Paper No. 10
Presentation Time: 10:15 AM
TWO-STAGE PRESERVATION OF EOLIAN STRATA THROUGH DUNE MIGRATION FOLLOWED BY INFILLING OF INTERDUNES PERMIAN CEDAR MESA SANDSTONE, SE UTAH
There are two commonly used models to explain preservation of eolian dune fields. Some dunes or draas are assumed to climb during migration in the same manner as climbing ripples in the bedform climbing model. When interdunes are stabilized groundwater, eolian stata can be preserved by rises in the water table beneath Stokes surfaces. However several features of the Cedar Mesa Sandstone in southern Utah do not fit either model. First, unconformity bounded units typically are upward fining, opposite to what should be expected through bedform climbing and difficult to explain with bypass surfaces. Second, high angle surfaces (5-20 degrees) climb both upwind and downwind entirely through the interval and laterally segments many units. Since conventional mapping of strata proved ineffective, detailed GPS mapping of a 5 km2 area in the was used to trace packages of eolian sediments and their intervening bounding surfaces through a 20 m thick super-bounding surface defined package in the Needles district of Canyonlands National Park. High angle bounding surfaces define dune complexes or draas that were spaced at intervals over a kilometer apart and were 650 to 800 m wide. Within the dune complexes, second-order surface dipping at up to 30 degrees separated sets of downclimbing dunes that marked alternations of barchanoid and transverse dunes during draa migration. The upper surfaces of the draas are eroded showing degradation as the intervening areas were filled with sand sheets and associated flood deposits. The dunes intertoungue with the sand sheets along the downwind margin of the draa, illustrating downwind migration of the dune masses over the inter-draa sand sheets. The Cedar Mesa illustrates a two stage history of eolian aggradation wherein draas are grow and migrate down wind , but then are deflated, with the intervening low filled with younger sand sheets to produce a layer of eolian sand.