Slike strani
PDF
ePub

а

13428. Yukon River, southwest bank just below Glenn Creek. Collector, J. B. Mertie, jr.

13429. East side of Woodchopper Creek about 1 mile in a straight line from Yukon River. Collector, J. B. Mertie, jr.

In addition to the invertebrate collections above enumerated, Blackwelder in 1915 found remains of a plant which was identified by F. H. Knowlton as Chondrites heerii Eichwald. This was found along the southwest bank of the Yukon about 112 miles above the mouth of the Kandik River.

Of the six genera of invertebrates that have so far been found in these rocks, Aucella crassicollis Keyserling may be said to be the type fossil. This fossil is particularly abundant at the locality on Woodchopper Creek, in beds of slate. Other undetermined species of Aucella are probably also present at this locality. Along the international boundary Cairnes 87 also made a number of collections of Lower Cretaceous fossils, from Sitdown Creek northward to the sixtysixth parallel. These were determined originally by T. W. Stanton, of the United States Geological Survey. (For explanation of numbers, see p. 48.) XIV k 25:

Nucula sp.
Astarte? sp.
Panopaea ? sp.

Undetermined pelecypod casts.
XIV q 31:

Aucella crassicollis Keyserling. XV a 31, 32:

Aucella crassicollis Keyserling.

Astarte sp.
XV h 30, 31, and 1 30:

Aucella crassicollis Keyserling.
XV j 30:

Aucella crassicollis Keyserling. This Lower Cretaceous horizon is represented at many places in Alaska. The locality nearest to the Yukon is in the Rampart district, where a great thickness of Aucella-bearing slate and quartzite is found. In northern Alaska Schrader 88 found Lower Cretaceous rocks on both the south and the north sides of the Brooks Range, and he gave to these two groups of rocks the designations Koyukuk and Anaktuvuk “series." In the recent work in northwestern Alaska 89 the Lower Cretaceous rocks north of the Brooks Range,

87 Cairnes, D. D., The Yukon-Alaska international boundary between Porcupine and Yukon Rivers : Canada Geol. Survey Mem. 67, pp. 105-107, 1914.

88 Schrader, F. C., A reconnaissance in northern Alaska : U. S. Geol. Survey Prof. Paper 20, 1904, pp. 74–77.

89 Smith, P. S., and Mertie, J. B., jr., Geology and geography of northwestern Alaska : U. S. Geol. Survey Bull. 815, pp. 196–207, 1930.

roughly equivalent to the Anaktuvuk group, were shown to continue westward in a more or less continuous zone from the Anaktuvuk River to the Arctic Ocean. This group is probably closely correlative with the Kandik formation of the Eagle-Circle district.

In the Chitina Valley of southern Alaska two formations, the Kotsina conglomerate and the Kennicott formation, both believed to be in part at least of Lower Cretaceous age, were described originally by Rohn.“ Aucella-bearing shales were found by Capps" at the head of the Chisana River and are also known to exist to the east at the head of the White River. They were also found by Mendenhall "2 in the valley of the Nelchina River, a stream which heads against the Matanuska River and flows eastward to the Copper River.

In southwestern Alaska, east of Kuskokwim Bay, rocks of Lower Cretaceous age are included in the group differentiated by Spurr 93 as the “Oklune series.” The Herendeen limestone of the Alaska Peninsula, as mapped by Atwood, R4 is also of Lower Cretaceous age. Lower Cretaceous rocks are known on Admiralty and Etolin Islands, in southeastern Alaska.

UPPER CRETACEOUS AND EOCENE SERIES

DISTRIBUTION

Rocks of Upper Cretaceous and Eocene age crop out in a continuous belt 1 to 15 miles wide for a distance of 85 miles from the international boundary northwestward to Woodchopper Creek and beyond. This belt extends southeastward from the boundary for an unknown distance into Yukon Territory. Along the Yukon these rocks crop out only at few places north and northwest of Calico Bluff. A small area of rocks at the head of the Charley River mapped by Prindle in 1911 as Upper Cretaceous (?) is also included here.

