内蒙古西乌珠穆沁旗哲斯组宏体化石新发现和沉积相分析

Sedimentary facies analyses and discovery of gastropods from Zhesi Formation in the south of West Ujimqin,Inner Mongolia and their significances

详细的沉积相分析表明内蒙古西乌珠穆沁旗南部哲斯组下部为一套滨浅海相地层,上部逐渐向陆相地层转变,代表向上变浅的沉积序列。在西乌珠穆沁旗西南毛登地区前人发现放射虫的哲斯组相同地点和层位发现了宏体的腹足类螺化石及双壳类化石,环境分析表明这些宏体化石应形成于近岸的潮间带或潮下带水体。对中二叠统哲斯组地层进行了横向对比研究,表明哲斯组在内蒙古中东部地区以浅海相和滨浅海相为主体,而非深海洋盆。

As the representative Middle Permian strata in Inner Mongolia, Zhesi Formation is widely distributed in Inner Mongolia, from Zhesiaobao area to the west, through Sunidzuoqi-Xilinhot-West Ujimqin, extending to Suolun of Wulanhot, which is important to decipher the tectonic setting of western Xing-Meng Orogenic Belt. In order to better understand the sedimentary environment of Inner Mongolia in Middle Permian and further discuss the tectonic setting, detailed sedimentological and paleontological studies were performed to the Zhesi Formation in the West Ujimqin and Maodeng area. The Zhesi Formation from West Ujimqin recorded a section from fine-grained sandstone and limestone at the lower part with abundant of brachiopoda, coral and bryozoon fossils, and conglomerate at the upper part, representing a shallowing upward sequence from shallow marine facies to marine-continental alternating and continental facies. The Zhesi Formation in Maodeng area also recorded a shallowing upward sequence from mudstone nodular bearing black mudstone at the bottom, sandstone in the middle and coarse-grained sandstone on the top. Radiolarian was reported from the bottom black mudstone of this section several years ago, which was thought to represent deep-sea environment and to be the important evidence for the existence of Paleo-Asian Ocean during the Middle Permian. Our new paleontological study identified gastropods and bivalve fossils from the bottom black mudstone, indicating that they were deposited in offshore intertidal zone or subtidal zone. Meanwhile, plant fossils were reported from nearby, indicating proximal deposition. So, both the new fossils we found and the plant fossils reported nearby do not support a deep-sea environment for the Zhesi Formation in Maodeng area. From invoking the worldwide studies of radiolarian, it can be preserved in all kinds of marine deposition, with no direct connection to deep sea environment. Hence, the radiolarian found in Maodeng area and its representing environment should be considered seriously. Detailed sedimentological and paleontological studies have been carried out on Zhesi Formation in several areas of Inner Mongolia. In Zhesiaobao area, fine-grained sandstone and limestone dominated the sequence, with famous Zhesi fauna found therein, indicating shallow marine facies. In Sunidzuoqi area, the Zhesi Formation was mainly composed of alluvial-fluvial conglomerate and sandstone, with some plant fossils. In the West Ujimqin area, the Zhesi Formation displays a transition from transgression systems tract through highstand systems tract to regression systems tract, representing the deposition during shrinkage of the basin, after which the whole area uplifted successively and came into a continental environment. Combining with previous researches and lateral sedimentary environment comparison from Zhesiaobao, Sunidzuoqi, Maodeng and West Ujimqin, shallow marine facies clastic rocks and limestone deposition dominated the Zhesi Formation in most areas, with continental coarse-grained clastic deposition only distributed in Sunidzuoqi area. Furthermore, the Zhesi Formation displays a shallowing upward sequence, with proximal coarse-grained clastic deposition on the top, indicating a transition from shallow marine to epicontinental or continental sediment. Therefore, the Zhesi Formation in Inner Mongolia do not support a deepsea environment, the Paleo-Asian Ocean have been closed before the Middle Permian.