SEDIMENTARY ENVIRONMENTS/Deserts 543 Accumulation and Preservation of Aeolian Strata The preservation of desert aeolian systems in the rock record is a three-stage process that requires: (1) the construction of a sand sea, (2) its accumulation to form a succession, and (3) its long-term burial Sand Sea Construction Aeolian sand sea construction is a function of sediment supply, sediment availability, and the transport capacity of the wind The ‘sediment supply’ is the volume of sediment of a suitable grain size for aeolian transport generated per unit time This sediment may form either a contemporaneous or time-lagged source of material with which to construct an aeolian system, and may be derived from a variety of preexisting aeolian or non-aeolian sources ‘Sediment availability’ is the susceptibility of surface grains to aeolian entrainment, and is influenced by stabilizing factors, such as the presence of vegetation, mud drapes, coarse-grained lags, an elevated water table, or surface binding and cementing agents The ‘transport capacity of the wind’ is a measure of the potential sand-carrying capability of the wind, which increases with wind power Where sediment availability is limited, the airflow will be undersaturated with respect to its potential sediment load, and the wind will be potentially erosional Conversely, an airflow that is fully saturated with sediment and undergoes deceleration must drop some of its load, thereby encouraging aeolian bedform growth Accumulation ‘Accumulation’ to generate a body of strata occurs when an aeolian system experiences a positive ‘net sediment budget’, such that upstream sediment ‘influx’ exceeds downstream ‘outflux’ and the accumulation surface rises over time Accumulation of migrating bedforms occurs as a consequence of ‘bedform climb’ with respect to the accumulation surface The ‘angle of climb’ is determined by the ratio between the rate of downwind bedform migration and the rate of rise of the accumulation surface (Figure 3) For most aeolian dune systems, the accumulation rate is small compared with the migration rate and the resultant angle of climb is low, such that, as bedforms move through space, they truncate the upper parts of the preceding bedforms in a train, and only the basal parts of the bedforms accumulate to form sets of cross-strata, a process called ‘subcritical climbing’ Bounding Surfaces in Aeolian Strata ‘Bounding surfaces’ are erosional surfaces that are generated as an intrinsic product of aeolian dune migration and climb, such that bedforms (or parts thereof) scour into pre-existing deposits as they move through space Three broad types of aeolian bounding surface are recognized to occur as a product of ‘autocyclic’ (intrinsic) migratory bedform behaviour ‘Reactivation surfaces’ result from periodic erosion of a dune lee slope, followed by renewed sedimentation as a consequence of a change in bedform migration direction, migration speed, asymmetry, or steepness (Figure 4) These changes are common because the airflow on lee slopes is often subject to modification and is rarely steady In some cases, the period of the flow fluctuation is regular and generates cyclic reactivation surfaces, for example due to diurnal or seasonal wind reversals Nested reactivation surfaces on two or more scales occur when cyclic sets of cross-bedding separated by reactivation surfaces are generated by the interaction of two or more forcing parameters operating with different periodicities ‘Superimposition surfaces’ result from either the migration of superimposed dunes over a larger parent bedform, or the Figure Generation and accumulation of sets of cross strata by migrating and climbing bedforms Accumulation requires a positive net sediment budget, whereby the influx (qi) exceeds the outflux (qo) and the accumulation surface rises through time The ratio between the rate of rise of the accumulation surface and the rate of downwind migration of the bedforms determines the angle of climb (a) of the accumulated sets of cross strata