A review of aeolian bounding surfaces, with examples from the Permian Minnelusa Formation, USA

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Abstract
The recognition and use of bounding surfaces continues to be a problem in the study of modern and ancient aeolian sediments. The economic problems involve the apparent partitioning of aeolian petroleum reservoirs by cementation along bounding surfaces and associated strata. Bounding surfaces have also become a significant part of outcrop description. Ideally, bounding surfaces in ancient rocks should be tied to modern analogs, of which many are now recognized. The currently popular approach to bounding surface description involves breakdown of rock units into orders based on theoretical relationships to draa migration. As described in this report, however, there are numerous bounding surfaces within aeolian genetic units, and between aeolian and non-aeolian strata that are not the product of draa migration. Indeed, conditions in which modern bedforms climb, and are preserved in perfect sequence are relatively rare, occurring most commonly where sand seas pile up at mountain fronts. Further, aeolian deposition and preservation is more complex and episodic than can be accommodated by the restrictive requirements of the climbing bedform model. The author’s preferred approach to the description of aeolian sequences is in terms of genetic units and subunits, with objective description of crossbed sets using either Campbell’s or Mckee & Wier’s methods. These methods tie in well to sequence stratigraphic concepts that are applied at higher heirarchial levels. Some examples of the application of bounding surface geometries and terminologies can be found in the Permian Minnelusa Formation of Wyoming, USA. The Upper Minnelusa Formation comprises a parasequence set of at least four parasequences. Each parasequence boundary was formed by marine transgression over an aeolian dunefield. The entire sequence is bounded by unconformities at the top and based. Most oil exploration takes place within the individual parasequences. Development drilling, however, usually making use of core, focuses on bounding surfaces at a much smaller scale. Perhaps surprisingly, the most important form of bounding surface control on reservior quality that can confidently be identified is fabric-selective cementation along primary bounding surfaces and strata. Other forms of cementation and decementation, not directly tied to bounding surfaces are also important.