The Feature And Formation Model Of Cretaceous Cold Seep Carbonate Concretions In Xigaze,Southern Tibet

Since cold seep are closely related to the gas hydrate and methane,which are viewed as the important energy and key greenhouse gas,now cold seep are more and more highly regarded by scientists.In this thesis,we will pay attention to the petrography,mineralogy,geochemistry of the cold seep carbonate concretions and discuss the composition,structure,carbon sources,and formation processes and model.According to the morphology,the carbonate concretions could be divided into four kinds,as nodular,tubular,tabular,and septarian carbonate concretions.The nodular concretions,ranging from 2cm×3cm to 15cm×30cm,are composed of micritic calcite cements?30⁵0%?,quarz and feldspar?40⁶5%?,foraminifers?2³%?,pyrite?5%?.The?¹³C values of cold seep carbonates are from-29.79 to-3.61‰VPDB.The gradual change of the?¹³C values and the content of the terrigenous elements in nodular concretions indicate that they are formed from the center to the margin.Based on the data of the?¹³C values,TOC,and other evideces,we suggest the nodular concretions are formed by anaerobic oxidation of methane.The tubular concretions,the size of which are different with each other,are parallel to or cross the bedding.They are mainly composed of micritic calcite cements?50⁸0%?,quarz and feldspar?20⁵0%?,foraminifers,and pyrite.The?¹³C values of the tubular concretions in the Gamba and Shangba section are from-27.37 to-23.85‰VPDB and from-8.85 to-3.47‰VPDB,respectively.Based on the data of the?¹³C values,TOC,and special shape,we suggest the tubular concretions are formed by anaerobic oxidation of methane,which is source form the hydrate release,and the growth of the the tubular concretions is from the margin to the center.The tabular concretions,23cm±in thickness,are parallel to the bedding.They are mainly composed of micritic calcite cements?85⁹0%?,quarz and feldspar?10¹5%?,and pyrite.The?¹³C values of the tabular concretions are from-21.87 to-6.67‰VPDB,and they are formed by anaerobic oxidation of methane.Septarian concretions,ranging from 6 cm×8 cm to 100 cm×30 cm,are parallel to the bedding.They consist of two parts:concretions matrix and septarian crack fillings.The concretion matrix is composed of micritic calcite cements?55⁷5%?,quarz and feldspar?25⁴5%?.The crack filling consist of early fabric calcite cements and later blocky calcite.Compared with the micritic and fabric calcite cements,the blocky calcite cements show higher FeO,and lower MgO content.Their?¹³C values,ranging from-26.81 to-1.86‰VPDB,increase from the micritic matrix in the margin to the blocky calcite cements in the center.Besides,the homogenization temperature that we get from the primary fluid inclusion is 176.9?.Based on the results above,the concretion matrix and fabric calcite cements are formed by anaerobic oxidation of methane,and the blocky calcite cements are formed during the burial process and are formed by heated pore water.The concretion matrix and fabric calcite cements recrystallized partly during their burial.