Carbonate Microfacies And Foraminiferal Morphogroup Study On Guadalupian-Lopingian Boundary In Eastern Sichuan

Guadalupian-Lopingian Boundary(the GLB event)in the Permian be esteemed as the first stage of the end-Permian mass extinction.Many fusulinids and shallow water creatures became extinction in the GLB event.After the event,the main benthic foraminiferal feeding modes,living locations and other ecological habits have changed in the Wujiaping Stage.As a part of the Yangtze platform,the Sichuan Basin developed relatively complete Permian marine carbonate rocks,which is an ideal place to study this boundary event.In this paper,benthic foraminifera in carbonate rocks of Middle Permian Maokou Formation and Upper Permian Wujiaping Formation in two fields(Erya section of Huaying,Guangan,Sichuan Province and Huilongchang section in Fengdu,Chongqing)in eastern Sichuan Basin are taken as the main research objects.Based on the analysis of carbonate microfacies and morphogroup,reconstructed the sedimentary environments from Guadalupian to Lopingian,and recovered the palaeo-ocean ecological information.To discuss the relationship between the evolution of palaeo-ocean environment near the Guadalupian-Lopingian boundary and marine benthic organisms.We recognize seven types of LBF microfacies based on the rock characteristics and benthic foraminifera combinations of the two main sections,and associate them with six sedimentary microfacies(Coastal plain,Bioclastic shoal,Open marine,Grain shoal,Dolomitized shoal,Marginal shallow shoal),four sedimentary subfacies(Coastal plain,Open marine,Grain shoal,Marginal shoal),three sedimentary facies(Coastal plain,Open platform,Marginal platform).In addition,seven morphogroups and five sub-morphogroups were classified according to the benthic foraminiferal information of the number of chambers,feeding strategies and living positions.According to the study of carbonate microfacies and morphogroup analysis,the water depth in the study area of the Maokou Stage gradually became shallower.The early Maokou Stage was mainly deposited in an open marine,morphogroup F was common.Grain shoal was developed in the middle Maokou Stage,morphogroup F was the main type of grain shoal,and other morphogroup A,B,C,D,G were visible.And bioclastic and marginal shoal was developed in the late Maokou Stage,morphogroup G appeared in large numbers.Then the crust risen and evolved into coastal plain.During the Wujiaping Stage,the sea invaded and water depth increased,and the open marine and grain shoal were mainly developed,which were characterized by a high proportion of morphogroup F,and the other morphogroup were rare.After the GLB event,the sedimentary environment from the Maokou Formation to the Wujiaping Stage changed little,but the ecological scene of benthic foraminifera changed significantly.With nutrient inputs increased,plant debris and the proportion of grazing herbivores decreased,and the proportion of suspension-feeders and deposit-feeders was higher,which proved the objective existence of the GLB event.Survivors,such as Codonofusiella,Eotuberitiina,prefer an attachment life style,require less oxygen,have a variety of feeding methods,and can adapt to a variety of living environments in the Wujiaping Stage.Fusulinid survived in the GLB event have increased bottom attachment surface area to resist wave,and extended semi-shells to enhance their feeding capacity,so they can adapt to the post-disaster environment.