STRATIGRAPHIC AND CLIMATIC TRANSITIONS WHERE THE KAYENTA FORMATION AND THE NAVAJO SANDSTONE (LOWER JURASSIC, COLORADO PLATEAU, USA) INTERTONGUE—A PRELIMINARY REPORT

The transition from the predominately fluvial Kayenta Formation (Jk; Lower Jurassic) to the Navajo Sandstone (Jn; Lower Jurassic) on the Colorado Plateau represents a long-recognized paleoclimatic drying trend as the Navajo erg expanded westward. Documenting the transition is critical to understanding the hydroclimatic evolution of the erg, where both local and distal variations in rainfall played a role in controlling the availability of water and the consequent variations in the formation of paleosols and in the biota. In south-central Utah, we documented the transition from the Jk to the Lamb Point Tongue of the Jn (Jnl), from the Jnl to the Tenney Canyon Tongue of the Jk (Jkt), and from the Jkt to the Jn proper. Drying from the upper part of Jk to the Jnl began 12.8 m below the sharp Jk-Jnl contact. The drying is evidenced by the presence of reworked eolian sand and disappearance of all traces of life, with the exception of scattered, small rhizoliths on one bed. Normal freshwater aquatic and terrestrial traces occur below that for at least 6.5 m. The Jnl contains typical eolian sedimentary structures; however, in one area, thin (< 40 cm) dolomite-cemented clastic lake deposits are present. These stacked lakes are reminiscent of previously reported stacked lacustrine carbonates in the older part of the erg farther east. In the same area, the Jnl is capped by a previously reported carbonate lake that extends at least 1.6 km. Where best developed, the lake is underlain by soft-sediment deformation of eolian strata. Only at the top of the Jnl (base of the Jkt) was the lake persistent enough to deposit carbonate; the location of that lake was likely determined by subsidence following abrupt dewatering of the underlying sand, consequent subsidence of the surface at that location, and continued flooding by the rising groundwater associated with return of the Jk rivers. Where studied, the drying from the Jkt began at the top of a paleosol complex 0.8 m below the Jn, consistent with a more-rapid drying from the Jkt to the Jn. Our preliminary conclusions are that the principal sources of water were the rivers that deposited the Jk and local precipitation. To date, no spring mounds indicating artesian springs, which are present as far west as Lake Powell, have been found in this area.