THE POST-LARAMIDE DISCONFORMITY IN NORTHWEST ARIZONA: A PROXY FOR THE EOCENE-OLIGOCENE TRANSITION?

Widespread, poorly exposed, Paleogene sediments with limited chronologic control are exposed in a broad east-west swath stretching from the western Grand Canyon to the Fort Apache Reservation in Arizona. In many places the post-Laramide disconformity is buried beneath volcanic rocks. Many of the Neogene deposits, originally termed “Rim gravels”, only survive as residual lag gravels that sometimes mask the arkosic parent sediments. The arkosic sediments were derived from the Laramide highlands that formerly ringed the southern and western margins of the Colorado Plateau. The precise nature of the disconformity is best preserved and exposed in paleocanyons on the Hualapai Reservation in the western Grand Canyon region and along the eastern Mogollon Rim. Weakly lithified sediments below the disconformity consist of deeply weathered, arkosic fluvial sands interbedded with occasional marls and limestones. In a few places a thick lateritic soil with well-developed ped structures is present. The limestone conglomerates above the disconformity contrast markedly in color, texture, lithology, and provenance with the older sediments. Fossil gastropods, charophytes, and ostracodes in the limestones suggest that the age of the preserved Paleogene sediments is late Paleocene and/or Early Eocene. The subsequent weathering interval that is characterized by the dark red lateritic soil is assumed to be middle to late Eocene in age. The magnitude of the preserved weathering interval suggests that the disconformity should be recognizable in the Tertiary landscapes of the Colorado Plateau. A corresponding geomorphic record in the region has yet to be widely recognized, with the exception of the Eocene surface in the adjacent southern Rocky Mountains. The disconformity may be represented by subtle field evidence across a broader region. Global and North American paleoclimate records imply that the rocks above the disconformity record the transition to early Oligocene aridity. Wider recognition of this disconformity may provide an improved means of unraveling the early Tertiary evolution of the Colorado Plateau landscape and establishing an improved Tertiary stratigraphic model. The challenge is to devise an improved, multidisciplinary strategy that can detect the subtle evidence for the extended erosional episode.