Sedimentary Characteristics,Paleogeography And Paleomarine Environment Of The Neoproterozoic Nantuo Formation In Shennongjia Area(South China)

Neoproterozoic Cryogenian(?720-635 Ma)is known for its global glaciation events,which are significantly different from the Phanerozoic glaciations.Neoproterozoic global glaciation events are mainly characterized by the large scale of glaciation,the extension of glaciation to middle and low latitudes,and the rapid climate change in a short period.Moreover,these global glaciations were accompanied by tectonic and environmental events such as the breakup of the Rodinian supercontinent and Neoproterozoic Oxidation Event(NOE).This period witnessed the emergence of metazoans and the ealiest stage of the complex eukaryotic radiation,therefore,the dramatic fluctuations of climate may have had a potential impact on the evolutionary events of life.Thus,the study of paleoclimate,paleo-environment and paleontology of the Neoproterozoic Cryogenian strata is important for understanding the extreme climatic environment and survival mechanism of organisms in this period.Recent advances have provided new insights into the development scale of Cryogenian glaciations,the cyclicity and the isochronicity of sedimentary records,which have raised new concerns and thoughts on Cryogenian glaciation research.The Nantuo Formation of the Nanhua System in the Yangtze region(South China)is a widely distributed and stable Cryogenian strata(named as Nanhua system in China).Recent years,Chinese scientists have carried out great sedimentological work on the Nantuo strata in the Yangtze region,and many important understandings and advances have been made.However,the poor of research on the Nanhua Systen in the Shengnongjia area at the northern margin of Yangtze block,as well as the relationship between Cryogenian biology and environment have prevented a full understanding of the development and impact of the global glacial events in South China.The Cryogenian Nantuo Formation in Shennongjia area is characterized by well-exposed stratigraphic sequences,diverse rock types and abundant sedimentary phenomena.The macroalgae fossils in the Nantuo Formation of Shennongjia are the key evidence for the existence of the"oases of life"during the global glaciations.This is the only macrofossil record reported so far in the Cryogenian over the world.Therefore,it is an ideal place to study the environmental and biological events of the period.Based on the basic geological works and samples collected in the field,combined with the application of conventional and new techniques in the laboratory,the sedimenatry environments of the Nantuo Formation in Shennongjia area have been achieved.Through detailed sedimentological work,the specific sedimentary environment and sediment transport mechanism for the Nantuo Formation were identified.Detrital and magmatic zircon isotope-dating was used to carry out provenance analysis,and glacier flow direction was identified by comparing with surrounding geological bodies.Finally,the dynamic evolution process of the Nantuo glaciation was analyzed based on the above data,and the deposition time of the Nantuo Formation was defined by using zircon geochronology.In addition,the element geochemical analysis of the fine clastic interlayer in the Nantuo Formation was carried out in combination with iron species and redox sensitive elements and other proxies.We further discussed the relationship between the oceanic redox conditions and the biological refuge under the background of Cryogenian global ice age.Our major findings and conclusions can be summaried as below:1.Based on the detailed field geological survey and section measurements in Shennongjia area,we found that the Nanhua System stratigraphic sequence in Shennongjia area is very different from east part to west part.In the eastern Shennongjia area near the E-zhong paleocontinent,only the Nantuo Formation developed in the Nanhua system,which overlies Mesoproterozoic Shennongjia Group by an angular unconformity,The Nantuo Formation in the central Shennongjia Muyu area also directly overlies the Shennongjia Group.In the Xiaojiuhu-Gaoqiaohe-Xiaguping area(i.e.,the western area of Shennongjia area),the Nanhua system is completely exposed,including the Gucheng Formation,the Datangpo Formation and the Nantuo Formation from bottom to top of the stratigraphic sequence,The"Songluo Biota"-bearing strata were traced in detail,and it was found that it was only distributed within a 2 km~2 area of the Songluo section,and the shale strata of the"Songluo Biota"was significantly different from the manganese-rich black shale of the Datangpo