Nodular limestone and its genesis from the Ordovician Yanwashan Formation in western Zhejiang Province
Zhang Xia1 Lin Chunming1 Ling Hongfei1 Jiang Shaoyong1 Li Yanli1Gao Likun1 Yao Yulai1 Liu Xiao21 State Key Laboratory for Mineral Deposits Research,School of Earth Sciences and Engineering,Nanjing University,Nanjing 210093,Jiangsu2 Planning & Designing Institute of East China Branch,SINOPEC,Nanjing 210036,Jiangsu
In this paper,we discussed the genesis of the nodular limestones from the Ordovician Yanwashan Formation in the Huangnitang section(the first GSSP) ,Changshan,Zhejiang Province on the basis of mineral component,texture,structure,chemical composition,and particularly the stable C-O isotopic composition of both the nodule and matrix. The research results indicate that there is an apparent difference between the nodule and matrix of the nodular limestones. The nodules are mainly composed of micritic calcite(70%～98%) ,while the primary component of the matrix is clay mineral(40%～78%) with only 20%～58% calcite. The δ13CVPDB values of calcite of the nodule and matrix are 0.5‰ to 1.4‰(average 0.9‰) and-0.4‰ to 1.1‰(average 0.2‰) ,respectively. The δ18OVPDB values of calcite of the nodule and matrix are-12.8‰ to-8.0‰(average-10.6‰) and-11.7‰ to-9.0‰(average-10.3‰) ,respectively. The nodule always shows higher δ13C value and formation temperature than those of the matrix for the same sample. All features imply that the nodular limestones from the studied section are formed due to a combined processes of the periodical dissolution of bottom current during the sedimentary period and the differential compaction and pressure solution during the diagenesis. There are three existing types of the nodular limestones:banded nodular limestone,discontinuous nodular limestone and random nodular limestone. The occurrence mode of the nodular limestone is determined by the insoluble residue content of country rock of nodules and the pressure solution intensity.