CONSTRAINING THE ABSOLUTE AGE OF THE LOWER PART OF THE JURASSIC NAVAJO SANDSTONE, SOUTHEASTERN UTAH

Erg deposits are notoriously difficult to date. Fossils are scarce and generally not age diagnostic. Fine-grained sediment in this environment, including volcanic ash, is rarely preserved. Age dating methods of detrital zircons may reveal provenance rather than age of deposition; magnetostratigraphy may be difficult because of the inherent gaps in the record. For these reasons, eolian sandstone in the Jurassic Glen Canyon Group (GCG) on the Colorado Plateau in the western USA, has mostly been dated only by inference. The GCG consists of, in ascending order, the Wingate Sandstone (Ss) in the east and partially correlative fluvial Moenave Formation (Fm) in the west; the Kayenta Fm; and the Navajo Ss. The Moenave and Kayenta formations are relatively well dated with detrital zircons, although the Kayenta Fm has yielded very different results depending on location. Magnetostratigraphic work has been consistent with the overall sedimentary succession but is not precise. Recent advances in U-Pb geochronology of carbonates permitted analysis of U-Pb ages from carbonate lacustrine deposits in the Navajo Ss in southeastern Utah. Two localities at the Navajo erg margin yielded dates of 200.5 ± 1.5 Ma (earliest Jurassic, Hettangian Age; 2 samples) and 195.0 ± 7.7 Ma (Early Jurassic, Sinemurian Age). The older ages are from 65 m above the base of the formation and are much older than previously suggested ages for the lower part of the Navajo Ss, which were generally Sinemurian to Toarcian. The results indicate that the base of the Navajo Ss is strongly time transgressive, becoming younger to the west. In addition, these and other data indicate that in the area of Moab, Utah, the Wingate Ss, Kayenta Fm, and the lower part of the Navajo Ss were deposited quickly, in as little as 1 million years or less.