4. INFAULTED BLOCK OF TAHKANDIT LIMESTONE (PERMIAN) IN MIDST OF WOODCHOPPER VOLCANICS (MIDDLE DEVONIAN) ON THE SOUTH BANK OF THE YUKON RIVER ABOUT 1 MILE BELOW THE MOUTH OF COAL CREEK

Elsewhere in this area the lithology of the Permian is about the same as that seen at the Nation River locality. Along the Yukon below Coal Creek the Tahkandit limestone is bordered by basalt and tuff of greenstone habit, but these beds are believed to belong to the Middle Devonian sequence and to be faulted to their present position.

STRUCTURE AND THICKNESS

The Permian (Tahkandit) rocks at the Nation River locality, like the rocks of the Nation River formation, are distinguished by a northeasterly trend, which is about at right angles to the trend of the other formations along the Yukon. They also are probably involved with the Nation River beds in a great arch, whose northwestern limb crops out just above the Nation River and whose eastern limb is seen in Trout Creek. The axis of this arch appears to trend about southwest by west. The arch is not simple but is modified by minor folds, which are perfectly exemplified by the Tahkandit limestone at the Nation River. Plate 10, A, a view of the northeast bank of the Yukon just above the Nation River, shows an anticline and a syncline in the Tahkandit limestone. This syncline is believed to be continuous with the syncline shown in Figure 6. The high butte shown at the extreme right in Plate 10, A, is an isolated outcrop of the limestone, which may represent the trough of another syncline lying parallel to the one shown in the center of the picture but farther northeast. These folds are evidently of the open type and appear to be fairly symmetrical.

The Tahkandit rocks are believed to grade downward into the Nation River formation, as indicated in Figure 6. The upper part of the Tahkandit limestone, however, has not been recognized, for the formation at the Nation River is overlain without apparent angular discordance by Upper Triassic rocks, and it is not possible to tell at this locality how much if any of the Tahkandit was eroded before the Upper Triassic sediments were laid down. Hence, no estimate of the total thickness of Tahkandit limestone can be given, but it may be stated with some assurance that about 527 feet of strata are exposed from the base of the Upper Triassic down to the shale that is believed to represent the top of the Nation River formation. Of this thickness 373 feet is limestone.

AGE AND CORRELATION

The Tahkandit limestone, like the Mississippian rocks, is very fossiliferous, and numerous collections have been made by geologists, including the writer, since 1896, when Spurr made his trip down the Yukon. This fauna has been tabulated and is presented below. The

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determination of the fossils, except in a few of the earlier collections, has been made by G. H. Girty. As with the Mississippian fossils, however, Girty's work has been more a rapid reconnaissance of the fauna with the object of determining its age than a detailed study involving the identification and description of new species. Many of the species are new and are apparently more closely related to Asiatic than to North American Permian forms. Nevertheless, 54 genera and at least 123 species have been recognized. This fauna was originally believed to be of Pennsylvanian age but is now assigned by Girty to a horizon low in the Permian. The arrangement of the localities in the table is more or less chronologic.

Zaphrentis sp.

Stenopora sp.

Polypora sp..

Lingulidoscina cf. L. utahensis.

Orthotetes sp.

Derbya? sp

Meekella? sp..