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than real, depending to a considerable extent upon differences in color, which are due to weathering and which do not persist laterally along the beds. The section from top to bottom is given herewith:
Section of Calico Bluff formation in south side of Calico Bluff, on Yukon River below Eagle
[Zones indicated by letters are shown in fig. 5 and pl. 8, B]
Covered. Lower part composed of drab shale, with a few
Thin beds of black shale and limestone, both of which
Thin, fissile black shale that weathers to a bronze color, ex-
Fissile black shale, weathering brown at base and red and
Dark-gray fissile shale that weathers yellow-brown..
Dark-gray fetid limestone that weathers light gray. Fossilif-
Alternating thin beds of dark-gray limestone and thin, fissile
Black slate, somewhat siliceous. Near the top is a 6-inch
Alternating beds of limestone and shale with some chert. In-
The base of this section, as here given, is about at the end of the point shown in Figure 5 as projecting into the Yukon. (See also pl. 8, B.) This basal line, as previously stated, is an arbitrary one, placed at about the level where, in going down in the section, the limestone and fossiliferous beds cease. This line therefore serves also as the upper limit of the lower Mississippian sequence, already described.
STRUCTURE AND THICKNESS
The structure of the rocks at Calico Bluff is that of an open syncline which plunges gently about N. 30° W. The other two belts of this formation to the northwest, one on the northeast side of the Yukon and one on the southwest side, are interpreted in a broad way as the northeast and southwest limbs of this same syncline. Strike faulting, however, has materially modified the synclinal structure. On the north side of the Yukon, north of Calico Bluff, occurs a rapid alternation of Upper Cambrian, lower Mississippian, upper Mississippian, and Upper Cretaceous rocks, standing practically on end. The lower Mississippian rocks are here repeated three times in the section, and the Upper Cretaceous rocks twice. Similarly, on the southwest side of the Yukon strike faulting is apparent, particularly opposite the mouth of the Tatonduk River, where the areal distribution of the lower and upper Mississippian rocks is exactly the reverse of what would be expected if normal synclinal structure were present. None of the four known upper Mississippian localities northwest of Calico Bluff, therefore, may be expected to contribute much stratigraphic evidence about this formation. The rocks at Calico Bluff, for some unknown reason, constitute a little stratigraphic island of relatively simple structure, surrounded on all sides by beds that are more intricately folded, as well as faulted, and all the structural and stratigraphic information available is concentrated at this one locality.
The northwestward plunge of the syncline is indicated on the south side of Calico Bluff (see fig. 5), where the strike of the rocks is east and the dip is 15°-35° N., averaging perhaps 20°. Minor crumpling is evident toward the east side of the bluff, but in general the rocks are little metamorphosed and show perfectly the original bedding planes, with incipient fracture cleavage only in some of the weaker beds.
The highest beds have been eroded at Calico Bluff, and it is therefore not possible to obtain a complete stratigraphic section. The thickness of beds from the arbitrary basal line to the top of the bluff at its south end is believed to be about 1.270 feet. The thickness from the base of zone b to the top, as measured instrumentally, is 968 feet.
In this connection it is of interest to examine the section made by Brooks, 62 who first studied in detail the rocks exposed at Calico Bluff and gave the following section:
Calico Bluff section
[As given by Brooks]
O. Gray to blackish shale, with few bands of limestone____ 300
with thin bands of black slate____.
M. Black shale and subordinate thin bands of limestone__
D. Black carbonaceous shale, with slaty cleavage developed
C. Black carbonaceous shales, with some thin black chert
B. Black cherts in 4-inch to 6-inch bands and interbedded
A. Black carbonaceous fissile shale_-_.
From an inspection of this section it is fairly certain that zones L and G correspond, respectively, to zones b and a of Figure 5. Brooks's section, from the base of his zone L to the top of the bluff, is given as 910 feet, and when it is remembered that this measurement was made without instruments, it shows a remarkable similarity to the instrumentally measured section of the same sequence from the base of zone b to the top, which, as measured by the writer, is 968 feet.
AGE AND CORRELATION
Fossils are very plentiful at Calico Bluff, as well as at the other four upper Mississippian localities, and nearly every geologist who has gone down the Yukon, from the time of Collier's trip in 1902 to the present, has made collections at these localities. Some of the fossiliferous zones at Calico Bluff are indicated in the section above given, but others are doubtless present though not seen by the writer, for it was noticed that some of the fossiliferous zones had a rather slight lateral extent along the bedding planes, and any one section would therefore necessarily miss some zones. The writer, like his
Brooks, A. H., and Kindle, E. M., Paleozoic and associated rocks of the upper Yukon, Alaska: Geol. Soc. America Bull., vol. 19, p. 288, 1908.
predecessors, collected additional fossil material from this unique locality at Calico Bluff. It has seemed best, however, to assemble and correlate all the collections, and the result is shown in the following table, which may be said to represent the intermittent collecting of seven geologists over a period of 24 years. A few of the earlier paleontologic determinations were made by Charles Schuchert, now of Yale University, but the great mass of determinative work has been done by G. H. Girty, of the United States Geological Survey. Girty himself made a trip down the Yukon in 1918 and collected some of the best of the material tabulated below. The localities are arranged roughly in the chronologic order of the collections. Although some of the locality descriptions are inadequate and some are slightly in error, they are published as given by the original collectors, with the exception that the terms right bank and left bank, as used in some of the locality descriptions, have been altered to north, east, west, or south bank, as the case may be. Interpretations of the locality made by the present writer are indicated in parentheses. Zones indicated by letters are those shown in Figure 5.