Climatic Transition from the Fluvial Kayenta Formation to the Eolian Navajo Sandstone (Jurassic, Colorado Plateau, U.S.A.)

The fluvial Kayenta Formation underlies the eolian Navajo Sandstone in the eastern part of the basin (SE Utah), and the two formations intertongue in the western part of the basin (SW Utah). The age of the transition is as much as 5 million years younger in the west, indicating a long transition from fluvial to eolian conditions as the Navajo erg expanded across the basin. There are three possibilities for the dynamics of this transition: (1) erg expansion proceeded more rapidly as the erg expanded westward owing to positive feedbacks of aridification; (2) erg expansion proceeded more slowly because rivers were more persistent in the west; and (3) the processes controlling the transition were independent in the older, eastern part and that in the younger, western part of the basin. In the eastern part of the basin, the timing of the transition may have been partially obscured by the lack of accommodation, but appears to have taken place gradually on a smaller scale, with repetitive interbedding of fluvial and eolian sediments and their associated, contrasting biotas. This contrasts with the large-scale intertonguing observed in the western part of the basin. There, the transition from normal fluvial conditions of the Kayenta to the Lamb Point Tongue of the Navajo Sandstone occurred over a longer stratigraphic thickness than the transition from the fluvial Tenney Canyon Tongue of the Kayenta Formation to the Navajo Sandstone; both transitions were marked by changes in the floras and faunas as indicated by trace fossils.