During
    the Late Cretaceous, a lake basin developed in the Zaysan Basin of eastern
    Kazakstan and has been there till the present. The oldest deposits of
    paleo-Lake Zaysan are the Tayzhuzgen Formation, as much as 136 m of red-bed
    mudstone, siltstone, shale, quartzose sandstone, conglomerate and freshwater
    limestone. The Cretaceous-Paleogene boundary in the Zaysan Basin is located
    stratigraphically very low in the sedimentary section deposited by paleo-Lake
    Zaysan, and it is not a complete section of the boundary, but likely one marked
    by one or more unconformities and/or a condensed section produced by slow
    sedimentation.