AEOLIAN ARCHITECTURAL ELEMENTS AND FACIES VARIABILITY IN A PROXIMAL SOURCE-TO SINK-TRANSECT, NORPHLET FORMATION, SOUTHWESTERN ALABAMA

Marked spatial variations in aeolian dune-field architecture have been identified between dune-field margins and interiors. Such transitions are recognized as the result of the interplay of dune-field autogenic dynamics and environmental boundary conditions. We studied the Upper Jurassic Norphlet Formation with the aim of inferring the relative influence of autogenic dynamics and environmental boundary conditions and characterizing a source-to-sink transect from the dune-field margin to the interior. Using three cores and detailed facies analysis, we characterized the stratigraphic configuration from the dune-field margin to the dune-field interior. Facies identified include interdune, wadi, aeolian dune, sandsheet and marine facies. Aeolian set and grainflow thickness were measured. Results show a distinct spatial and temporal transition in facies during Norphlet deposition. Spatially the dune field transitioned from a fluvially dominated margin with small aeolian sets to an aeolian dominated dune-field center with the thickest overall sets. An increase set thickness and decrease in the total number of first order bounding surfaces moving away from the sediment source was identified, with set thickness ranging between 1 to 6 m in the erg center. Preserved grainflow thickness shows a positive correlation with distance from the sediment paleosource, with a significant population between 0.5 and 1.6 cm thick for the middle and distal locations. Temporal changes are recorded by vertical facies variability, which transitioned upward from wet interdune strata to wadi and dry dune facies, to an interval of sandsheets that grades to marine deposits. The sequence implies a relative sea level rise at the end of the Norphlet deposition. Interpreted climatic conditions suggest a wetter fluvio- aeolian system prevailed in the updip location that changed to a dryer dune dominated area in the dune field center. The transition to the uppermost Norphlet section characterized by sandsheets and marine siltstones, indicate a combination of decrease in sediment availability, increase in sea level, and increase in water table level, leading to cessation of aeolian deposition.