Stokes surfaces and the effects of near-surface groundwater-table on Aeolian deposition




Stokes surfaces in aeolian deposits are caused by wind scour of unconsolidated material to a roughly planar horizon controlled by near-surface water-tables (Stokes, 1968). A water-table forms a downward limit of scour through the cohesion of damp or wet sand near water-table, and through early cementation by evaporites precipitated in the sediments as water evaporates near the sand-air interface. Study of modern analogues reveals that Stokes surfaces exist in a variety of depositional settings, including a coastal offshore prograding sand sea (Jafurah, Saudi Arabia); a coastal onshore prograding sand sea (Guerrero Negro, Mexico) and a continental sand sea (White Sands, New Mexico, USA). These modern analogues indicate that our concept of Stokes surfaces must be broadened to include the following: (i) modern analogues for Stokes surfaces described here cover areas on the order of 25 km2. These may be as representative of similar surfaces in ancient rocks as hypothesized plains of deflation requiring removal of entire sand seas; (ii) Stokes surfaces occupy a continuum in scale from local to extensive, and erosional surfaces of different magnitude may be stacked closely in the sediments; (iii) Stokes surfaces, although erosional in nature, are commonly associated with deposits both above and below the Stokes bounding surface which plainly reveal the influence of a near-surface groundwater control on wind sedimentation. Moreover, the erosional relief of the bounding surface itself (as well as other features) reveals the influence of a groundwater-table; (iv) Stokes surfaces may be diachronous, representing the lateral shift of a zone of scour within a sand sea rather than simultaneous removal of all dunes from the area encompassed by the erosional surface; (v) Stokes surfaces and associated deposits are often laterally transitional to surfaces and deposits of adjacent depositional environments, including interdunes, tidal flats, lagoons, beaches, lakes and non-aeolian sabkhas. Finally, modern examples from different depositional settings suggest that while most Stokes surfaces have many features in common (such as erosional ridges due to early cementation), there are some features which may, with further study, be revealed to be distinctive of an individual depositional setting.