Abstract

During the Early Jurassic, the Navajo Sandstone erg, possibly the largest that has existed on Earth, spread gradually southward and eastward through Utah and into Arizona, and across erg-margin deposits of the fluvial Kayenta Formation. The transition was cyclic, with at least three and locally as many as thirteen fluvial tongues of ephemeral fluvial deposits extending into the erg. The present study examines this transitional interval in the Kanab area of southern Utah, where there are three distinct fluvial–eolian cycles. These cycles can be tentatively traced for some 200 km eastward, suggesting allogenic control on the deposition of these deposits. Facies assemblages are interpreted in terms of cycles of increasing aridity. Cycle development generally began with the accumulation of ephemeral fluvial deposits in terminal-fan environments. Deposits of channelized flow pass upward into sheet-like deposits recording unconfined flow, and the cycles culminated in short periods of rapid eolian bedform accumulation, followed by intense regional deflation during times of maximum aridity, with the formation of erosional super surfaces. It is likely that the climatic control of this succession was orbital forcing, but given the lack of detailed chronostratigraphic control for the Kayenta–Navajo interval it is not possible to suggest a likely orbital frequency.