Stratigraphic Evolution of the Eolian Navajo Sandstone, SE Utah

Abstract

The eolian Navajo Sandstone crops out extensively across southern Utah, and has been studied as a reservoir analogue, but sub-division and correlation between exposures is hindered by the paucity of non-eolian facies within the stacked dune deposits. The Navajo Sandstone was studied in detail in the Moab area of SE Utah, in order to reconstruct the depositional environments, characterise erg development and enable stratigraphic subdivision of the thick, dominantly cross-bedded succession. The results of previously unpublished logged sections, bounding surface mapping and dune reconstruction studies are presented here. In the Moab area, the Navajo Erg succeeded the Kayenta perennial fluvial system. This transition is commonly abrupt, with little interaction between the fluvial and eolian systems, but locally a more gradational transition zone exists, comprising ephemeral fluvial, extra-erg dunefield and playa environments, possibly reflecting the effects of underlying salt uplift and climatic fluctuation on the fluvial system. Above this, the Navajo succession is divided into three, locally four, stratigraphic units, separated by super bounding surfaces, indicating that it was formed by four separate phases of erg construction and demise. These sharp, laterally continuous surfaces record prolonged periods of deflation of the erg to the water table, and are characterised by reddening, polygonal fissures, root mottling, rhizoliths and lacustrine carbonates. Widespread horizons of soft sediment deformation occur at consistent stratigraphic levels, recording synchronous periods of sediment liquefaction. The individual stratigraphic units are characterised by different dune styles, showing a vertical change from simple to compound and back to simple dunes, together with a change in preserved set size and related interdune deposits. The character of the units appears to be consistent over a distance of at least 60km in the Moab region. These variations reflect differences in sand supply and relative water table elevation during successive erg phases. Reconnaissance mapping suggests that these divisions may be recognised up to 100km further west. Super surfaces are not apparent, however, within the thick Navajo successions in SW Utah, including Zion Canyon, suggesting a permanent erg in the west, which expanded and contracted eastwards. However, vertical evolution in dune style and preserved set thickness may provide a means of correlating between these areas.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018