Controls on accumulation in modern and ancient wet eolian systems

Wet eolian systems are those in which the water table is shallow and the floors of the interdune areas are within the capillary fringe. Sediment accumulation within a wet eolian system occurs as a result of a relative rise of the water table, and these accumulations are characterized by the presence of both dune and interdune-flat deposits. Surfaces capping the accumulations form under conditions of falling or static water table and are the result of erosion or sediment bypassing, respectively. Together an accumulation and its bounding surface are termed a wet eolian sequence. The Middle Jurassic San Rafael Group of eastern Utah is interpreted as the product of a coastal wet eolian system. Four complete sequences are correlated along a 365 km transect of eastern Utah stretching from near Dinosaur National Monument to Bluff. The areal extent of these sequences suggests that the controls on relative change in the level of the water table were regional in nature, and related to relative sea-level fluctuation caused by eustasy, regional and local subsidence. The rate of relative sea-level rise balanced against the rate of sediment supply controlled the depositional facies that form the accumulation. However, the rate of sediment supply was not an independent variable, and appears to parallel the rate of relative sea-level rise. The probable cause of this relationship is that as the sea transgressed, it delivered sediment to the coastal areas upwind of the San Rafael Group. Because both marine sequences and wet eolian sequences accumulate with a rising relative sea level and form bounding surfaces on the relative sea level fall, they can be tied into the same sequence stratigraphic framework. The gypsum dune field at White Sands National Monument is a modern example of a continental wet eolian system. The dune field is characterized by dunes separated by damp interdune areas migrating to the northeast under the influence of southwesterly Spring winds. Accumulations observed in shallow trenches in the interdune areas are composed of inclined packages of cross-stratified dune and flat-bedded interdune strata. The average inclination of the upper bounding surface of the interdune accumulations is a tenth of a degree, which reflects the angle at which the dunes are climbing under the influence of a rising water table. The water table at White Sands is experiencing a relative rise as a result of sediment compaction and subsidence within the tectonically active Tularosa Basin. Lateral variations in the angle of climb result from temporal and spatial variations in the depth of scour. Temporal variations are the result of fluctuations in the water table level controlled by annual changes in precipitation. Spatial variations in the depth of scour reflect variability in the size of the interdune areas. In larger interdune areas, any point on the surface is exposed to the erosive effects of the wind for a longer period of time than in a smaller interdune area, thus resulting in a greater scour depth.