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MIDDLE CAMBRIAN OR OLDER ROCKS

DISTRIBUTION

The rocks here described as Middle Cambrian or older constitute a mappable unit that lies geographically and stratigraphically between the red beds and the Middle Cambrian limestone. The known exposures are restricted to the Tatonduk River.

LITHOLOGY

This formation consists of beds of dark-gray limestone and dolomite from 6 inches to 3 feet thick, interbedded with shale and argillite, the latter perhaps somewhat calcareous. Much of the clay shale is nodular. There are also a few beds of limestone grit, a finegrained phase of the limestone conglomerate described on page 66. Two such beds, the upper 25 feet thick and the lower 5 feet thick, separated by red shale, occur at the very base of this formation and show that the formation grades downward without a stratigraphic break into the red beds. The apparently concordant structure suggests that this formation also grades upward into the massive beds at the base of the Middle Cambrian limestone, but this relation can not be regarded as proved.

STRUCTURE AND THICKNESS

This formation is not continuously exposed in the bluffs along the Tatonduk River, but the visible exposures indicate a rather uniform westerly dip, ranging from 20° to 30°. Geographically, this formation occupies a belt along the Tatonduk about half a mile wide, measured across the strike, and the maximum thickness can not, therefore, much exceed 1,000 feet. This estimate, however, may be materially lessened if the structure should be found to be more intricate.

AGE AND CORRELATION

No fossils have yet been found in these rocks, and their age can therefore not be given on paleontologic grounds. They underlie the Middle Cambrian limestone and are thus Middle Cambrian or older. Much uncertainty is involved in a more definite age assignment, because of the possibility that an unconformity may exist at the base of the Middle Cambrian limestone. No positive evidence of such an unconformity was seen on the Tatonduk River, but Cairnes's data concerning conditions farther north along the boundary, particularly at Jones Ridge, suggest strongly that such an unconformity exists. Cairnes describes the rocks of his Tindir group along Tindir Creek and between Ettrain and Hard Luck Creeks as consisting of dolomite, limestone, quartzite, slate, shale, and greenstone. The lithology suggests that the formation here described as Middle Cambrian or older may be correlated with a part of Cairnes's Tindir group.

MIDDLE CAMBRIAN LIMESTONE

DISTRIBUTION

The Middle Cambrian limestone crops out in the hills on the north side of the Yukon north of Calico Bluff and continues northward to the Tatonduk River and for an unknown distance beyond.

LITHOLOGY

The Middle Cambrian limestone is rather varied in character and appearance. Some of it seems to be pure limestone, but much of it is silicified to a greater or less degree. No dolomite was noted, but parts of the formation may have an appreciable content of magnesium. Textural varieties are also conspicuous, such as oolitic limestone and limestone conglomerate. The pure limestone is usually white and finely crystalline. The silicified varieties have the same general appearance but contain varying amounts of quartz or of chert. Where much secondary quartz is present the rocks simulate a granular white quartzite. At one locality the partly silicified limestone contains disseminated through the rock a shiny black conchoidally fracturing mineral that suggests a hydrocarbon similar to gilsonite.

This limestone formation is thin bedded at the top but very thick bedded in its lower half, making conspicuous cream-colored bluffs along the north side of the Tatonduk River. Viewed from the south bank of the river the limestone in these bluffs appears to grade upward into thin beds of gray chert, but the closer inspection afforded by swimming along the bluff's on the south side shows that on that side there is a sharp change from thin-bedded limestone to thinbedded chert, with a covered zone a few feet wide that may conceal a dip fault parallel to the westward-dipping beds.

Where the Middle Cambrian limestone plunges southwestward toward the Yukon, north of Calico Bluff, a cold sulphur spring issues beneath the base of the limestone and discharges into a little gulch that drains southward to the river. This spring, which is about 100 yards from the Yukon, discharges from two vents some 20 or 30 feet apart, which are conspicuously colored with a white sulphurous deposit. The odor of sulphureted hydrogen is strong and may be detected 100 yards or more from the spring. A sample of the sulphur water was analyzed in the chemical laboratory of the United States Geological Survey by Margaret D. Foster, who reports the following composition :

Analysis of water from cold sulphur Spring 11.4 miles N. 433° E. of Eagle,

Alaska

Parts per million
Silica (SiO2)----

14
Iron (Fe)-----

.24 Calcium (Ca)---

274 Magnesium (Mg)-

156 Sodium and potassium (Na+K) (calculated)

352 Bicarbonate radicle (HCO3).

725 Sulphate radicle (80.).

544 Chloride radicle (Cl)--

660
Nitrate radicle (NO3)---

Trace.
Total dissolved solids at 180° C -

2, 426 Total hardness as CaCO3 (calculated) It seems rather probable that a part of the contained solids of this water has been leached from the overlying limestone.

1, 325

STRUCTURE AND THICKNESS

This limestone is not continuously exposed along the banks of the Tatonduk River, but exposures seen indicate a rather consistent westward dip at 20° to 30°. Southward from the Tatonduk River this limestone continues across wooded hills to the Yukon, bends sharply eastward, and finally veers northeastward, forming a well-defined anticline that plunges rather steeply toward the Yukon just north of Calico Bluff. The evidence at present available indicates that the east end of the anticline is cut off by a fault.