Formation in western Shennongjia area,so we propose that these strata belong to the Nantuo Formation rather than the interglacial Datangpo Formation.2.Based on detailed glacial sedimentology study of the Nantuo Formation in eastern Shennongjia area,five major rock types were identified,representing different glacial sedimentary environments:(1)Massive diamictite(nearshore glacial sea gravity flow facies or subglacial facies),(2)conglomerate(nearshore glacial sea gravity flow facies),(3)dropstone-bearing sandstone(distal glacial raft facies),(4)siltstone and(5)black shale(normal open Marine facies).The sequence from massive diamictite to siltstone or black shale represents a complete glaciation sequence.In the thickest and relatively complete Songluo section(?300 m),three major glaciation sequences were identified.Based on the analyses of sedimentary facies distribution and stratum thickness,the palaeogeographic pattern of the Shengnongjia area is established,which features topographical high in the east and low in the west of Shennongjia and accprdingly we inferred that the Nantuo glacial flow advanced from the east to the west of the study area.The rapid phase transformation and thickness change of the upper Nantuo formation in the basin are attributed to the paleo-topographic difference caused by the erosion of the underlying strata by the ice sheet during the glacial maximum and the dynamic fluctuation of the glacier fronts.The macroalgae-bearing black shale indicates the normal marine environment.Due to the small sediment supply and the weak effect of glaciers,the water environment was transparent and clear,providing favorable conditions for the survival of the macroalgae.At the same time,local depressions in the shallow sea environment provided favorable conditions for the preservation of large algae fossils of the Songluo Biota.3.We carried out systematic detrital zircon U-Pb and granitic gravel geochronology for the Nantuo Formation in the eastern Shennongjia area,and compared it with the former zircon U-Pb geochronology of the surrounding geological bodies of the same age.The results show that the Nantuo glaciation is in the time range of?655-630 Ma,which is consistent with the geochronology reported for other areas of South China.The detrital zircon ages of the Nantuo Formation are different in time and space.Detrital materials from the lower part of the Nantuo Formation in the Songluo section are mainly derived from the underlying Shennongjia Group,and thus mainly from the near source.The Nantuo sequence in the upper part of the Songluo section has a similar provenance to the Nantuo sequence in other sections,and its age is dominated by the Neoproterozoic zircon age,which is consistent with those ages obtained from the rest of the Yangtze Block.Multi-dimensional analysis of detrital zircon ages shows that the detrital zircons of the Nantuo Formation in Shennongjia were mainly derived from the Kongling terrane,secondarily by the Wudang terrane,suggesting that there were different ice sheet supplies,and that the Nnatuo glaciers may have originated from multiple highland glaciers.4.The black shale containing“Songluo Biota”at Songluo and the mudstone interlayer of the Nantuo Formation at Qinglinkou section in the Three Gorges area were selected for geochemical comparison study.The data of iron species and trace elements in the Nantuo black shale interlayer at Songluo show that the Songluo area was in an oxic-to dyoxic bottom waters,showing high primary productivity and normal seawater salinity.The interbedded mudstone of the Nantuo Formation at Qinglinkou are characterized by oxic bottome waters,low productivity and fresh-salt water environment.The characteristics of major-trace elements show that the source rock of black shale at Songluo is mainly granite,whreas the source rock of the Qinglinkou section is mainly intermediate-acid igneous rock and basite,which can be well compared with the surrounding geological bodies,and all of these source rocks should have had moderate intensity chemical weathering.The normal marine salinity levels for the Songluo Biota indicate that the glaciers were far from sedimentary basins,and this normal salinity and oxic-dyoxic environment provide favorable conditions for the survival of macroalgae.In contrast,the low salinity at Qinglinkou indicates that glacial meltwater was injected in large quantities,although it was located in an oxic shallow sea environment.It is not conducive to biological survival because Qinglinkou section was located near the glacier and has not yet escaped from the glacial deposition system,thus showing extremely low productivity.