科研之友
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摘要
Extensive outcrops of Navajo Sandstone in the southwestern United States expose eolian dune deposits that are subdivided in a complex array of foresets and bounding surfaces. In the Glen Canyon region, and other places, this architecture is frequently disrupted by large-scale, soft-sediment deformation features. These features have been attributed to episodic liquefaction events that affected saturated sand below the level of the interdune surface. Though erosional truncation of deformation features indicates that liquefaction often occurred in the uppermost levels of Navajo dune deposits, very few paleotopographic disruptions due to subsurface deformation have been documented. Navajo Sandstone outcrops in West Canyon, Utah, provide unusually comprehensive exposure of architectural details linking large-scale deformation features and associated interdune deposits, enabling a well constrained appraisal of-their genesis. At-this location, a 23 m succession of-sandstone, mudstone, carbonate, and chert deposits overlies a zone of deformation that extends, laterally, for hundreds of meters. This horizontally stratified lens occupies an abrupt synform along a bounding surface between successive crossbeds that otherwise appears as a featureless, sub-horizontal plane. Large-scale foresets below this bounding surface oversteepen at the margins of the synform and grade downdip into contorted stratification and structureless expanses. %26lt;br%26gt;The authors propose that liquefaction in the Jurassic erg caused localized subsidence of a minor portion of a dry interdune surface to a position several meters below the contemporary water table. A succession of hyperpycnal sand flows, lacustrine evaporites, and eolian sheet and dune deposits filled this depression prior to the advance of large dunes across the site. The process/response dynamics evident in this outcrop suggest that deformation may have exercised significant, non-systematic control over depositional architectures in areas of the erg prone to liquefaction. Similar dynamics are unknown from modern desert environments and their intrinsic scale defies laboratory simulation; therefore, close investigation of these ancient features is essential for exploring the full range of depositional controls that may be encountered in other ancient eolianites on Earth and in eolian accumulations on other planets.
- 出版日期2013-11-15
