Pyrite morphology in the first member of the Late Cretaceous Qingshankou Formation, Songliao Basin, Northeast China

► Different pyrite forms were found in mudstones of K2qn1 in the Songliao basin. ► The framboid size distribution indicates lake anoxic events during K2qn1. ► During K2qn1 the anoxic lake event continuing for about 196 ka. ► The different S:Fe ratios may represent different stages of pyrite formation.