Spatial and temporal variation in the evolution of ancient aeolian dune-field. The Pennsylvanian Piauí Formation (Parnaíba Basin), Brazil

The sedimentary record of aeolian systems is the result of a complex interplay between the size of the dune and interdune morphologic elements, the rate of bedform migration and the rate of aggradation. Since these parameters are not constant over the time and space, the resulting preserved deposits display a complex architecture characterized by sets of variable thickness and climb angle. Such complexity is observed in both wet and dry aeolian systems of the Pennsylvanian Piauí Formation and is studied here using a numerical reconstruction of the controlling parameters, including dune wavelength and angle of climb each of which varied both spatially and temporally. Through facies analysis and stratigraphic correlation, three genetic units were identified. Spatial variation in the first unit is characterized by a dry aeolian system in which the dimensions of dunes increased to the NW, suggesting deceleration of saturated winds from a dune-field margin to its center. The second and third units are characterized by wet aeolian systems marked by progressively thicker sandsheet deposits and a reduction of dune/interdune thickness ratio. In both units, the spatial variations are marked by a reduction in the dimensions of dunes to the NW, suggesting progressively lower sand availability most likely related to higher water table or its capillary fringe from a dune-field center to its margin. The temporal variation was inferred from the drying-upward cycles observed in all three units. Although the first unit is marked by aeolian cosets overlying thinner simple dune sets, the second and third units are composed by sandsheet deposits at the base overlain by the deposits of progressively larger dunes, in some case culminating with the preservation of aeolian cosets of strata. Such observation suggests that both dry and wet units are marked by an increase in sand saturation allowing the preservation of larger dunes over time. The results of this study suggest a highly dynamic nature of the aeolian dune system where variations in sand supply, availability and wind energy exerted a direct effect on the aeolian system, triggering expansion or contraction of the dune-field. These expansion and contraction phases observed in the Pennsylvanian Piauí Formation are related to glacio-eustatic changes documented during the Pennsylvanian period.