Sedimentary Environment Analysis Of Neoproterozoic Luoquan Formation In Western Henan Province

The characteristics and distribution of the global Neoproterozoic glacial events are hot issues in the Precambrian geological research.The Luoquan Formation of the Neoproterozoic,in the Southern margin of North China,is a special set of conglomerate layers,the genesis of which is important to study the Neoproterozoic glacial events.Based on the four field survey profiles,the sedimentary characteristics and sedimentary facies of the Luoquan Formation in the western Henan Province were analyzed by sedimentary and stratigraphic techniques.The surface structure characteristics of the quartz particles in the Luoquan Formation were observed by scanning electron microscopy(SEM),and the evolution process of the sedimentary environment of the Luoquan Formation was analyzed and finally determined.The research results are as follows:(1)The Luoquan Formation is a special layer of conglomerate dominated by purple-red and gray-green.The composition of the conglomerate is complex,including sandstone,quartzite,mudstone,and dolomite,etc.The sorting and roundness of which are poor.Glacial pavement can be seen at the base of the Luoquan Formation of some profiles.The lower part of the Luoquan Formation is massive conglomerate deposits,while the middle part is stratoid conglomerate deposits,and the upper part is layered conglomerate deposits.Based on the glacial relics underneath of the Luoquan Formation and the sedimentary characteristic of its lower part is disordered massive complex conglomerate deposits,it is determined as glacial sedimentary facies.Sandstone lenses and dropstone-structure occur in the middle and upper part of the Luoquan Formation,which can be attributed to glacial-water sedimentary facies.(2)Surface morphological characteristics of quartz grains in the sediments of the Luoquan Formation in Longtan area of Lushan were observed and studied by SEM.The characteristics closely related to the sedimentary environment analysis are striation,flat cleavage surface,large conchoidal fracture,small conchoidal fracture,and V-shaped percussion crack.Quartz grains characteristics and its frequency indicate that this is a set of glacial deposits,which were later influenced by slight water flow,diagenesis,and weathering.(3)The sedimentary evolution of the Luoquan Formation begins with the movement of glaciers in the early stage,during which the underlying sediments are excavated,abraded and compressed to form obvious glacial pavement structures on the underlying strata surface;then the glaciers begin to melt and the debris materials carried by the glaciers rapidly deposit;the water body deepens further with the glaciers continue to melt.The debris-rich ice-water mixture rolls,flows and creeps along the original undulating ice surface to form sandstone lens body.Some of the ice bodies float on the water body,and the gravel debris carried inside is continuously deposited,forming dropstone-structure and developing glacial-water facies deposits.The sedimentary evolution process of the Luoquan Formation indicates that there were glacial activities in the North China Plate during this period.This study provides a certain reference value for determining the sedimentary environment evolution of the Neoproterozoic glacial period and solving the glacier distribution problem of the North China plate in Neoproterozoic.