EARLY SILURIAN GRAPTOLITE ZONES IN THE NORTHERN QILIAN MOUNTAINS

The Early Silurian strata rich in graptolite fossils are well developed in the Northern Qilian Mountains area. The Angzanggou-Hanxia section is one of the standard Silurian sections in northwestern China. Based upon the "Geological Bulletin of the Qilian Mountains", this paper synthesizes and summarizes the data obtained through twenty years of regional geological surveys and studies on special subjects. In the paper, the Early Silurian graptolite zones in the Northern Qilian Mountains are combined into twelve zones, which can be correlated with the graptolite zones of the Qinling area, the standard graptolite zones of China and the standard graptolite zones of Britain. They reflect the current vertical relationships between the graptolite zones in the area. The appearance and localities of the fossil assemblages in various graptolite zones have also been introduced briefly. The first Early Silurian graptolite zone in the area is the Glyptograptus persculptus zone. The strata containing this zone lies conformably on the Upper Ordovician. The highest graptolite zone is the Oktavites spiralis zone, and no graptolite fossils have been found above the strata containing the O. spiralis zone, which show a comformable relationship with Middle Silurian strata of shelly facies.