Early Cretaceous spore-pollen assemblages from the Shahezi Formation in Well SK-2 and their paleoclimate significance

Well SK-2 is located in the Anda City of Heilongjiang Province, China. A total of 4134.8 m continuous core in Well SK-2 provides a unique chance to study the Lower Cretaceous in the Songliao basin, being an important part in the continental scientific drilling program. Based on the distribution of pollen and spores in the Well SK-2, two assemblages are established for the upper part of the Shahezi Formation. They are named as Leiotriletes sp.−Cyathidites australis−Chasmatosporites sp. assemblage (at the depth of 3832.94~3901.35 m) and Klukisporites triangulus−Aequitriradites sp.−Pristinuspollenites sp. assemblage (at the depth of 3395.46~3493.24 m). The geological age of the Shahezi Formation is assigned to Hauterivian – Early Barremian (Early Cretaceous) according to the characteristics of the assemblages. The reconstructed vegetation changes from coniferous forest and tussock to evergreen broad-leaf forest, to coniferous forest and tussock, and to coniferous and broad-leaved mixed forest during the time interval. The reconstructed climate changes from humid warm temperate to semi-humid south subtropical, to humid warm temperate, and to semi-humid temperate.