Study Of Paleobiological Characteristics And Sedimentary Environment Of The Shiniulan Formation In Dejiang Area,Guizhou Province

The Lower Silurian stone cows are widely distributed,with unique rocky sequence and a large number of brachiopods,gastropods,corals,cephalopods,and crinoid fossils,which can be compared and divided in a wide range of strata.It also provides good geological data and evidence for understanding the sedimentary environment of the Lower Silurian and the ancient rock features.Based on the regional geological survey project and the previous geological work report and literature data,this paper focuses on the comparative study of the chronological stratigraphy,biostratigraphy and rock strata in the Dejiang-Sinan area and the Shiniulan Formation in each area.The classification,the determination of rock-rock properties,and the revealing of the rock properties,paleontological characteristics,carbon-oxygen isotope characteristics and sedimentary environment characteristics of the Shiniulan Formation have obtained the following points:1)The Shiniulan Formation is widely distributed in the study area,mainly distributed in the northwest part of the study area and in the eastern and southern areas.The overall thickness variation is small,except for the Guandongdong village in the northeastern Wenping town,which is 87.4m.It is 127.7-169.4m,with an average of about 130m,and the overall exposure is better.Integral contact with the overlying Majiaochong Formation,the disappearance of limestone,honeycomb coral and tubular coral layer in the field,and the appearance and lowering of yellow-grey-green-medium-thick silty mudstone and thin layer of mudstone Shiniulan Formation component boundary.The disappearance of the gray thin-layered argillaceous limestone with the grayish-grey thin layer of calcareous shale and the gray-blue-gray thick layer of limestone,gray-medium-thick layer bioclastic limestone.The appearance of gray medium-thick layered-like lenticular limestone is a two-component boundary mark,and a steep ridge is often formed on the topography,often starting from the boundary of the stone at the bottom of the steep ridge.In the northwestern and central parts of the study area,the main lithology of the lower part of the Shiniulan Formation is gray-yellow thin shale and gray-white thin layer of argillaceous limestone interbed,gray-dark gray mid-stratified nodular limestone,gray-dark gray thin-The middle layer contains argillaceous limestone,gray thin layer of calcareous shale and yellow silty mudstone interbed,gray-yellow thin shale ash green silty shale;the main lithology of the lower part of the north eastern,eastern and southern parts is Light gray-grey medium-thick layer argillaceous limestone,light gray-grey medium-thick layer-blocky mud-microcrystalline limestone,light gray-grey middle layer containing bioclastic microcrystalline limestone,light gray-gray-Thick layers containing bioclastic sandstone.The main lithology of the upper middle part is gray thick layer fine-grained shell limestone,light gray-gray middle layer containing silty sand containing bioclastic micro-mud limestone,gray thin-thick layer containing bioclastic mud-microcrystalline limestone,gray middle micro-muddy bioclastic limestone,gray thick layer-blocky muddy sand fine-microcrystalline limestone;in the central and northeast parts,see gray thin-middle micro-fine crystal bio-crush Dust limestone,gray-green thin mudstone,gray-green shale,gray-gray-green argillaceous siltstone.2)The paleontological fossils found in the section of the Shiniulan Formation in the study area are Borealis sp.,Borealis rhomboidalis Rong,Xu et Yang,Pentamerus sp.,Pentamerus borealis?Eichwald?,Nucleospira,Eospirifer?plicatus Rong Xu Et Yang,Fardenia?lauta Rong Xu et Yang,Isorthis sp.Coral Brachylasma sp.,B.zhenganense He et Huang,Favosites sp.,Catennipora sp.,Halysites sp.,Amplexoides sp.Cephalopods?Eushantungoceras.Gastropods Raphistomina sp.,Homotoma sp.,Umbonellina globulosa Yu,Bellerophon sp.Sea lily Petalocrinus sp.,Cyclocyclicus sp.3)Based on the study of regional paleontological collection and identification,combined with the previous data research,a total of 9 biostratigraphic units were identified in the study area.There are 5 brachiopod biostratigraphy,Borealis-Kritorhynchia combination belt,Paraconchidium-Virgianella combination belt,Pentamerus dorploplanus-Clintonella kneichouensis combination belt,Nucleospira-Nalivikinia combination belt,Salopina minuta-Spinochonetes notata combination belt.The coral biostratigraphic unit two,Brachylasma sibiricum-Cystiphyllum siluriense combination belt,Codonophyllum-Maikottia combination belt.The nautilus biostratigraphy,Sichuanoceras-Neosichuanoceras combination belt.The sea lily biostratigraphic unit,Spirocrinus-Petalocrinus combination belt.4)Combined with the ¹³C and ¹⁸O isotope chemical identification values of the Shiniulan Formation in the study area,the calculation and calibration were carried out to obtain the sampling layer paleo-salt value?Z?and paleotemperature quantification?t?.The analysis shows that the paleo-salt value?Z?.The value ranged from 121.81 to132.92 and the average value was 128.61,both of which were greater than 120,indicating that the section of the Shiniulan Formation is a marine sedimentary environment with higher salinity.The paleotemperature quantification?t?is 21.10-28.43?,and the average value is 24.04?,reflecting the dry heat or warm climatic conditions of the section deposition period.5)The sequence facies paleogeographic map of the Shiniulan Formation in the study area based on the third-order sequence system domain is drawn,and the main two sedimentary facies of the Shiniulan Formation in the study area are highlighted.In the upper part of the Shiniulan Formation,the bottom of the carbonate platform and the bottom shelf of the Shiniulan Formation,and the subfacies of the carbonate platform are identified as the subfacies of the open platform subfacies and the platform marginal slope.The subfacies of the continental shelf are shallow water continental shelves.Subphase and subsurface of the mixed continental shelf.The microfacies of the open platform subfacies are shoal microfacies,intertidal microfacies,and bioclastic shoal microfacies.The microfacies of the subfacies of the platform margin are the reef microfacies,the upper slope microfacies,and the platform edge groove microfacies.The microfacies of the lower slope microfacies and the meristor shoal microfacies,the microfacies of the shallow water continental subfacies are shallow water gray continental shelf microfacies,and the microfacies of the subcontinent of the mixed continental shelf are gray matter-mud mixed continental shelf microfacies.6)The Shiniulan Formation in the Dejiang-Sinan area of Guizhou is mainly the evolution model of the continental shelf-platform margin-carbonate platform.The paleogeographic evolution is mainly affected by sedimentary topography and is less affected by sea level changes.The high water level in the eastern part of the study area is low,the low water level in the western terrain is high,and the sea water is gradually shallower from west to east.The depth of sea water is slightly shallower from early to late in the stratigraphic deposition with small fluctuations.