Intermontane erg environment and arid climate indicated from associated eolian and alluvial fan facies during the Late Cretaceous, South China

Abstract

Late Cretaceous eolian sandstones found in several basins in South China were previously interpreted as indicators of desert environments. To better understand the environment and climate conditions of that period more accurately, a comprehensively in-depth study of the eolianites was undertaken across multiple basins. Two primary lithofacies were identified within the eolianite-bearing sequences. The eolian facies is classified into four subfacies, namely dune, interdune, sand-sheet, and Gobi, while the alluvial fan facies are categorized into three subfacies, i.e. ephemeral channel, sheet flood, and debris flow. These two facies signify recurring interactions of eolian and water-laid systems, highlighting instances of eolian dune migration and deflation in conjunction with the intrusion of alluvial flows. The coexistence of the two facies in conjunction with the spatial distribution and morphometric characteristics associated with paleowind directions, suggests the following: 1) Eolian sandstones were formed within local intermontane ergs confined to narrow basins, negating their formation in broadly expansive desert or sand sea environments; 2) The alluvial fan facies evolved under localized ephemeral arid conditions; 3) South China experienced a relatively prolonged arid climate during the middle Late Cretaceous period, as opposed to the alternating humid and arid conditions. It is argued that the paleowind directions, recovered from the azimuths of cross-beddings in eolianites are reflective of the planetary wind system that was present in South China at that time, given their consistence with the strikes of basins and tectonic axial orientations.