THE LATE PALEOZOIC ICE-HOUSE AND THE COASTAL DESERTS OF WESTERN TROPICAL PANGEA; NEW INSIGHTS FROM THE SOUTHERN MARGIN OF THE PERMIAN CEDAR MESA SANDSTONE ERG IN THE CANYONS OF THE SAN JUAN RIVER, SE UTAH

In SE Utah the Early Permian (Wolfcampian) Cedar Mesa Sandstone is spectacularly exposed by the canyons of the San Juan River. Six new 80-125m-thick measured sections characterize the southern extent of an erg dominated by eolian dune, sand sheet, and fluvial interbeds. New subsurface, satellite imagery, and previous work define the extent and facies composition of the adjacent and intertounging strata of the coastal playa system to the E-SE. Prominent bedding packages in the eolian units are the result of Late Paleozoic Ice House sea-level and climate fluctuations. The Cedar Mesa does not intertounge with the underlying Halgaito Shale, and surface and subsurface data show that the Cedar Mesa eolian interval expands beneath the Cedar Mesa Plateau by adding units to the base, suggesting the onlap of a paleo-high, consistent with the location of a forebulge related to thrusting on the Uncompahgre front. The southern margin of the Cedar Mesa erg can be divided into three informal units. A basal eolian-dominated unit, a medial unit with a higher percentage of sand sheets and extradunal deposits, and an upper interval with interbedded eolian dune intervals and extradunal facies. Paleosols with rhizoliths, pedogenic carbonate, and non-marine trace fossils are common in the sand sheet and extradunal facies and in the tops of eolian dune strata. This suggests fresh or brackish water within the dune sea and indicates that incursions of the playa evaporites were limited. Preserved eolian dune topography with abundant rhizoliths indicate a more vegetated system, which was surrounded by sand sheets indicating frequent environmental change. Evaporites and associated facies crop out for more than 50-60 km along the SE margin of the erg, and currently are exposed along the side of Raplee Anticline and stretch as far south as Monument Valley region. Significantly, the distribution of these evaporite facies degrades ground and surface water quality in this region. The strata of the eolian margin and erg center are valuable surface analogues for oil and gas reservoirs hosted in “wet” eolian systems. Finally, the variation in this system further refine our understanding of climatic variability on the tropical, downwind margin of western Pangea during the Late Paleozoic Ice-house.