Tertiary Planation in Idaho

Abstract

Field work by members of the U.S. Geological Survey and others has led to the recognition of an ancient erosion surface or peneplain in parts of Idaho, Montana, and Wyoming. Differences of opinion have arisen regarding the age and character of this peneplain. The problem is regional in character, necessitating more extended field work before a final solution may be reached. The paper gives a discussion of the problem in the light of studies by the author in southeastern Idaho. It is concluded that the idea of post-middle Miocene planation across earlier Tertiary and older rocks, as advocated for Idaho by Blackwelder and Rich, is not proved by the available evidence. There seems a probability that the erosional history of western and northern Idaho and of western Montana was somewhat different from that of southeastern Idaho or of western Wyoming. For southeastern Idaho the evidence seems to favor pre-middle Miocene planation succeeded by: (1) middle Miocene deformation; (2) late Miocene and perhaps early Pliocene erosion; (3) Pliocene deformation, erosion, and deposition; and (4) early Pleistocene deformation or rejuvenation and subsequent erosion.