Ordovician integrative stratigraphy, biotas, and paleogeographical evolution of the Qinghai-Tibetan Plateau and its surrounding areas.

The Ordovician rocks on the Qinghai-Tibetan Plateau represent the oldest non-metamorphic strata, and are critical to understanding the history of regional geology and biotic evolution of the entire plateau. Strata of Floian, Darriwilian, Sandbian, Katian and Hirnantian are represented in the plateau with a hiatus of variable duration occurring underneath the basal Ordovician across the area. Five stratigraphical regions, including the Himalaya, Gangdise-Zayu, Qiangtang-Qamdo, Songpan-Garze, and Karakoram-Kunlun-Altun, are differentiated for the Ordovician strata, which are correlated with their equivalents in the Sibumasu, Indochina, Qaidam-Qilian, Tarim-Tianshan, and the Yangtze (western margin) stratigraphical regions. On the Qinghai-Tibetan Plateau, graptolites, conodonts, and cephalopods are the most common and useful fossils for the Ordovician biostratigraphy. The Ordovician biotas of the Qinghai-Tibetan Plateau bear some distinguishable palaeobiogeographical signatures, among which the cephalopods are characterized by the flourishing actinocerids of North China affinity in the Early-Middle Ordovician, and by the thriving lituitids and orthocerids of South China affinity in the Middle-Late Ordovician. Fossil occurrences and their palaeobiogeographical evolution provide critical evidence bearing on the reconstruction of the geological history of the Qinghai-Tibetan Plateau and surrounding terranes in northeastern peri-Gondwana. The stratigraphical successions of the Cambrian-Ordovician transition in the Himalaya and Lhasa and nearby Sibumasu terranes were significantly affected by the Kurgiakh Orogeny, which resulted in the extensive unconformity between the Ordovician and the underlying rocks in most areas of the Qinghai-Tibetan Plateau. In southern Xizang, a warm-water biota of Middle Ordovician age was recovered from oolitic limestones, suggesting a likely palaeogeographical location in low-latitudes near the equator. In the Himalaya and Sibumasu regions, the Upper Ordovician was typified by the occurrence of red carbonates with distinctive reticulate structures, which are correlative to their equivalents in the Yangtze region of South China, and might be deposited under similar geological conditions. The global end-Ordovician glaciation and sea-level drop likely caused the wide absence of late Katian strata in western Yunnan of China and the Shan State of Myanma, and may have also affected deposition in the Xainza and Nyalam areas of the Qinghai-Tibetan Plateau during this time interval. [ABSTRACT FROM AUTHOR]