246 F. C. SCHRADER — GEOLOGICAL SECTION IN NORTHERN ALASKA 



Keyserling, or a closely related form and undoubtedly of Lower Cre- 

 taceous age. The series is to be correlated with the Koyukuk series, to 

 be next described, though the lithologic difference between the two series 

 is somewhat marked. 



KOYUKUK SERIES (LOWER CRETACEOUS) 



Character and occurrence. — The Koyukuk series constitutes the southern 

 45 miles of the section lying principally between the 66th parallel and 

 the Arctic circle, on the Koyukuk river. The series, however, is known 

 to extend much farther southwestward, and may with further discovery 

 prove to have a very wide extent over the Koyukuk basin. The rocks 

 of the series consist of impure pink and reddish limestone, dark shale, 

 slate, and some sandstone or arkose, all more or less associated with or 

 intruded by igneous rocks, denoting volcanic activity during and subse- 

 quent to Low r er Cretaceous time. The series is represented as limited 

 on the northeast by the Bergman series, which in a general way it seems 

 to underlie, but may later be found to be closely connected with it in 

 point of geologic age. Owing to the various breaks in the sequence of 

 outcrops, and the changed attitude of the rocks, no estimate of the thick- 

 ness of the Koyukuk series can be given as yet. It may be noted, how- 

 ever, that at the point where the fossils were collected, near the southern 

 end of the section, the limestone alone exhibits a thickness of about 800 

 feet. 



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

 some faulting, but the prevailing dip seems to be northward, roughly 

 speaking, at an angle of 40 degrees. A profuse jointing trends nearly 

 north 25 degrees west and dips steeply northeast, while a well marked 

 cleavage dips 75 degrees southeast. 



Age. — The age of the Koyukuk series is supposed to be the same as 

 that of the Anaktoovuk series, Lower Cretaceous. This assignment is 

 based on the evidence of fossils collected in the impure limestone near 

 the southern end of the section, and which were found to be undoubtedly 

 of Lower Cretaceous age by the presence of Aucella crassicollis Ke} 7 serling, 

 thus correlating the Koyukuk series with the Anaktoovuk series, both 

 containing Aucella beds typical of Lower Cretaceous in Alaska. 



BERGMAN SERIES (CRETACEOUS) 



Character and occurrence. — The series consists of a comparatively uni- 

 form group of rocks, covering a large area in the Koyukuk basin and 

 forming in large part the rolling Koyukuk upland already noted. It 

 succeeds the Koyukuk series on the north, and has a north and south 



