SEAWATER DOLOMITIZATION IN GEOLOGIC RECORDS

The seawater dolomitization is extensively developed in geologic records not only in younger Eocene strata(e,g.Enewetak Atoll in the Pacific),but also in older Sinian Dengying Formation(e. g.northeastern Yunnan).The importance and universality of the seawater dolomitization were greatly underestimated in the past research of dolostone origins.Since Saller(1984)firstly reported the example of normal seawater dolomitization on the Enewetak Atoll,considerable progress has been made in the research of the seawater dolomitization(Kastner,1984;Land,1985,Smart et al., 1988b;Tucker,1990;Zhu & Luo,1993).
On the basis of the comprehensive research on petrology,sedimentary facies,palaeogeographic framework and sedimentary geochemical signatures of the peritidal sediments in the Sinian Dengying Formation in northeastern Yunnan in recent years,the authors suggest that the thicker dolostone beds(400-1000m thick)in the study area are not primary but have a diagenetic replacement cri-gin.One of the main mechanisms for the formation of the dolostones is the seawater dolomitization in early diagenetic stage.
The seawater dolomitization is mainly controlled by eustatic sea level,depositional patterns and palaeoclimates.The main mechanisms giving rise to the seawater dolomitization of the Dengying stage in northeastern Yunnan include:(1)the pumping of the cold oceanic currents in the northwest-ern Qinghai-Xizang Ocean into the eastern sediments and(2)the Kohout convection of the sea water in the southeastern Yangtze marginal sea toward the western sediments.The mutual superim-position of the above-mentioned pumping and convection permits the huge volumetric sea water to pass through the sediments in the Yangtze sac area,especcially those on the southwestern margin of the Yangtze platform,thus resulting in the large-scale seawater dolomitization in these sediments.
The textural order index(Ⅰ)of the dolomite resuited from the seawater dolomitization ranges between 0.68 and 0.82.The carbon and oxygen isotopic compositions indicate a marine carbonate origin,δ¹³C=+1.8-+2.6‰ PDB(sources of seawater CO₃^-2).δ¹⁸O=+2.1-+2.7‰ PDB (sources of cold oceanic curents,Eocene,Enewetak Atoll)or δ¹⁸O=-5.1-6.5‰(influenced by geothermal fluids and meteoric waters,Sinian Dengying Formation,northeastern Yunnan).
The seawater dolomitiZation model gives a reasonable explanation for the key problems about dolomite origins such as the massive dolomitization origin of the regional thicker carbonate sedi-ments,abundant Mg⁺ source and the transport of the excessive Ca⁺.