Paleoecology of Early Jurassic Navajo Sandstone Interdune Deposits
The Lower Jurassic Navajo Sandstone represents a desert that covered more than 366,000 square kilometers. Localized interdune deposits commonly occur along the eastern edge of this desert that include carbonates, bioturbated layers, and plant fossils. Previous studies of these deposits focused on specific fossil types or isolated sites. This study involved a comprehensive analysis of the paleoecology of interdune deposits with an integrated approach combining paleontology, sedimentology and geochemistry. The methods used in this study were devised to test specific paleoecological and preservational questions, including water sources and geochemistry, sedimentation mechanisms, fossil identification and paleoecology, taphonomy, and diagenesis. Three hypotheses were developed to test for the water source of the interdune deposits. The first was an ecosystem with diluted marine brines derived from upwelling Paradox Formation evaporites. These brines would have created low-diversity salt-stressed ecosystems concentrated along fault systems that paralled salt-cored anticlines. The second was an interdune ecosystem with limited water availability, derived from upland rainfall from the Uncompahgre Highlands to the east. This would have sustained interdune environments along the easternmost edge of the erg at many stratigraphic levels. Limited water supply would have created drought conditions with xeric plant communities. The final water source hypothesis was a climate change event that increased rainfall across the Navajo Sandstone depositional basin, resulting in dune stabilization and widespread mesic ecosystems at specific depositional horizons. The results of this study have shown that interdune deposits are concentrated along two stratigraphic levels in the Moab, Utah area. Stable isotopes indicate freshwater environments with laminated lacustrine carbonates with freshwater ostracodes. Plant fossils indicate freshwater interdunes were stable for sufficient intervals to grow large conifers without seasonal variation of wood growth. Vertebrate fossils reported by other studies represent cosmopolitan species rather than desert-adapted species. The water source for these deposits was widespread rainfall during two climate change events. Ecosystems from mesic regions extended into the Navajo desert during these climate events. These regional events were similar to oscillations that occurred during the Kayenta Formation/Navajo Sandstone transition and may be linked to global elevated carbon dioxide events in the Early Jurassic.
Wilkens, Nathan Daniel, "Paleoecology of Early Jurassic Navajo Sandstone Interdune Deposits" (2008). Canyonlands Research Bibliography. Paper 2.
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