The Characterization And Forming Mechanism Of Trace Fossils In The Third Member Of Majigou Formation,Ordovician,Northwest Henan Province

Amount of ichnofossils occur in the 3^rd member of Majiagou formation.In order to study the modification effect and the way that how bioturbation works,field observation,polar-scope?PL?,cathodeluminescence scope?CL?,X-ray diffraction?XRD?,plunger test of porosity and permeability,and carbon and oxygen isotope test are conducted.Results show that:?1?Thalassinoides ichnofabric,Helminthopsis ichnofabric and Chondrites ichnofabric occur in the 3^rd member of Majiagou formation;In general,the connectivity of Thalassinoides ichnofabric,both in vertical and horizontal,is well;The horizontal connectivity of Helminthopsis ichnofabric is pretty well but the vertical connectivity is worse.And with the increase of BI,the horizontal connectivity is getting well;The horizontal connectivity of Chondrites ichnofabric is well but the vertical connectivity is pretty low;?2?Burrow-related dolomitization presents multi-stages and multi-types diagenetic characteristics.Two models of dolomitization were built including seapage reflux dolomitization,and burial dolomitization,which occurred in burial stage.Based on the morphology,morphology evolution of dolomite,tectonic evolution and burial history,three diagenetic stages have been divided.The first diagenetic stage occurred in contemporaneous-penecomtemporaneous-shallow burial diagenetic environment.During the early dolomitization,only micritic dolomite was formed.The micritic dolomite acted as the center of later replacement of dolomitization.The second diagenetic stage can be subdivided into A and B sub-stage.During the A sub-stage,due to the replacement around the micritic dolomite crystal nucleus,euhedral and subhedral dolomite with a planar surface was formed.During B sub-stage,the continuous replacements lead to anhedral mosaic dolomite.The third diagenetic stage?late dolomitization?mainly occurred in the epigenetic environment.And during the later uplifting,especially near the earth surface,lime-mud in high porosity zone of burrow fillings and micro-cracks could be corroded and then filled with later precipitated sparry calcite.?3?The evolution process of bioturbation induced pores are consisted of?1?Burrowing of trace makers,this process,also increases the porosity and permeability of contemporaneous-penecomtemporaneous sediments,provides a basement for the follow carbonate sediments and carbonate diagenesis;?2?Diagenesis which occur in contemporaneous-penecomtemporaneous period.These diagenesises contribute to the fill of carbonate mud in burrows,and due to the effect of CO₂,some if the carbonate mud will be dissoluted then increase the porosity and permeability of sediments;?3?Shallow burial diagenesis:carbonate mud are transformed to carbonate rock and the burrow infills are dolomited forming euhedral and subhedral dolomite crystal then increasing the rock property.?4?Middle-deep burial diagenesis which include burial dolomitization,burial dissolution and cementation;In general,middle-deep diagenesis could decrease rock property.?5?Epidiagenesis which mainly consists of cementation and dissolution;Dissolution are characterized by burrow selective dissolution and cementation are characterized by recrystallization of calcite in fractures and dissolution crevice;In general,epodiagenesis will increase the rock properties;?4?Both porosity and permeability of plungers which are ichnofabrics bearing are higher than those without ichnofabrics;The average porosity of ichnofabrics bearing is 0.97%and the permeability is 8.2155×10^-3md;Porosity of ichnofabrics bearing is twice over matrix and permeability is about 27 time than matrix;With the increase of BI,both porosity and permeability will increase first the decrease and when the BI is about 3 or 4 the maximum will arise;For which may result from over-dolomitization in burrows;The modification of Thalassinoides ichnofabric is the highest which result from the extending area both in vertical and horizontal;Helminthopsis ichnofabric will locally increase rock property because of the limitation of vertical extending area;When the BI is about 3 the property of Helminthopsis ichnofabric will come to the maximum;The modification of Chondrites ichnofabric is the worst because of the limitation of vertical and horizontal extending area but it can increase rock property locally.