Significance of interdune deposits and bounding surfaces in aeolian dune sands

Bounding surfaces and interdune deposits provide keys for detailed interpretations of the development, shape, type, wavelength and angle of climb of aeolian bedforms, as well as overall sand sea conditions. Current alternate interpretations of bounding surfaces require very different, but testable models for sand sea deposition. Two perpendicular traverses of Jurassic Entrada Sandstone, Utah, reveal relations among crossstrata, first-order bounding surfaces, and horizontal strata. These field relations seem explicable only as the deposits of downwind-migrating, climbing, enclosed interdune basins (horizontal strata) and dune bodies consisting of superimposed smaller crescentic dunes (cross-stratified deposits). 4 1:7 km traverse parallel to the palaeowind direction provides a time-transgressive view showing continuous cosets of cross-strata, first-order bounding surfaces and interdune deposits climbing downwind at an angle of a few tenths of a degree. Changes occur in the angle of climb, cross-strata structure, and interdune deposits; these,reflect changes in depositional conditions through time. A 1.5 km traverse perpendicular to the palaeowind direction provides aview at an instant in geological time showing firstorder bounding surfaces and interdune deposits forming flat, laterally discontinuous lenticular bodies. The distribution of interdune sedimentary structures in this traverse is very similar to that of some modern interdune basins, such as those on Padre Island, Texas. Hierarchies of bounding surfaces in an aeolian deposit reflect the bedform development on an erg. The presence of three orders of bounding surfaces indicates dune bodies consisting of smaller, superimposed dunes. The geometry of first-order bounding surfaces is a reflection of the shape of the interdune basins. Second-order bounding surfaces originate by the migration of the superimposed dunes over the larger dune body and reflect individual dune shape and type. Third-order bounding surfaces are reactivation surfaces showing stages in the advance of individual dunes. The presence of only two orders of bounding surfaces indicates simple dunes. Modern and Entrada interdune deposits show a wide variety of sediment types and structures reflecting deposition under wet, damp, and dry conditions. Interdune deposits are probably the best indicators of overall erg conditions and commonly show complex vertical sequences reflecting changes in specific depositional conditions. I N T R O D U C T I O N Aeolian cross-stratified deposits are universally interdune deposits record the history of a n erg (sand punctuated by various bounding surfaces, the most sea), but until recently interpretive efforts were extensive of which are commonly overlain by horiconcentrated upon dune cross-stratification. Boundzontal interdune deposits. Together with duneing surfaces and horizontal strata within aeolian deposited cross-stratification, bounding surfaces and cross-stratified units were widely noted, but the * Present address : Department of Geological Sciences, bounding surfaces were not generally attributed to University of Texas, Austin, Texas 78712, U S A . systematic dune development, nor were the hori0037-0746/81/1200-0753 $02.00 zontal strata interpreted as interdune deposits.