Quantitative Characterization of Ichnofabric and its Significance of Indicating Sedimentary Environment: An Example from the Pinghu Formation-Baoshi Formation in Pingbei Area, Xihu Sag

Key words: 
• Key words: ichnofabric /
• quantitative characterization /
• sedimentary environment /
• Xihu Sag

Abstract: Abstract: [Objective] The abundant ichnofossils were developed in the Pinghu Formation and Baoshi Formation of the Eocene in Pingbei area, Xihu Sag. It is of practical significance for the division of sedimentary system in Xihu Sag to study the characteristics and distribution of its ichnofabrics and its indication of sedimentary environment. [Methods] Through core observations, the study identified ichnofossils and ichnofabrics in the Pingbei Area. The IDIPA technique was employed to objectively quantify ichnofossil abundance and accurately determine both the bioturbation index and the ichnofabric index. By integrating these findings with ichnological indices, this study summarizes the characteristics of the planar distribution and vertical evolution of ichnofabric in the study area. [Results and Discussions] (1) The research identified 6 ichnogenera of ichnofossils and divided them into 4 types of ichnofabrics, including Planolites ichnofabric, Thalassinoides ichnofabric, Teichichnus ichnofabric and Beaconites ichnofabric. (2) The study area features a high landform in the northwest and a low landform in the southeast. Seawater invades from south to north, and the sedimentary environment transitions from the subtidal zone to the intertidal zone, and finally to the supratidal zone, where the highest terrain is located in the northwest. The evolution pattern of the corresponding ichnofabrics follows the sequence Thalassinoides → Teichichnus → Planolites → Beaconites, consistent with the results of regional sedimentary zoning and paleoclimate analysis. (3) From the Pinghu to Baoshi Formation in the study area, it was found that the increase in the size, diversity, and abundance of ichnofossils. The tiering transitioned from simple and shallow to complex and deep. The variety of organisms and the degree of bioturbation increased, and the biological habits evolved continuously, indicating that the sedimentary environment changed from from anoxic to oxic. [Conclusions] Based on the characteristics of ichnofossils, ichnofabrics, and sedimentary structures, the response relationship between the ichnofabrics and the sedimentary environment was established in the Pinghu Formation and Baoshi Formation in Pingbei area. The research results provide new evidence for the changes in sedimentary environment from the perspective of ichnology in the Eocene of Xihu Sag.