From the structural data available on the Tatonduk River the thickness is estimated at 800 to 1,200 feet, say 1,000 feet; but if the structure is more intricate than is now supposed, the thickness may be less. The top of the Middle Cambrian limestone, however, has not been definitely recognized, and it is possible that a complete section at some other locality will show a thickness greater than 1,200

а feet.

AGE AND CORRELATION

Fossils were found in this limestone, apparently in the upper half of the formation, as follows:

25AMt148 (2062). Northeast side of Yukon River, at an altitude of about 1,400 feet above the river, 3.1 miles N. 212° E. of Eagle:

Nisusia or Jamesella sp.
Dorypyge, 2 sp.
Albertella mertieii.
Stepotheca rugosa.
“Ptychoparia,” 2 sp.

Ogygopsis sp.
These fossils were examined by C. E. Resser, of the United States
National Museum, who determined them to be of lower Middle Cam-
brian age. Resser's report is as follows:

All of the foregoing species are quite similar or possibly identical with forms in the Langston limestone or the Ross Lake shale and hence belong to a horizon well down in the Middle Cambrian.

One part of this collection consists of a darker, more crystalline limestone, as compared with the whiter and finer-grained limestone which contains the more abundant fossils. Both apparently contain the same fauna, and in both the fossils silicify on weathering.

This material does not correlate with that secured by the Canadian Geological Survey along the international boundary. There are two distinct horizons represented by the Canadian material, the lower one possibly, being a Cambrian horizon somewhat above Doctor Mertie's horizon; but the other is well up in the Ozarkian, containing a species or two of Symphysurina.

Of other collections made along the international boundary, Resser has seen only those of L. D. Burling, and it may well be that a reexamination of the Cambrian collections made by Cairnes will show that his horizons are more nearly equivalent to those of collection 2062, just described.

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UPPER CAMBRIAN LIMESTONE

DISTRIBUTION

Limestone containing Upper Cambrian fossils has been recognized at two localities. The first is a thin plate of limestone that crops out along the north side of the Yukon north of Calico Bluff. The second occurrence is in the headwaters of the North Fork of Shade Creek at the top of the great mass of limestone that extends from Shade Creek northeastward to the Tatonduk River. The geographic position of these two limestones suggests that they may be continuous, but this relation has not yet been proved.

LITHOLOGY

The limestone along the north bank of the Yukon is a massive light-gray finely crystalline rock, with conglomeratic and oolitic phases, in general appearance not unlike the Middle Cambrian limestone farther up the hillside. (See pl. 4, A.) The conglomeratic phase of this limestone contains small pebbles of gray limestone and a few well-rounded pebbles of shiny black chert. The limestone at the head of Shade Creek consists of alternating thin layers of light and dark gray rock.

STRUCTURE

The limestone band north of Calico Bluff strikes about N. 30° W. and dips perhaps 60° SW., toward the Yukon. It was not measured instrumentally but appears not to exceed 300 feet in thickness. Evidently this limestone forms the uppermost bed of a pitching anticline, composed mainly of Cambrian strata and previously men

tioned in connection with the Middle Cambrian limestone. It is overlain by beds of lower Mississippian age, and this relation is believed to be due to faulting.

The Upper Cambrian limestone in the head of the North Fork of Shade Creek strikes northwest and dips gently southwest. It lies at the top of a mass of undifferentiated limestone. It is underlain presumably by older Cambrian strata. On the other hand, the sequence is not continuously downward all the way north to the Tatonduk River, for both Silurian and Middle Devonian fossils have been found in the undifferentiated limestone on the Tatonduk River. The Upper Cambrian limestone is directly overlain by graptolitebearing beds of Ordovician (Normanskill) age.

AGE AND CORRELATION

Two fossil collections have been made, as follows: 28AMt263. Limestone bluff along northeast bank of Yukon River, north of the north end of Calico Bluff :

Archaeocyathus? sp. 28AMt262. North Fork of Shade Creek, 1.15 miles N. 2942° W. from “ Hug" boundary triangulation station (McCann Hill):

Acrothele sp. Kirk's report on this material is as follows: No more definite age assignment of these lots of fossils is possible than to call them Cambrian. The fossil in lot 263 appears to be very like the Upper Cambrian forms referred to Archaeocyathus. Such scant evidence as there is would point to the Middle or Upper Cambrian age of the containing beds.

Archaeocyathus does not occur in any of Cairnes's Cambrian collections, but Acrothele is found in four of his eight collections, all four determined as Upper Cambrian. Lying apparently at the top of this mass of limestone at the head of Shade Creek, and being directly overlain by Ordovician beds, this Acrothele-bearing limestone seems therefore most logically regarded as Upper Cambrian. These two limestones, therefore, are believed to represent about the same stratigraphic horizon, and both of them are correlatable with the beds from which some of Cairnes's collections were obtained, notably with those on Jones Ridge.

ORDOVICIAN SYSTEM

Rocks of Ordovician age are known to be present along the international boundary but have not been recognized as such along the Yukon between the boundary and Circle. Cairnes 88 in his work along the boundary made several collections of Ordovician fossils, including one collection of graptolites. The invertebrates include forms from

** Cairnes, D. D., The Yukon-Alaska international boundary between Porcupine and Yokon Rivers ; Canada Geol. Survey Mem. 67, pp. 66–69, 1914.

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