Lithofacies Paleogeography In Hirnantian Stage Of Late Ordovician In Upper Yangtze Region

Upper Yangtze region experienced two stages on the early Paleozoic: Yangtze craton basin(€-O2)and the back bulge basin(O3-S). In this area, it developed Late Ordovician Kuanyinchiao Formation or Nanzheng Formation, which laid a solid foundation of the research on lithofacies paleogeographic a nd sedimentary response of Hirnantian stage. The global glaciation happened in Hirnantian of the late ordovician, The lithofacies, biologic facies and sedimentary environment of Hirnantian Kuanyinchiao Formation in Upper Yangtze region, were the response to the glacial event.According to sedimentary characteristics, combining paleontologies, Kuanyinchiao Formation in Upper Yangtze region can be divided into shore facies, tidal flat facies, shallow facies, continental shelf facies.Considering lithologies, shelf facies can be further divided into clastic shelf facies, carbonate shelf facies, mixed product shelf facies and deep-water shelf.Shore facies developed in the peripheral area of Chuanzhong uplift, Hannan ancient land, Motianling ancient and Ziyang- Pingli heights. Lithological combination is fine grained quartz sandstone, calcareous sandstone and mud coarse grain in the quartz sandstone. A small amount of biological developed there, giving priority to brachiopod fossil, individual incompletely, and much as the shell fragments, a small amount of Hirnantia fossils developed in north of Yangtze massif.Tidal flat facies developed in the peripheral areas of Q ianzhong uplift and western of Sichuan- Kangdian uplift. Lithology combination is mainly silty limestone, silty argillaceous limestone and calcareous siltstone, develop ing a large number of biological fossils like brachiopods Dalmanella cf. Testudinaria, Eostropheodonta cf. Hirantensis, Rafinesquina sp., etc., also see a small flap gill class Modiolopsis sp., trilobites Dalmanitina sp and crinoid stems, moss, insects, snails, bivalves and calcium, a handful of areas to see Hirnantia fossils lied in the lower part of the guanyinqiao formation. In the peripheral tidal flat phase of Qianzhong uplift, the lithology is given priority to with grain limestone, including shale less sparry calcarenite, bioclastic limestone, oolitic sparry calcarenite, developing complete corals,brachiopods and trilobite fossils.Shallow facies developed in tidal flat facies, it is because of the coastal tidal flat phase, and the local energy higher gradually happened shallow.Shallow facies distribution in the surrounding area such as Fenggang, Yanhe and Renhuai.Lithology combination was mainly grain limestone, including shale less sparry calcarenite, bioclastic limestone, oolitic sparry calcarenite, etc.In biology, seeing complete coral fossils(Sinkiangolasma sp. And Brachyelasma sp.) with a small amount of relatively complete Hirnantia fossils of brachiopods and trilobites(Dalmanitina sp.).Clastic shelf facies developed in Q ianzhong uplift to the north of tidal flat facies Shizhu-Wulong-Tongzi-Weixin-Ganluo, and the southern of Hannan ancient land shore facies to Wangcang-Nanjiang-Wanyuan, lithology combination with manganese iron qualitative mudstone, containing silty mudstone and silty mudstone, argillaceous siltstone, developmental biology sort is more, the Yangtze massif north rim mainly by Hirnantia fossils, brachiopods, snails, and in the southeast of chongqing- suck region of clastic shelf phase, creature more than the former, the major categories for Hector nantes, brachiopods, trilobites.It is worth mentioning that in this region, in addition to the development of Hector nantes period unique biological, bottom and top appeared in group the guanyinqiao Hirnantia and graptolite nantes biota fossils symbiotic phenomenon, reflects the change the continuity of the sedimentary environment.Carbonate shelf facies developed in the Chuanzhong uplift shoreland siltstones phase to the east of Zitong- Yongchuan-Leibo, lithology combination was mainly marl, bioclastic limestone, the development of wrist, Hirnantia, trilobites and graptolite fossils.Mixed shelf facies deposition occurs from south of Dalianghui-Q iaoting to the north of Shizhu-Wulong-Daozhen-Tongzi-Weixin and the east of Zitong-Yongchuan-Leibo, the generalized mixed product shelf facies lithology for muddy limestone and calcareous mudstone and silty mudstone interbedding, marl and calcareous mudstone or calcium shale interbed is given priority to;Narrow mix product shelf facies lithology mainly raw crumbs limestone, argillaceous limestone, containing carbonaceous silty mud limestone, clay, limestone, biological limestone calcareous shale.In biology, biological individual integrity, common brachiopods Triplesia sp.?Fardenia sp.?O xoplecia sp.? Dalmanella sp. Tetrephalerella sp.?Plectothyrella sp., trilobites Dalmanitina nanchengensis Lu? Leonaspis sp.and planktonic life of graptolite and camp C limacograptus sp.?Paraorthograptus sp.?Diplograptus?Orthograptus sp.?Akidograptus sp and so on.Deep-water shelf facies developed in the Nanchuan-Qijiang of southeast of C hongqing, lithology combination is given priority to with carbonaceous mud limestone, carbon content is higher, the more dirty hands, shows the water deeper sedimentary environment, biological combination is relatively single, give priority to with floating graptolite creatures,Hirnantia basic invisible, only a small amount of shell fragments.In general, Upper Yangtze region late ordovician sed imentary environment as a whole the guanyinqiao formation belongs to the coastal environment, shallow water, mostly in situ deposits or short distance transportation, biological prosperous, more for Hirnantia fauna, trilobites, snails, sea lilies, floating pen stone.Because in the study area, whether in the coastal zone, or shallow sea area are growth Hirnantia shellfish fossils, Hirnantia period, due to the effect of southern gondwana glaciers events, ice water invasion into the Yangtze north rim of district.Also because the glacier events lead to sea levels drop, shallow water on the Yangtze region, early deep-water shelf into shallow water shelf, biological also by planktonic phase graptolite rapidly into a shell phase Hirnantia, and further expand the early uplift, and lack of formation.Therefore, K uanyinchiao group sedimentary environment than Wufeng and streams the dragon group black graptolite facies shale anoxic stranded deep-water shelf environment more shallow, relative sea level and water temperature is low, is Hirnantia period glacier events directly affect the results.