| Popis: |
The White Rim Sandstone of Leonardian age forms prominent topographic benches west of the Colorado River in Canyonlands National Park. It has been described as being of eolian or marginal-marine origin by several authors without reference to specific evidence, except for large-scale generally conspicuous cross-stratification. A detailed study of the sedimentary structures of the sandstone, including gross geometry, cross-stratification, ripple marks, trace fossils, and facies relations, revealed a subaqueous environment of deposition, probably sublittoral marine. The formation contains numerous offshore bars that were constructed by surf and longshore currents moving from the northwest, as shown by bar trends and cross-stratification analyses. A large, elongate bar with 200 ft of relieve extends for about 10 mi through Elaterite Basin in an arcuate north-northwest trend. Excellent exposures of sedimentary structures show that the bar is preserved in its original geometric configuration. The sandstone grades abruptly into find-grained lagoonal redbeds on the east, forming a stratigraphic oil trap. The shall w-water bar apparently was constructed on the nose of the Monument upwarp,which was slightly positive and provided shoal conditions at the time of sedimentation. Numerous smaller bars with 10-20 ft of relief form in a northwest-trending swarm Pre-Moenkopi (Triassic) redbeds, were deposited across the White Rim Sandstone, draping over the large bar. Subsequent erosion prior to Moenkopi sedimentation produced local angular unconformities along the margins of the bar. The Elaterite bar is almost uniformly saturated with interstitial heavy hydrocarbons, and asphalt seeps are numerous within exposures of the bar. Adjacent offbar sandstones are largely devoid of hydrocarbons. Apparently the source of the oil was in Kaibab or Sinbad limestones in the Henry Mountains basin on the west. The petroleum migrated updip through the White Sandstone and was retained in the bar because of its geometric configuration and the permeability barrier created by the updip shale-out of the White Rim Sandstone just shoreward (east) of the bar. |
