The Burial Environment of Nothosaurs in the Middle Triassic Pod-uan Formation,Guojiawan,Southwest of Guizhou: Implications from carbonate lithofacies and carbon and oxygen isotopes

Key words: 
• lithofacies analysis /
• carbon and oxygen isotopes /
• Anisian Stage /
• earthquake /
• Guojiawan section

Abstract: The burial paleoenvironment of Nothosaurs during the Anisian Stage of the Poduan Formation,Guojiawan area,southwest of Guizhou province,is chacterized in detail. Analysis of carbon and oxygen isotopes in the lithofacies showed that the Poduan Formation consisted of an open platform and a platform-edge reef. The reef edge slope subfacies comprises five sedimentary microfacies types. Whole rock isotope analysis revealed a δ¹⁸O_(PDB) value of -6.28‰ to -2.11‰,averaging 4.36‰;the δ¹³C_(PDB) was +0.50‰ to +2.86‰,at an average of 1.58‰. Both values are consistent with the carbon and oxygen isotopic composition in southwestern Guizhou province and adjacent region during the Anisian Stage. The study showed that the ancient seawater temperature was 16℃ to 20℃,and the salinity was between 125.77 and 131.62,indicating a normal warm marine environment. The slight negative deviation of the δ¹³C_(PDB) and δ¹⁸O_(PDB) was caused by the mingling of fresh water with the seawater. Based on a comprehensive analysis,the following burial process of the Nothosaur is proposed. Earthquakes led to local instability of the carbonate platform,causing a debris flow. This is evident from the large number of Nothosaur bones deposited on the reef edge slope,ultimately forming the present-day fossil bed. This study provides more information for the analysis of Triassic biota in southwestern Guizhou province and adjacent areas.