MESOZOIC ROCKS, KOYUKUK AND BERGMAN SERIES. 77 



KOYUKUK SERIES (LOWER CRETACEOUS). 



Character and occu?Tence. — The rocks of the Koyukuk series consist of pink and 

 reddish impure limestone, dark shale, slates, and some sandstones or arkose, with 

 occasional associated igneous rocks. The latter include dioritic dikes, amygdaloids, 

 and andesitic tuffs, which denote volcanic activity during and subsequent to Lower 

 Cretaceous time. 



The series is represented as occupying a small, roughly quadrangular area near 

 the southern edge of the geologic map, tying principally between the sixty-sixth 

 parallel and the Arctic Circle on Koyukuk River. It, however, is known to 

 extend southwestward considerably beyond the limits of the map, and ma} 7 with 

 further investigation be found to occupy a large part of the Koyukuk Basin. It is 

 represented as limited on the northeast by the Bergman sei'ies, which in a general 

 way it seems to underlie, and with which further study may show it to be closely 

 connected in point of age. 



Owing to wide breaks in the sequence of outcrops and the changing attitude of 

 the rocks, no attempt will be made at present to form an estimate of the thickness of 

 the series. It may be noted, however, that at one point where fossils were collected, 

 near the southern edge of the map, on the right (or west) bank of Koyukuk River, 

 the limestone exhibits a thickness of about 800 feet. 



Structure. — The Koyukuk series has been variously disturbed by folding and by 

 some faulting, but the prevailing dip, so far as observed, is northward, roughly 

 speaking, at an angle of 40° to 45°. A profuse jointing trends N. 25° W., and dips 

 steeply northeast, and a well-marked cleavage strikes northeast and dips 75° SE. 



Age. — The age of the Koyukuk series is supposed by Doctor Stanton to be the 

 same as that of the Anaktuvuk series. This determination is based on fossils collected 

 by the writer in the impure limestone near the southern edge of the area mapped, on 

 the right bank of the river, and at another locality a few miles farther down the 

 stream, to the southwest. In each instance Doctor Stanton states that Lower 

 Cretaceous age is indicated by the presence of Aucella crassicollis Keyserling. 



Correlation. — See "Correlation" under Anaktuvuk series, page 76. 



BERGMAN SERIES (CRETACEOUS?). a 



Character and occnrrrence. — The term Bergman is here employed to designate a 

 group of comparatively uniform rocks covering a large area in the Koyukuk Basin and 

 forming in large part the Koyukuk Plateau (see p. 44). This series lies north of the 

 Koyukuk series and has a north-south extent of 60 to 70 miles. On the north 

 it rests unconformably on the schists of the Totsen series, while on the south it is 

 apparently infolded with the Koyukuk series, to which it is supposed to be closety 

 related and which it is supposed to succeed in geologic age. 



"The name Bergman is that of a trading post on Koyukuk River situated within the area occupied by the series, at a 

 point where its rocks have become exposed by the incision of the valley into the plateau. 



