Phycosiphon in Daye Formation of Early Triassic in Huaxi Area, Guiyang Morphological structure analysis and its Paleoenvironmental Significance
Abstract Phycosiphon is abundant and well preserved in the middle portion of Daye Formation in Huaxi district of Guiyang in Early Triassic, which provides good materials for studying its ecological behavior and morphological structure. The Phycosiphon burrow system of Daye Formation is composed of "U"-shaped leaf-shaped pascichnia and linear repichnia. Statistical analysis of width, height and edge tube diameter of 267 "U" leaves in 61 samples shows that the ratio of width to height of "U" leaves ranges generally from 50% to 60%, and the ratio of edge tube diameter to leaf height ranges from 12% to 18%. Combined with linear correlation analysis, it is considered that there is a certain relationship between Phycosiphon trace maker size and leaf size; through the analysis on the behavior of Phycosiphon trace makers, it is considered that the concave direction of leaves can indicate the foraging direction of Phycosiphon trace makers. In morphology, "U"-shaped pascichnia and linear repichnia can form 4 combined types: antler- shape, mass-shape, irregular-shape and chain-shape. Antler- shape and mass-shape are mostly formed in front of turbidity flow, indicating low-energy still water environment; irregular-shape is generally formed after turbidity current, indicating water environment with high deposition rate and relatively high energy; However, chain-shape develops in both environments, which reflects the random foraging characteristics of Phycosiphon trace makers. The early Triassic study area is located in the slope of shallow sea peri-platform slope, with frequent turbidity current, and the whole area is in an oxygen-poor and low-energy environment. The development of Phycosiphon in the second member of Daye Formation is one of the characteristics of "anachronistic facies" after P-T event.