LITHOLOGY

Little additional work was done on this group of rocks during the season of 1925; they were examined at several localities on the north side of the Yukon, in Yukon Territory, and the northern boundary of the group was modified in the area northeast of Eagle. So far as known, these rocks are all of fresh-water origin and represent conditions of sedimentation not unlike those that existed at the time the Nation River formation was laid down. In fact, these rocks are so similar lithologically to the rocks of the Nation River formation ihat it is difficult to differentiate between them at the localities where the two formations adjoin.

5Rohn, Oscar, A reconnaissance of the Chitina Valley and Skolai Mountains, Alaska : U. S. Geol. Survey Twenty-first Ann. Rept., pt. 2, p. 431, 1900.

01 Capps, S. R., The Chisana-White River district, Alaska : U. S. Geol. Survey Bull. 630, P. 52, 1916.

* Mendenball, W. C., A reconnaissance from Resurrection Bay to the Tanana River, Alaska : U. S. Geol, Survey Twentieth Ann. Rept., pt. 7, p. 309, 1900.

** Spurr, J. E., A reconnaissance in southwestern Alaska : U. S. Geol. Survey Twentieth Aon. Rept., pt. 7, p. 167, 1900.

** Atwood, W. W., Geology and mineral resources of parts of the Alaska Peninsula : U. S. Geol. Survey Bull. 467, pp. 39–41, 1011.

Where seen along the Yukon, the Upper Cretaceous and Eocene rocks consist of impure greenish-gray to almost black sandstone, graywacke, sandy shale, and beds ranging from grit to coarse conglomerate. A short distance above the international boundary, in the hills north of the Yukon, the conglomerate consists of pebbles from a quarter of an inch to 3 inches in diameter, in a brownish matrix. The pebbles here were mainly of vein quartz and chert, with some quartzite, quartzite schist, graphitic phyllite, and decomposed granitic or dioritic material. This is about the average character of the conglomerate. At some places, however, the conglomerate is coarser, and boulders as large as 3 feet in diameter have been observed. At most localities all these rocks are loosely consolidated and therefore by weathering form on top of the ridges gravel deposits that simulate high bench gravel. At places, however, as for example in the valley of the Seventymile River, these rocks have been more than ordinarily metamorphosed and occur as hard, well-indurated sandstone and conglomerate.

A small area of coarse conglomerate and sandstone is shown at the head of the Charley River. These rocks consist of gray more or less carbonaceous sandstone, interbedded with very coarse conglomerate containing boulders as much as 6 feet in diameter. Like most of the strata of this series, they carry plant remains.

The sequence of rocks in this series, as interpreted by Prindle,95 comprises shale and sandstone with beds of lignite at the base of the formation, overlain by sandstone, shale, and conglomerate, with thick beds of conglomerate at or near the top.

The upper or conglomeratic beds appear to be the proximate though not the ultimate source of a part of the gold in the placers of the Seventymile River and of Fourth of July, Coal, Woodchopper, and other creeks along the south side of the Yukon between Eagle and Circle. This statement, which was made originally by the writer " in 1923, in discussing the origin of the gold deposits of interior Alaska, has not been clearly understood by some of the mining men of this district, who apparently have been led to believe

*5 Prindle, L. M., A geologic reconnaissance of the Circle quadrangle, Alaska : U. S. Geol. Survey Bull. 538, pp. 32–34, 1913.

96 Mertie, J. B., Jr., The occurrence of metalliferous deposits in the Yukon and Kuskokwim regions : U. S. Geol. Survey Bull. 739, p. 164, 1923.

that all the creeks which cut this formation ought therefore to have placer gold in the present stream valleys, derived from these conglomerates by recent erosion and stream sedimentation. This belief, of course, is incorrect. As examples may be cited Fourth of July and Crowley Creeks, south of Nation; the former contains workable gold placers, but the latter contains little or no gold in its stream gravel.

