珠江三角洲全新世沉积物磁性特征及早期成岩作用分析

Magnetic characteristics and early diagenesis of Holocene sediments in the Zhujiang River Delta

磁性矿物的早期成岩作用是沉积物埋藏后的重要过程,辨别早期成岩作用,才能更好地解释地层的矿物磁性变化。本研究对珠江三角洲顺德平原全新世钻孔MZ孔进行沉积相和室温磁性分析,并辅以热磁分析鉴定磁性矿物,以探讨钻孔不同深度和沉积相的早期成岩作用阶段。结果表明,MZ孔全新世地层自下而上包括感潮河道、河口湾和三角洲相。室温磁性特征与沉积相缺乏明显关联,表现出强烈的早期成岩作用。此外,全新世晚期岩芯磁性特征还受人类活动影响。该孔早期成岩作用以磁性矿物溶解和形成自生黄铁矿为主。在三角洲前缘相的上部和河口湾相底部保存了硫复铁矿。根据矿物组合推测以4.51~4.56 m和30.4~30.5 m两个深度为代表的硫复铁矿形成机制不同,即三角洲前缘相中硫复铁矿可能形成于早期成岩作用的硫酸盐还原阶段,而河口湾相的硫复铁矿形成于甲烷厌氧氧化阶段,后者的含量随着深度增加逐渐增多。上述现象说明,沉积环境可以通过影响有机质和硫酸根离子的供应量,决定磁性矿物所达到的早期成岩作用阶段。

Magnetic mineral diagenesis is an important early diagenetic process after the burial of sediments and its proper identification is the precondition of interpretations for the mineral magnetic properties in the sediments.This study carried out analyses of sedimentary facies,room temperature magnetic and thermomagnetic properties in a Holocene Core MZ collected in the Shunde Plain of the Zhujiang River Delta to identify the vertical changes in the assemblage of magnetic minerals,so as to explore the early diagenetic stages and possible linkage to the sediment-ary facies.The results show that the Holocene sedimentary sequence of Core MZ includes tidal channel,embay-ment,and deltaic successions from bottom upward.The magnetic properties at room temperature lack correlation with sedimentary facies and demonstrate features of strong early diagenesis.In addition,the magnetic properties of the late Holocene sediments were strongly influenced by the human activities.The early diagenesis mainly in-cludes the dissolution of magnetic minerals and the formation of authigenic pyrite.Greigite was also identified in the upper section of the delta-front succession and the bottom of embayment succession.The concentration of greigite increases with depth in the embayment succession.According to the magnetic mineral assemblages,we in-fer different formation mechanism of greigite in the two successions.We suggest that the greigite in the delta-front facies was formed in the sulfate reduction stage of early diagenesis,whilst it was formed in the anaerobic oxidation stage of methane in the embayment facies.These phenomena indicate that sedimentary environment has impacts on the early diagenetic stage of magnetic minerals by controlling the availability of organic matter and sulfate.