Development of longitudinal dunes under Pangaean atmospheric circulation

Abstract

As a result of the large difference in heat capacity between land and ocean, global climate and atmospheric circulation patterns over the supercontinent of Pangaea were significantly different from those of today. Modelling experiments indicate a seasonal alternation in cross-equatorial flow induced by the seasonal reversal in the direction of the monsoonal circulation; however, there are large discrepancies between model-generated surface wind patterns and the reported palaeowind directions from aeolian dune records. Here, we present the spatial distribution of dune slip-face azimuths recorded in the Lower Jurassic aeolian sandstones over a wide area of the western United States (palaeolatitude: ∼19–27∘ N). The azimuth data for dune slip faces reveal a bidirectional and oblique angular pattern that resembles the internal structures of modern longitudinal dunes. Based on the spatial pattern of slip-face directions and outcrop evidence, we suggest that most Lower Jurassic aeolian sandstones were NNE–SSW- to NNW–SSE-oriented longitudinal dunes, which likely formed as the result of a combination of westerly, northwesterly, and northeasterly palaeowinds. The reconstructed palaeowind pattern at ∼19–27∘ N appears to be consistent with the model-generated surface wind pattern and its seasonal alternation. The reconstructed palaeowind patterns also suggest an influence of orbitally induced changes in atmospheric pressure patterns over Pangaea.