PERENNIAL INTERDUNE LAKES PRESERVED IN THREE DIMENSIONS WITHIN THE JURASSIC NAVAJO SANDSTONE, SOUTHEASTERN UTAH

The Navajo Sandstone is a quartz arenite representing a great desert or sand sea (erg) in the Lower Jurassic (~195 million years ago), and is exposed extensively in southeastern Utah. Limestone and heavily calcite-cemented sandstone form lenses in the Navajo Sandstone. At Horseshoe Canyon (Canyonlands National Park) and North Wash southeast of Hanksville, these lenses are prominent and numerous. The strata suggest deposition by standing water. These lenses preserve the three-dimensional contours of an interdune lake bottom. At both locations there is approximately 15 meters of relief on individual beds that can be used as paleolake-depth indicators. The morphology of these lenses suggest that the interdune spaces were closed depressions as opposed to long broad, interconnected corridors. The slow growth rate of large microbial mounds preserved on the lake bottom requires several thousand years of sediment-free open water; this is evidence of a hiatus in dune migration for that time. The presence of such lakes in the middle of an erg suggests an extended pluvial episode. The increased water budget raised regional groundwater levels, and allowed formation of interdune lakes in interdune depressions. A well-defined edge to the limestones representing the shoreline suggests near-constant lake levels. The slow drainage of groundwater mounds created beneath dunes after seasonal precipitation would have provided enough water to maintain the local water table at a constant level throughout the year. Comparison to the modern interdune lakes of the Nebraska Sandhills suggest that during long-lived pluvial episodes, such lakes likely dotted much of the Jurassic erg landscape.