Trace Fossils And Their Paleoenvironmental Significance Of The Middle Triassic Guanling Formation, Southwest Of Guizhou Province

As the most serious biological extinction event since Phanerozoic Eon,the mass extinction event at the turn of Permian-Triassic directly disintegrated the Paleozoic marine ecosystem.After a long recovery period in the Early Triassic,a brand-new marine ecosystem with active benthic,endophytic and carnivorous animals prospering together began to be established in the Middle Triassic,which marked a critical period for the comprehensive reconstruction of the Mesozoic and Cenozoic marine ecosystems.During the Early Triassic to the early Middle Triassic,due to the anomaly of marine environment,there are relatively few solid fossil records.During this period,trace fossils developed in South China,which is an important material for studying the paleomarine environment in the early Mesozoic.In this paper,the trace fossils of Guanling Formation of Middle Triassic in Southwest Guizhou were taken as the research object,and the benthic ecosystem and sedimentary environment in the early Middle Triassic were studied through the analysis of trace science,sedimentology and geochemistry.(1)According to the observation of outcrop profile in southwest Guizhou,Guanling Formation is mainly composed of a set of shallow sea and saline sea limestone,dolomite and a small amount of clay rocks,with horizontal bedding,core stone,bottom formwork structure,ripple structure,bird’s eye structure and mud crack structure.The sedimentary microfacies of Yongningzhen and Dingxiao sections are analyzed emphatically,and 10 microfacies are mainly developed: bioclastic marl,limestone,spherical-like marl,bioclastic micritic limestone,micritic limestone with bird’s eye structure,bioclastic gravel mudstone,banded limestone,aggregated granular,spherical-like mudstone and bioclastic algae mass micritic limestone.(2)The grain size of strawberry-like pyrite in the producing horizon of trace fossils is analyzed,and it can be divided into six forms: standard pyrite,aggregate,autogenous crystal,flaky body,banded and recrystallization.According to the statistical results of particle size,using the maximum particle size method,average particle size method and binary graphic analysis method,it is considered that the Guanling Formation of Middle Triassic in Southwest Guizhou is an oxidation-suboxidation sedimentary environment.(3)Twelve genera and 17 species of trace fossils were systematically described:Arenicolites ichnosp,Bergauria perata,Bifungites fezzanensis,Cruziana furcifera,Didymaulichnus rouaulti,Planolites montanus,Megagrapton tibeticum,Helminthoida hieroglyphica,Palaeophycus tubularis,Palaeophycus heberti,Psammichnites sintanensis,Planolites beverleyensis,Rhizocorallium irregulare,Rhizocorallium ganxiensis,Rhizocorallium kueichouensis,Rhizocorallium jenense,Thalassinoides xizangensis.There are only 3 relic genera in the first member of Guanling Formation in Yongningzhen,and 8relic genera in the second member.There are 7 trace genera in Dingxiaozhen section.On the whole,compared with the first stage,the structure of the second stage trace fossils changed from simple to complex,and the trace differentiation also increased,which indicated that the environmental pressure of the second stage was improved.(4)Combined with the study of trace fossils and sedimentary microfacies,the first member of Guanling Formation in southwestern Guizhou is a limited platform deposit,and the second member is an open platform-biological shoal deposit.The improvement of shallow sea environment in Middle Triassic Anisian period promoted the recovery of marine life and provided favorable conditions for the comprehensive reconstruction of marine ecosystem.