Basic characteristics of key interfaces in Upper Ordovician Wufeng Formation - Lower Silurian Longmaxi Formation in Sichuan Basin and its periphery, SW China

WANG Yuman,WANG Hongyan,QIU Zhen,SHEN Junjun,ZHANG Qin,ZHANG Leifu,WANG Canhui,LI Xinjing

Table 1 Boundary types and characteristics of Wufeng Formation-Longmaxi Formation

Boundary types Recorded geologic events Major characteristics Research methods Application effect References Characteristic graptolite belt Climate change and ecological differentiation of graptolite fauna The boundary recognition is determined by the specific graptolite occurrence. It is greatly affected by sedimentary environment, graptolite preservation conditions, cognitive level of researchers on characteristic graptolites and other factors, and the variation of boundary lithofacies is not obvious. It is isochronous and stably distributed in a large area. Determine the first occurring layer of charac- teristic graptolites through detailed outcrop survey or core observation. High degree of research and application. In the infralittoral and deeper water areas, graptolites are rich and highly differentiated, and the application effect is better. Graptolites are rare in nearshore, shallow water and near provenance areas, and the application effect is worse. [1-6, 9, 13-15] Hirnantian glacial shell layer Climate change and mass extinction in Hirnantian glacial period The characteristics of lithofacies, biological assemblage and well logging response are distinct and easy to identify. It is isochronous and stably distributed in vertical and horizontal directions. Determine by detailed outcrop survey or core observation and well logging response characteristics. High degree of research. In the infralittoral and deeper water area, Hirnantian is rich in fossils and has better application effect. In nearshore, very shallow water or paleo- uplift areas, Hirnantian has rare or no fossils, and the application effect is worse. [2-3, 7, 12, 14-15, 18] Specific bentonite Plate movement (Guangxi movement) and volcanic activity It is isochronous, extensively distributed, and easy to identify. It is greatly affected by the scale of volcanic activity, the source of volcanic ash and the cognitive level of researchers. Through the detailed survey of typical outcrop section, determine the bentonite dense section; coupling with the calibration of key wells, provide reference to key areas. Standard sections need to be established in different exploration areas for calibration or reference. It preliminarily meets the requirements of exploration evaluation and geological research. Combined with characteristic graptolite, the effect is better. [16-17, 19-21] Concretion Periodic strong activity of sea basins It is isochronous and easy to identify. The formation and distribution environments are generally adjacent to ancient land (i.e. provenance area), in deep-water area, and in foreland period. It only appears in local areas of a sea basin, with poor distribution stability, large differences in mineral composition and well logging response characteristics, and is only available in local exploration areas. Through detailed outcrop survey, determine lithofacies association and GR responses, and then provide reference to bottom hole. It can act as the boundary of fourth-fifth order sequences in some areas. It is in the stage of exploratory research, and has not yet been applied substantively. [22-24]