PALEOCLIMATIC IMPLICATIONS OF FACIES TRANSITIONS IN THE NAVAJO AND KAYENTA FORMATIONS

Traditional interpretations of the paleoclimatic significance of the Navajo Sandstone, based upon analogy to the modern Sahara Desert, propose a dramatic decrease in precipitation throughout the basin of deposition, enabling widespread eolian dune activity. In recent years, however, various reports of features indicating high water table conditions in the early Jurassic erg call for a more nuanced analysis.

Gradational transitions from fluvial to eolian deposition at the Kayenta/Navajo boundary are apparent in many outcrops on the Colorado Plateau, supporting the traditional view. Abrupt and alternating transitions also appear, suggesting that diversion of fluvial systems was also important. At some locations, sub-aqueous interdune facies and soft-sediment deformation features occur throughout the Navajo section, indicating persistent high water table conditions. Evidence of intradunal soft-sediment deformation suggests that these conditions may have derived from direct rainfall as well as stream incursion and the concentration of regional groundwater flow along preferred pathways.

It appears that, though precipitation decreased through time in the Navajo basin of deposition, direct precipitation continued to overprint the regional groundwater flow with dramatic local effects. The influence of a drier climate on fluvial depositional systems was enhanced by increasingly efficient diversion of regional drainage by the expanding erg. Highlands to the south and east may have continued to receive abundant rainfall. Eolian dune migration was favored by the drier climate and by a pre-Cretaceous biota devoid of stabilizing grasses, enabling more widespread dune migration under wetter climatic conditions than displayed by any modern analogue.