一、引言早二叠世及晚二叠世早期,我国天山南、北山、内蒙、东北大部分及蒙古人民共和国南部为一连通的古海槽—“中国—蒙古海”(或称“中蒙海”)。该海槽曾与古北极海和特提斯海相互贯通。海域内广泛地沉积了碳酸盐岩、陆源碎屑岩和火山岩地层,并富产独特的古北极海—特提斯海型混生动物群(中蒙海动物群)。北山区居于中蒙海的中部,其范围是河西走廊以北,中蒙边界以南,甘新省界以 I. INTRODUCTION In the early Permian and early Late Permian times, the southern part of China’s Tianshan Mountains, northern mountains, Inner Mongolia and northeast China and the southern part of the People’s Republic of China are connected with the ancient sea trough “China-Mongolia Sea” (or “ ”). The trough interlinked with the ancient Arctic Sea and the Tethys Sea. Carbonates, terrigenous clastic rocks and volcanic rocks are widely deposited in the sea area, and the unique ancient Arctic Sea-Tethys sea-type mixed fauna (Central Mongolia Sea fauna) is rich. Beishan District is located in the middle of the Central Mongolia Sea, its scope is north of Hexi Corridor, south of China-Mongolia border, Gansu Province