Biostratigraphic And Paleogeographic Study Of Permian Fusulinids And Shanita Fauna (Foraminifera) From The Baoshan Block, Western Yunnan

Based on fossil collections in recent years, a comprehensive study has been conducted to reveal the faunal composion, biostratigraphic and paleogeographic implications of the Permian fusulinids and Shanita fauna (foraminifera) from the Baoshan Block in western Yunnan, China. Systematic study reveals that the Early Permian fusulinids occur in the top part of the Dingjiazhai Formation of Dongshanpo Section and Aluotian Section, in northern and southern Baoshan Block respectively. These fusulinids comprise 11 species of Eoparafusulina and Pseudofusulina, and is Sakmarian in age. The Mid-Permian fusulinids occur in the Shazipo Formation of Bawei Section and Xiaoxinzhai Section in southern Baoshan Block. In Bawei Section, Mid-Permian fusulinids include 31 species of 11 genera, and could be subdivided into Yangchienia-Nankinella Assemblage Zone and Chusenella-Rugosofusulina Assemblage Zone in ascending order. The age of these two zones could be referred to Murgabian-Midian. In the Xiaoxinzhai Section, contemporary fusulinids contain 36 species of 10 genera and could be subdivided, from bottom to top, into Schwagerina yunnanensis Range Zone, Eopolydiexodina Abundance Zone, Sumatrina annae Range Zone and Verbeekina abundance level. The former two zones are referable to Murgabian in age and the later two to Midian.The Shanita fauna in northern and southern Baoshan Block remarkably differs in faunal composition. The Shanita fauna in Woniusi Section and Hewanjie Section in northern part is rather restricked in stratigraphic range and confined in the basal part of the Hewanjie Formation. Shanita amosi, Sh. intercalaria, Sh. bronnimanni and Hemigordiopsis biconcavus are the most abundant species in these two sections. In contrast, Shanita aff. amosi and Hemigordiopsis renzi are the most abundant in the Shanita fauna from Bawei Section in southern part. Besides, the Hemigordiopsis has a much wider stratigraphic range in Bawei Section. Comparative study suggestes that this fauna mainly occurs in Midian to Dzhulfian (Capitanian to Wuchiapingian) strata in SE Asia. The occurrence of this fauna in the basal part of the Hewanjie Formation indicates that the Late Permian to Middle Triassic hiatus which has been previously supposed probably does not exist.Paleogeographically, the studied Sakmarian fusulinids could be correlated to the Kalaktash fusulinid assemblage in Central Asia, implying peri-Gondwana environment. Thus, these Sakmarian fusulinids probably belong to Gondwanan Province in Tehtyan Realm. The Murgabian to Midian fusulinids in this block yield elements of Superfamily Verbeekinacea and Eopolydiexodina, thus should definitely belong to Western Tethyan Subprovince. From Sakmarian to Midian stage, the change in fusulinid faunal composition signifies the warming-up of the environment. The Sakmarian fusulinids are dominated by just two genera of Family Schwagerinidae, indicating temperate environment; elements of Superfamily Verbeekinacea, which is typical in tropical/subtropical area, first appear in the Murgabian stage, suggesting the cusp of environmental amelioration; while in the Midian age, the Verbeekinacea already predominates over the Schwagerinidae, indicating much warmer water masses.Furthermore, the Murgabian-Midian fusulinids from the Baoshan Block were statistically compared with the coeval fusulinids from other Gondwana-derived blocks as well as South China. Results demonstrate that the fusulinids from Baoshan and Lhasa Blocks are similar in composition and with relatively low generic diversity, which probably signify relatively high latitude of these two blocks. Both Sibumasu (sensu stricto) and Salt Range yield fusulinids with low generic diversity and no Verbeekinacea genera, which may imply cooler environment. Meanwhile, the fusulinid faunas from other Gondwana-derived blocks has higher generic diversity and probably live in much warmer environment. It is especially true for fusulinids from Iran/Transcausia which has comparable generic diversity with South China.In terms of paleobiogeographic affinity, the Murgabian-Midian fusulinids from Baoshan Block, Iran/Trancausia, Turkey, Qiantang Block, Lhasa Block, Pamir and South Afghanistan should belong to Western Tethayn Subprovince, because of the presence of Verbeekinacea and Eopolydiexodina. Moreover, the South Afghanistan, Qiangtang Block, Pamir and Lhasa Block all yield Lepidolina and/or Colania, which is common in Cathaysian Tethyan Subprovince. These four areas, therefore, should communicate to certain degree with Cathaysian Tethyan Subprovince during Midian stage, probably represent a transitional zone between the Western and Cathaysian Tethyan Subprovinces.