Study On Lithofacies Paleogeography And Coal Forming Model Of Anyuan Formation In Pingle Depression

This paper is based on the predecessors' study combining with existing geological exploration data and sample test analysis,and is guided by the theories of sedimentology,coal geology and petrology.The petrological characteristics and lithofacices paleogeography of the Late Triassic Anyuan formation in Pingle depression are studied,the sedimentary environment is restored.What's more,the coal controlling factors are analyzed and the coal forming model is summarized.Sedimentological studies indicate that the Anyuan formation is mainly composed of marine and terrestrial facies clastic rocks.The bottom of Zijachong member is alluvial formation.Due to transgression,the middle and upper part of the Zijiachong member and Sanjiachong member is lagoon-tidal flat deposit.Sea water retreated in Sanqiutian member,sedimentary environment turned into shallow water delta.Through the analysis of the ancient types of plants,it is found that the hygrophilous plants and thermophilic plants account for most of the plant community and the percent is about 75%,so the sedimentary climate of the Late Triassic Anyuan formation is a tropical subtropical climate with abundant rainfall in the study area.Through the calculation of the salinity by the B element method,it is found that the paleo salinity ranges from 10.309‰ to 15.281‰ with an average value of about 12.968‰,which represents a brackish water sedimentary environment.The salinity of the water is low in the brackish water range,which has the character of desalination of seawater.Based on the relative sea-level change,ancient climate and plant analysis,tectonic and sedimentary environment,coal controlling factors,two kinds of coal forming patterns are put forward.Zijiachong sedimentary period is the main coal forming period,as the gulf lagoon-tidal flat coal forming pattern under the background of transgression,and peat swamp is the main coal forming environment and coal forming has a wide distribution,big thickness and a single structure;the coal forming pattern of Sanqiutian member is shallow water delta under the background of regressive and coal is mainly produced in delta plain distributary interchannel swamp,and coal forming has a thin thickness and a high ash content.