Mechanisms for the Accumulation and Preservation of Eolian Dune Successions
Many past and current facies models that attempt to account for the preserved sedimentary architecture of eolian successions invoke bed-form climbing as a mechanism to explain the accumulation of thick preserved successions of eolian dune and interdune strata. In its most basic form, the bed-form-climb model envisages a train of contemporaneously active bed forms separated by interdunes that undertake gradual and progressive accumulation as they migrate over time, thereby accumulating a series of stacked sets of cross-bedded dune and interdune strata. Crucially, accumulation via such a mechanism requires a sustained episode of accumulation that is concurrent with on-going bed-form migration. Although bed-form climbing can be convincingly shown to have been responsible for some preserved successions (e.g. parts of the Jurassic Navajo Sandstone), others are more likely to have accumulated in response to punctuated accumulation whereby the preserved record represents numerous sequences, each bounded by supersurfaces that represent potentially protracted episodes of bypass or deflation. Accumulation via bed-form climbing and accumulation to form individual sequences separated by supersurfaces have traditionally been considered competing end-member models. However, several studies now convincingly demonstrate that both mechanisms for accumulation can occur and that transitions between accumulation via bed-form climb and via sequence and supersurface generation can occur both spatially and temporally for some preserved eolian successions (e.g. parts of the Permian Cedar Mesa Sandstone). Alternative mechanisms for the accumulation of eolian successions are also possible (and arguably likely), including the generation of sets of cross strata in response to the migration of bed forms of pseudo-random height and scour depth and in the absence of net deposition (i.e. a zero angle of net climb). Research questions addressed by this study include consideration of: (i) the proportion of the known eolian preserved record that arose via sustained episodes of bed-form climb versus episodic sequence and supersurface generation; (ii) the specific set of controlling conditions that allow for sustained bed-form climb versus episodic sequence accumulation and preservation; (iii) the application of 4D models to account for complex patterns of spatial and temporal transition between different mechanisms of accumulation and preservation.
AAPG Datapages/Search and Discovery Article #90189 © 2014 AAPG Annual Convention and Exhibition, Houston, Texas, USA, April 6–9, 2014