SEDIMENTARY ENVIRONMENTS/Deserts 545 Supersurfaces and the Preservation of Aeolian Sequences Aeolian accumulation ceases when the net sediment budget switches from positive to neutral or negative Under such circumstances, a positive angle of bedform climb is replaced either by a zero angle of climb (‘bypass’), when the accumulation remains static over time, or by a negative angle of climb (‘deflation’), when an undersaturated airflow cannibalizes the dunes and the accumulation surface falls over time (Figure 5) Both bypass and deflation result in the generation of a ‘supersurface’, a fourth type of bounding surface that caps the underlying accumulation The accumulation defines a ‘sequence’ and its bounding supersurface defines a ‘sequence boundary’ Distinguishing supersurfaces in aeolian strata from other types of bounding surfaces is often difficult Sedimentary features associated with supersurfaces include desiccation cracks, polygonal fractures, bioturbation, rhizoliths (calcified roots), and salt structures, all of which yield important palaeoenvironmental information regarding the nature of the desert surface at the time of supersurface formation Supersurfaces often have great lateral extent and continuity and may bound entire sand sea successions Thus, supersurfaces, which often result from changes in ‘allogenic’ (external) controlling factors, such as climate or tectonic setting, are considered to be of a higher order than the autogenically generated bounding surfaces that they truncate Long-term preservation of an aeolian succession requires that the body of strata is placed below some regional ‘baseline of erosion’, beneath which erosion does not occur, the principal agents promoting preservation being subsidence, water table rise, sea-level rise, and surface stabilization Ancient Desert Systems Ancient aeolian desert systems exposed in outcrop are widely recognized from all seven continents, and a sizeable database of case examples now exists In particular, the extensive Permian to Jurassic outcrops of the Colorado Plateau in the western USA (Figure 6) have been the focus of intensive study in recent years Dry Aeolian Systems Dry aeolian systems, in which accumulation is controlled solely by aerodynamic configuration, are widely documented in the rock record Accumulation in such systems occurs once dunes have grown to cover former interdune flats The angle of climb of the system is determined by the ratio between the rate of accumulation (i.e rise of the accumulation surface) and the rate of bedform migration Steep angles of climb enable larger proportions of the original bedforms to be preserved, although climb angles rarely exceed 1 and typically only the basal 10–15% of a climbing bedform is preserved as a dune set (Figure 3) Thus, the facies preserved within aeolian dune sets record the processes that operated on the lower part of the dune lee slope at the time of accumulation, and are dominated by windripple, grainflow, and grainfall strata Additionally, superimposition and/or reactivation bounding surfaces may be present within climbing dune sets bounded by interdune migration surfaces Supersurfaces within dry aeolian systems develop as a consequence of changed aerodynamic conditions, with deflation usually occurring when the airflow passing over a dune-field is not fully laden with sediment The Page Sandstone of Utah and northern Arizona is a dry aeolian system represented by separate supersurface-bounded sequences, each composed of climbing dune strata and an Figure Models for the generation of supersurfaces in aeolian systems (A) Bypass supersurfaces occur where the angle of dune climb reduces to zero, although dune migration may continue (B) Deflationary supersurfaces occur as a result of erosion, often to the level of the water table Such surfaces may exhibit evidence of widespread plant and/or animal colonization