Middle Jurassic palaeoclimate changes within the central Ordos Basin based on palynological records

Detailed palynological analyses covering the interval of 897.0–430.0 m of well GX15 in the Wushenqi, central Ordos Basin, were performed with aims to gain knowledge on the vegetation and assess changes in palaeoclimate. In total, 149 species belonging to 64 genera have been identified from the Yan'an to lower Zhiluo Formations (897.0–629.0 m) and palynomorphs (spore and pollen) are absent in the upper Zhiluo Formation (above 629.0 m). Based on the abundance variation of major spore and pollen taxa through the succession, two palynological assemblage zones and four subzones are established within the interval 897.0–629.0 m. The zones are as follows; the Cyathidites – Pinuspollenites zone ( Cyathidites – Deltoidospora – Pinuspollenites subzone, Cyathidites – Osmundacidites – Pinuspollenites – Cycadopites subzone, Cyathidites – Cycadopites subzone, Cyathidites – Pinuspollenites – Cycadopites subzone) and high Cyathidites zone in asce nding order. These zones are assigned to the Middle Jurassic (Aalenian to Bathonian) by comparison with palynological assemblage zones in other areas within China. Two distinct palaeovegetation and palaeoclimate types are reconstructed based on their ecological habits: (1) The vegetation was a mixed lowland fern–upland conifer forest, indicating a warm and humid climate during the deposition of the Yan'an Formation; (2) The vegetation was characterised by a lowland fern forest, still suggesting similar climate, but with reduced humidity during the deposition of the lower Zhiluo Formation. Such climate may be related to global warming, resulting from strong greenhouse effect, monsoon circulation, and volcanic activity.