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that underlies the Calico Bluff formation illustrates some of these difficulties. At its type locality its structural relation to the Calico Bluff formation is unequivocal, but its lower stratigraphic limit has not been recognized, and fossils are absent. Across the Yukon, below the mouth of Shade Creek, a few nondiagnostic fossils have been found, but the structural relations to overlying or underlying formations can not be discerned. And 75 miles to the northwest, in the Woodchopper area, occur more chert and chert conglomerate which with no fossil evidence and little stratigraphic evidence are interpreted as the base of this formation. Obviously, the correlation of these and other chert occurrences along the Yukon as a single formation is based more upon vague geologic interpretation than upon facts, particularly as at least one other chert formation, of Middle Devonian age, is also present in this same area.

The chert formation here considered is definitely known to underlie conformably rocks of upper Mississippian age. It is believed to be no older and perhaps younger than the Rampart group and Circle volcanics, which in this paper are considered to be lower Mississippian. Although elsewhere in Alaska similar cherty rocks have been termed Mississippian or Upper Devonian, the structural and stratigraphic relations of this formation to adjoining formations along the Yukon indicate a much closer kinship with the Mississippian than with the Devonian rocks of interior Alaska; and therefore this formation along the Yukon is referred to the lower Mississippian, not with the purpose of indicating a close correlation with the Waverly or Madison epochs of the United States, but merely to show that these rocks are younger than the Woodchopper volcanics, older than the Calico Bluff formation, and possibly in part contemporaneous with the Rampart group and Circle volcanics.

Fossils were found in the siliceous slate and chert at the mouth of Shade Creek by E. M. Kindle, in 1906. This collection (No. 844) has recently been examined by G. H. Girty, of the United States Geological Survey, and has been found to contain Enchostoma sp. and Lingula cf. L. spatulata. These fossils are regarded by Girty as possibly Devonian but more probably Mississippian. Inasmuch as such forms have not been found either in the Middle Devonian or in the upper Mississippian faunas of the Yukon, it would seem quite appropriate to regard them as lower Mississippian, to conform with the age assignment which the stratigraphic and structural relations indicate.

The rocks of this formation are correlated primarily with a similar "chert formation " in the Tolovana district,59 about 225 miles to the west. At that locality the base of the formation was composed of

59 Mertie, J. B., jr., Gold placers of the Tolovana district: U. S. Geol. Survey Bull. 662, pp. 239-244, 1918.

chert conglomerate and chert breccia, which may be correlated with the rocks seen on Coal and Woodchopper Creeks. The upper beds were mainly chert, with some siliceous slate and shale and silicified limestone; and this part of the sequence is believed to be the lithologic counterpart of the chert underlying the Calico Bluff formation. The "chert formation" of the Tolovana district was described originally as not older than Middle Devonian nor younger than Pennsylvanian, but fossils subsequently found indicate that it is Mississippian. Therefore it is correlative paleontologically also with the rocks at the north end of Calico Bluff and at the mouth of Shade Creek.

In northern Alaska a somewhat similar formation of chert conglomerate with interbedded layers of quartzite in the lower part and slate and shale in the upper part was described originally by Schrader 60 under the designation Stuver "series " and was considered pre-Devonian. Subsequently the Stuver "series" was shown by Smith and Mertie 1 to overlie the Upper Devonian rocks and to underlie the upper Mississippian rocks of northern Alaska and was therefore assigned to the basal part of the Noatak formation, of lower and upper Mississippian age. This basal part of the Noatak formation, previously called the Stuver "series," is therefore believed also to be approximately correlative with the chert formation of the Yukon. In this connection, however, it should be remembered that northern Alaska constitutes a different geologic province from interior Alaska, and it is therefore not surprising that the Noatak formation differs materially in lithology from rocks of similar age on the Yukon.

Other formations composed mainly or partly of chert are known elsewhere in Alaska, but accumulative data tend to show that rocks of several ages may be of this general lithologic character. The Upper Triassic chert of northern Alaska, described by Smith and Mertie, is a good example of one of the newly discovered chert formations that can not be correlated with the chert formation of the Yukon. The Middle Devonian cherty rocks at the head of the North Fork of Shade Creek constitute another example. Hence, in the future the burden of proof will bear heavily upon the geologist who attempts to correlate chert formations in Alaska upon lithologic evidence alone.

