LONG-LIVED PLUVIAL EPISODES DURING DEPOSITION OF THE NAVAJO SANDSTONE

The Navajo Sandstone of the American Southwest was deposited approximately 190 million years ago in a giant, subtropical dune field near the western margin of Pangea. At the northern edge of the Paria Plateau, near the Arizona-Utah border, the Navajo contains at least two 20 m-thick intervals of bioturbated dune cross-strata. Both of these intervals were churned by insects; the upper one was also trampled by reptiles. The upper unit extends throughout our 115 km2 study area. We interpret this suite of structures as the record of a pluvial episode that was climatologically similar to the period of “greening” in the Sahara 4,000-10,000 years ago. Although dunes continued to migrate freely, the distribution of trace fossils shows that plant life in wet interdunes sustained high levels of animal activity on the dunes for many thousands of years. Independent evidence for heavy rainfall events is present in the form of numerous slumps within avalanche-dominated sets of cross-strata. A high percentage of the rainfall on the Navajo erg recharged the water table, leading to the development of highly dilute, local groundwater flow systems that discharged into interdune areas. This ecological/ depositional system appears to be without direct modern analogs. Under Quaternary pluvial conditions in North Africa, modern plants were able to stabilize dunes, but in the Navajo Desert, dune migration and sediment aggradation continued unabated during pluvial periods. Although some unknown environmental conditions may have prevented dune stabilization, we consider it much more likely that, with the evolution of the angiosperms, the ability of land plants to stabilize dunes has improved greatly since the Early Jurassic.