Two considerations must be borne in mind—first, no reason exists for believing that gold is universally distributed in those conglomerates, for here as elsewhere only certain streams or even certain parts of some particular stream draining from a mineralized area contain auriferous gravel; second, these rocks have been highly folded at many places, particularly in this zone south of the Yukon, with the result that even if the conglomerate beds had originally carried equally distributed deposits of gold, gold would no longer be equally distributed areally. The gold in this mineralized belt from Eagle to Circle came originally from mineralized rocks at the border of the granitic rocks and also, in part, from the granitic rocks themselves within the drainage basins of the streams, and it has continued to accumulate as stream placers ever since these mineralized rocks were exposed to surficial erosion. The indurated conglomerates are but one stage in this process. Gold placers may therefore exist in any of these streams within whose drainage basins occur bodies of mineralized rock, but the chances for the formation of gold placers may be said to be appreciably enhanced in the areas where these conglomerates now occur, for in such areas both the original mineralized rocks and the fossil placers of the conglomerate are available as sources of gold.

STRUCTURE AND THICKNESS

No studies have yet been made that are sufficiently detailed to warrant a structural section across this belt of rocks. The rocks are known, however, to be everywhere folded and at some localities intensely deformed. The structure of the Upper Cretaceous and Eocene rocks south of the Yukon is believed in general to be much more complex than that of the Lower Cretaceous rocks north of the Yukon, notwithstanding the fact that the Lower Cretaceous rocks are older and probably were subjected to deformational processes before the Upper Cretaceous and Eocene rocks were laid down. This apparent anomaly is due in reality to the closer proximity of the Upper Cretaceous rocks to the seat of volcanism farther south. The same conditions were observed in the Paleozoic rocks, those south of the Yukon being much more metamorphosed than rocks of similar age north of the Yukon. This interpretation of the structure of the Upper Cretaceous rocks involves axiomatically the idea that vol

97

canism such as produced the great Mesozoic batholith to the south and led to the deformation of the late Paleozoic and Triassic rocks was renewed in Tertiary time. Evidence of Tertiary volcanism in southwestern Alaska has already been presented by the writer; and although no stratigraphic evidence of invasion of Cretaceous rocks by granitic bodies has been recorded in the Yukon-Tanana region, the evidence from southwestern Alaska, based on the types of metallization, has been extrapolated by the writer as into the Yukon-Tanana region to prove that Tertiary intrusions of granitic magmas occurred in that region, particularly in the Tolovana and Rampart districts. Hence the idea of a period of deformation in Tertiary as well as in Mesozoic time is not at variance with present interpretations.

The evidence of the intense deformation of the Upper Cretaceous and Eocene rocks is cogent at numerous localities, particularly along the Seventymile River, where these rocks lie nearest to the volcanic rocks to the south. On Barney Creek, a tributary of the Seventymile, the conglomerate beds stand nearly vertical at places, and even farther north, in the valley of Fourth of July Creek, they are nearly vertical at one exposure. Along the south side of the Seventymile River below the falls, where these rocks strike about N. 55o W., they dip consistently south at angles from 30° to 70° and are highly indurated. They consist here largely of sandstone and conglomerate, with some lignitic beds, but farther up Crooked Creek from the Seventymile the bedrock changes by degrees to soft shale and sandstone. This sequence indicates, if Prindle's interpretation of the order of deposition is correct, that these beds along the Seventymile River, though highly deformed, are at least right side up, the conglomerate resting normally upon the stratigraphically underlying shale and sandstone.

The beds of heavy conglomerate, however, are apparently duplicated by folding and crop out repeatedly in the area between the Seventymile and Yukon Rivers. This duplication of beds by folding and perhaps also by faulting unquestionably explains the great distance across the strike of these rocks from the falls of the Seventymile to the Yukon. It is therefore entirely possible that no great thickness of rocks exists in this belt, even though the greatest distance across the strike is nearly 15 miles. On the other hand, all of the sequence may not be present in that portion of the belt where the distance across the strike is the least. Prindle 99 estimated the thickness of the sequence between the Seventymile and Yukon Rivers at

07 Mertie, J. B., Jr., and Harrington, G. L., The Ruby-Kuskokwim region, Alaska : U. S. Geol. Survey Bull. 754, pp. 71-72, 1924.

to Mertie, J. B., Jr., The occurrence of metalliferous deposits in the Yukon and Kusko kwim regions : U. S. Geol. Survey Bull. 739, pp. 156–158, 1923.

1° Prindle, L. M., A geologic reconnaissance of the Circle quadrangle, Alaska : U. $. Geol. Survey Bull. 538, p. 33, 1913.

« PrejšnjaNaprej »