北祁连东段石炭纪岩相古地理基本特征

Characteristics of Carboniferous Lithofacies and Paleogeography in Eastern Part of North Qilian

本区早石炭世，秦岭海域海水以天水－静宁古海峡为通道侵入本区，向北、西超覆，形成“”型海湾环境，沉积了以海湾－泻湖微相及潮坪相为主夹少量碳酸盐台地相含煤岩系。晚石炭世，海侵进一步扩大，红土洼期－羊虎沟期为“”字型海湾；太原期，北祁连海与华北、华南及天山海域连成一片，沉积了滩相、潮坪相及三角洲相为主的含煤岩系。沉积作用向银川以北、武威以西超覆。

The studied area is located in the eastern part of the North Qilian Caledonian foldbelt where the Carboniferous system with rich paleontological fossils develops perfectly and outcrops well. and the studies of the biostratigraphy have a long history.The two-fold division scheme for Carbonifcrous is adopted in this paper, i. e., the Lower Carboniferous Series is composed of Qianheishan, Chouniukou and Tsingyuan Format ions; and Upper one consists of Hongtuwa, Yanghukou and Taiyuan Formations.The Qianheishan formation is equal to Tournaisian, Chouniukou to Visean, Tsingyuan and Hongtuwa to Namurian, Yanghukou and Taiyuan to Westphalian and Stephanian respectively. The evolution of the Carboniferous lithofacies and paleogography is described as follows: 1. This area was an epicontinental sea characterized by gulf. To the north was old Alaxa land; to the South Longxi uplift of Ordos.2. During the Qianheishan-Tsinsyuan period. the marine water invaded the area from Qinling through the Tianshui-Jingning strait, making it a gulf in the shape of ; in the Hongtuwa--Westphalian, the invasion of this area became progressive overlap regulary. resulting in developing a '-shaped' gulf. In the Stephanian, the sea water expanded from North China into North Qilian, and conjoined with South China and Tianshan seas respectively.3. The coal-bearing alternative marine-continental facies of barrier coast consists mainly of elastic rocks and the region is composed of the plain facies zones of delta and seashore.The Lower Carboniferous contains gypsum. Although the fauna realm of the Visean, Namurian and Westphalian is similar to that of Euramerian, the sedimentary association is of the Qilian type. Taiyuan Formation belongs to the North China type characterized by the Early Cathaysian flora.4. Based on both the paleomagnetism and the paleoecology, this area was situated in the low latitudal zone in the Carboniferous period. It changed from the dry-hot in Tournaisian into damp-hot climate in the rest of the Carboniferous period.The Stdphanian stage is a main coal-forming period in this area.This area, the northern margin of Gansu-Qinghai-Tibet plate, acted as an ocean during the Early Paleozoic Era.The ancient oceanic crust should be subducted beneath the Center Qilian. and then, the collision between the Alaxa-Tarim and Caidam took place in the Calcdonian. finally, resulting in the forming of the Hexi-Corridor transitional zone and the Caledonian foldbelt of the North Qilian.After that,the historical geology of Carboniferous in this area began to develop in the continental crust.