The Ordovician Palaeogeography of Eastern Asia

Abstract

Following introductory notes (1), the Ordovician formations in the Hwangho basin and Okcheon trough (2-3), the Ordovician trilobite, cephalopod and graptolite facies of the Yangtze basin and the Tsinling axis (4), the Lungshan group in Kwantung and Kwansi and the Ordovician formations in Indochina and the Yunnan-Malayan geosyncline (4-6) are successively described. The distribution of Ordovician cephalopod faunas and the areal variation of average thickness of the Ordovician system in Eastern Asia are shown in Figures 1-3. As in the Cambrian period, the Tienshan and Himalayan geosynclines were the main routes of migration through Eurasia (7).Finally, it is concluded that the Tomkolian and Wanwanian series designate the base of the Ordovician system in Eastern Asia. Like the Ozarkian system the Cambrovian pulsation cannot be maintained in the Palaeozoic stratigraphy of Eastern Asia. Judging from the variation of facies and thickness, the differential movements among tectonic elements in Eastern Asia became reduced in strength from Sinian to Korean (Chosen) and from Cambrian to Ordovician (8).In view of the fact that the Chosen group with the Toufangian series at the top is generally overlain by the Heian (Pyeongan) group para-unconformably in the grand Hwangho basin, the epi-Toufangian Middle Palaeozoic interval was the most stable land period for the part of Asia where little influence was recognizable either of the Caledonian or Variscan orogeny. The Kwansi disturbance or the late Silurian orogeny has, however, occurred in the Tsinling range in Central China and in the Nanling range in South China, although the Triassic Akiyoshi orogeny was stronger in many places of China, Korea and Southeast Asia.