NEW CONCEPTS IN EOLIAN STRATIGRAPHY, PRESERVATION OF MEGADUNES

Eolian sandstones are thought to largely accumulate through bedform climb, as dunes gradually rise during migration in the same way a bed of climbing ripples is deposited. Many dune fields may form in this way. However, as more ancient eolianites are studied in detail, it has become apparent that the accumulation and partial preservation of dunes, and more commonly megadunes (draas) shapes the internal stratification of many eolianites. The megadunes can be thicker than 120 m and spaced kms to 10’s of kms. Examples from Permian Cedar Mesa Sandstone, Triassic Wingate Sandstone, and Jurassic, Navajo, Entrada and Page Sandstone exposures on the Colorado Plateau illustrate how the architecture of eoianites is shaped. The process is easiest to see in parts of the dune fields where ponds and wet interdune deposits can be found in the lows that remained between the megadunes. Initial accumulation of large dunes that may climb across wet interdune, sand sheet or smaller dune deposits forms the initial core of the megadune. The megadune grows through three processes, (1) migration of large foresets that indicate migration of the dune form, (2) preservation of downclimbing dunes on the overall downwind side of the dune, and (3) preservation of dunes climbing the upwind side of the megadune. Transverse and oblique megadunes have been identified and it is likely that unidentified longitudinal dunes can be found in the rock record. Erosion of megadunes produces an irregular surface with 10’s to 100’s of meters of relief that separate the eroded megadunes from younger strata. Younger strata fill the intervening lows and, in some cases represent different environmental conditions.

In older dune fields today, the primary form of preservation is not the migrating dune, climbing as it migrates, but large megadunes that may be being reshaped by younger megadunes growing in a different wind regime. It seems likely that ancient eolianites formed through similar processes, where megadunes were preserved as the fundamental element of eolian architecture.