated, closely folded and cleaved and show all the typical indications of intense compression induced by lateral thrusting. Except in some of the more massively resistant greenstones and in the easily recrystallized limestones, flow cleavage is commonly present. Nevertheless, bedding also can be discerned at many places, particularly in the thin beds of quartzite, and where such bedding was recognized close folding is also prevalent. Where the thrusting movements were localized, as along fault and shear zones, closely appressed and even recumbent folds are clearly visible.
Numerous observations of the attitude of the cleavage planes and of the axial planes, both of the drag folds in the less resistant rocks and of the more diagnostic appressed folds of competent beds, were made from Fortymile downstream to Eagle. Bedding planes also were recorded where visible. It can not be stated that any absolute uniformity exists in these observations. Nevertheless about 80 per cent of the cleavage planes and axial planes of the folds dip south at angles of 15° to 65°, with an average of about 30°. The strike of the dominant structural feature, which here is the cleavage, is about N. 70° W. The bedding planes, where visible, appear to dip both to the southwest and to the northeast, but it is worthy of note
that the dip is prevailingly southward along the side of the river, where the rocks are most metamorphosed. In other words, cleavage and bedding in the metamorphic rocks are prevailingly parallel. These data suggest to the writer that the rocks have been thrust northeastward and overturned by enormous tangential forces acting from the southwest, and their degree of metamorphism is explained as a concurrent effect of this prodigious thrusting. The cause of the forces applied is believed to be a great batholith of Mesozoic granitic rocks that lies to the southwest.
At the boundary and just to the east are three groups of rocks, all mapped as a part of the undifferentiated Paleozoic sequence, that present a difficult stratigraphic problem; these are the metamorphic Paleozoic rocks along the river, the slate-quartzite group, and the band of limestone. Fossils found in the Paleozoic metamorphic rocks give some idea of the age of those rocks, but no fossils have yet been found in the other two formations, and the relative ages of the three must be deduced, if at all, from available structural data. The metamorphic rocks at this locality were regarded by Cairnes as preCambrian and were included as part of his Yukon group; and the slate-quartzite group, which he believed to be of Cambrian or preCambrian age, he correlated with his Tindir group, exposed farther north along the boundary; the limestone band was believed to be Cambrian. Cairnes's sequence, then, was metamorphic rocks at the base, overlain by the slate-quartzite group, overlain in turn by the belt of limestone.
The following data on the geologic structure may shed some additional light on this correlation. All three of these formations appear to be parallel to one another, trending about N. 55° W. The rocks of the Paleozoic metamorphic group are schistose, but where the bedding is visible or may be inferred the bedding planes also dip prevailingly southwestward. The angle of dip of the cleavage and bedding planes here, as farther upstream, averages about 30°.
Just below the mouth of Eagle Creek a well-developed fault zone may be seen exposed in the bluff along the north bank of the Yukon. This is not a clean-cult fault but rather a zone of dislocation along which adjustments consequent upon great thrusting movements took place. Doubtless numerous similar fault zones are present in this area, but it is doubtful if any will be discovered where the rocks are so well exposed and where the structural interpretation is so apparent. The rocks involved in this fault zone are the Paleozoic metamorphic rocks, which at this locality are greenstone and greenstone schist, and the rocks of the slate-quartzite belt. The strike of this fault zone is about N. 55° W., and the attitude of the axial planes of numerous drag folds shows that the thrusting came from the south
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 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
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