地质记录中的海水白云岩化作用

SEAWATER DOLOMITIZATION IN GEOLOGIC RECORDS

关键词:  白云岩化作用模式  海水白云岩化作用  Enewetak Atoll始新统  云南东北部灯影组

Key words:  dolcmitization model  seawater dolomitization  Eocene strata on the Enewetak Atoll  Dengying Formation in NE Yunnan

摘要:

地质记录中的海水白云岩化作用是广泛存在的。它不仅发育于时代较新的始新世地层内(如太平洋中的Enewetak珊瑚岛地区),而且也发育于时代较老的震旦纪灯影期地层内(如云南东北部地区)。在以前的白云岩成因研究过程中,海水白云岩化作用的普遍性和重要性被大大地低估了。自从Saller(1984)首次报道了正常海水白云岩化的实例以来,海水白云岩化作用的研究已取得了长足的进展,如Kasner(1984);Land(1985);Smart(1988);Tucker(1990);朱同兴、罗安屏(1993)等。
近年来,作者通过对云南东北部地区震旦系灯影组潮缘沉积物的岩石学、沉积相、古地理格局和沉积地球化学标志等综合研究,认为该套巨厚的白云岩层(400-1000m)的成因不是原生的,而是成岩交代形成的,其主要的形成机制之一就是成岩早期的海水白云岩化作用。
海水白云岩化作用的发育主要受海平面变化、沉积作用类型以及古气候等条件的限制。造成滇东北地区灯影期海水白云岩化作用的主要机制可能是位于其西北部的青藏大洋内的冷洋流向东部沉积物内泵吸,和位于其东南缘的扬子边缘海的海水向西部沉积物内循环对流(柯特对流)。上述的泵吸作用与对流作用的相互叠加可使巨大体积的海水通过扬子海域,尤其是扬子台地西南边缘(滇东北地区)沉积物,从而使其发生巨大规模的海水白云岩化作用。由海水白云岩化作用形成的白云岩的结构有序度中等,I=0.68-0.82,其碳、氧同位素组成均显示海水碳酸盐来源,δ¹³C=+1.8-+2.6‰(PDB),δ¹⁸O=+2.1-+2.7‰(PDB)(冷洋流来源,Enewetak Atoll,始新统)或-5.1——6.5‰(受地热和大气淡水双重影响而偏负值,滇东北地区;灯影组)。
灯影期海水白云岩化作用模式合理地解释了区域巨厚沉积物的块状白云岩化成因、充足的镁离子来源以及多余的钙离子的搬运等白云岩成因研究过程中的关键性问题。

Abstract:

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⁺.