Eolian topography as a control on deposition incorporating lessons from modern dune seas: Permian Cedar Mesa Sandstone, SE Utah, USA

摘要

The dominant current model for the accumulation of eolian strata in the rock record emphasizes translation and climb of bedforms and the preservation of continuous beds. Studies of modern dune fields, however, show that, when large dunes form, they create a topographically irregular surface; later episodes of eolian activity commonly fill the low-lying areas between the older dunes, resulting in beds that are laterally discontinuous. Strata in part of the Permian Cedar Mesa Sandstone in southeastern Utah require this model. In this new model, inherited eolian topography influences subsequent deposition. Detailed GPS mapping of an 8 km(2) area was used to trace eolian strata through a 20-25-m-thick unit in the Cedar Mesa Sandstone. The unit is bounded above and below by subparallel erosion surfaces that have up to 5 to of relief. Within the unit, large transverse draa (or megadune) deposits, are spaced 1 km apart. Locally (over 0.5 km wide areas), the draa deposits fill the entire 25 m thickness of the unit, only to be completely replaced by interdraa deposits a few hundred meters away. The draa surfaces were eroded and exposed prior to the filling of the intervening lows with alternations of barchanoid dune deposits, sand sheets, and ponds, which formed in the topographic lows. The Cedar Mesa Sandstone illustrates aggradation wherein the important factor is the preserved eolian topography. The draas formed an undulating stable surface, from which subsequent deposition filled the interdraa low. Large eolian dunes that influenced subsequent deposition are also observed in several Pleistocene to Recent dune seas. This mechanism offers an alternative to the existing models that require continuous migration of large dunes.