CALICO BLUFF FORMATION

DISTRIBUTION

The type locality of the Calico Bluff formation is at Calico Bluff, on the west bank of the Yukon River, about 8 miles due north of

Schrader, F. C., A reconnaissance in northern Alaska: U. S. Geol. Survey Prof. Paper 20, pp. 60-62, 1904.

Smith, P. S., and Mertie, J. B., jr., Geography and geology of northwestern Alaska: U. S. Geol. Survey Bull. 815, pp. 155-157, 1930.

Eagle. (See pl. 8, B.) The rocks of this formation are also exposed in a narrow zone on the north bank of the Yukon north of Calico Bluff, and this zone apparently continues N. 30° W. into the lower valley of the Tatonduk River and beyond in the same general direction. Another narrow belt of the same rocks occurs just west of the mouth of the Seventymile River and also continues N. 30° W., cropping out again on the west bank of the Yukon about opposite the mouth of the Tatonduk,

LITHOLOGY

The rocks at Calico Bluff consist essentially of alternating beds of limestone and shale with some slate and, being only gently folded, afford an excellent opportunity for measuring a detailed section.

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FIGURE 5.-Sketch of south side of Calico Bluff, Yukon River, showing folded Mississippian rocks. a, b, c, d, and e indicate some of the more prominent beds. A ragged ridge of Middle Cambrian limestone on the opposite side of the river is shown at the right

For convenience in referring to this section, a sketch of the south side of the bluff is included as Figure 5, in which some of the more prominent beds are designated by letters. The section given below was measured instrumentally from the base of zone b to the top of the bluff. The section from zone b down to the water's edge at the eastern point of the bluff was not measured instrumentally, but the thickness of beds involved was estimated by two roughly quantitative methods, which checked each other very closely. The limestone and shale are thicker at the base of the section and become progressively more thin bedded toward the top. As a result, the upper part of the section consists of a great number of thin beds of alternating limestone and shale, some of which have here been grouped together. It is doubtful whether any two sections of Calico Bluff made by different men would match, for much of the variation is more apparent

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
beds of limestone___
Slope of gray shale, with a little limestone. Contains in the
upper part two or three beds of sandstone that weathers a
light ocherous yellow, each 3 or 4 feet thick_-
Calcareous shale, light gray or darker gray, weathering to a
light gray. Upper 20 feet covered___.
Massive black limestone, noticeably carbonaceous. Weathers
light gray. Zone e----

Thin beds of black shale and limestone, both of which
weather light and dark gray. These rocks are fossilif-
erous, but the shale contains invertebrates of genera dif-
ferent from those found in the limestone, indicating an
oscillating condition of sedimentation and of marine life__
A cliff-forming group of beds. Consists of thin-bedded limy
shale that weathers to a chocolate color at the top; a 2-foot
bed of white-weathering limestone in the center; and black
shale at the base____.

Thin, fissile black shale that weathers to a bronze color, ex-
cept in the middle of the sequence, where it appears choc-
olate-colored for 10 or 15 feet__.
Upper 56 feet is mainly thin-bedded limestone with some
shale, weathering to a light chocolate color. Lower 27 feet
is fissile black shale that weathers blackish brown.
Zone d-------

Fissile black shale, weathering brown at base and red and
yellow farther up, giving a general bronze color to the beds_
Alternating thin beds of white-weathering limestone and fissile
black shale. Fossils occur in a thin band at the very top--
Dark-gray fetid limestone that weathers light gray. Fossilif-
erous at the base_-_.

Dark-gray fissile shale that weathers yellow-brown..
Beds of limestone as much as 3 feet thick, alternating with
shale. Contains about midway of the sequence a 2-foot bed
of fossiliferous limestone_-_-

Dark-gray fetid limestone that weathers light gray. Fossilif-
erous. Zone c‒‒‒‒

Alternating thin beds of dark-gray limestone and thin, fissile
black slate that is probably calcareous. Beds 2 to 8 inches
thick

Black slate, somewhat siliceous. Near the top is a 6-inch
bed of fetid black fossiliferous limestone that weathers
white. Zone b-----

Alternating beds of limestone and shale with some chert. In-
cludes 1 thick zone of black siliceous slate, which is zone

[blocks in formation]
[blocks in formation]

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.

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