Mechanism of Deep Burial Dolomitization of Massive Dolostones in the Middle Maj iagou Group of the Ordovician, Ordos Basin

ZHANG Yong sheng

There is a suit of massive dolostones(as MD hereafter)which develop in the middle Majiagou Group of the Ordovician in the Ordos basin.It is mainly distributed in Majiagou Fm.Ⅳ,and it has several hundred metres at the zone of Dingbian County to Eteke Qi in the west part of Ordos basin, with maximum thickness 431 m in Dingtan Well 1.It becomes thinner and thinner from west to east,and is intercalated with limestones from the middle to eastern part of Ordos. The MD consist of coarse silt-sized to fine sand-sized dolomite crystals.The mottled texture is well developed in such dolostones,and the boundary of dolomitic spots is commonly stylolites.It shows that mottled dolomitization is obviously controlled by stylolites. The dolomites have uniform dar red colour under cathodoluminecence. The analytical data of 67 samples shows the ordering of dolomites averages 0.85,and the molar concentration of CaCO₃ averages 50.65%;δ ¹³ C averages 0.6‰(PDB),δ¹⁸ O averages -7.4‰(PDB).The characteristics of cathodoluminecence and X-ray indicate approximate ideal dolomites are formed in stable diagenetic environment of slow conclear and crystalization.The homogeneous temperature of 21 primary inclusions in the host crystals is from 104℃ to 355℃(uncorrected by pressure),and the temperature of 104℃ is the lowest one.According to the earth temperature gradient of 3.98℃/100m in this area,the maximum burial depth of dolomitization is at least 2 600 m. Besides,the composition of inclusions has two parts-organic and inorganic and there is a certain of CH 4 both in gas phase and in liquid phase. All characteristics above prove that the huge thick dolostone body is formed in deep burial dolomitic environment.

Mechanism of Deep Burial Dolomitization of Massive Dolostones in the Middle Maj iagou Group of the Ordovician, Ordos Basin

Laboratory of Salt Lake Resources and Environment, Institute of Mineral Deposits,Chinese Academy of Geological Sciences,Beijing 100037

Received Date: 1999-03-30

Rev Recd Date: 1999-05-03

Publish Date: 2000-09-10

Key words:

Abstract: There is a suit of massive dolostones(as MD hereafter)which develop in the middle Majiagou Group of the Ordovician in the Ordos basin.It is mainly distributed in Majiagou Fm.Ⅳ,and it has several hundred metres at the zone of Dingbian County to Eteke Qi in the west part of Ordos basin, with maximum thickness 431 m in Dingtan Well 1.It becomes thinner and thinner from west to east,and is intercalated with limestones from the middle to eastern part of Ordos. The MD consist of coarse silt-sized to fine sand-sized dolomite crystals.The mottled texture is well developed in such dolostones,and the boundary of dolomitic spots is commonly stylolites.It shows that mottled dolomitization is obviously controlled by stylolites. The dolomites have uniform dar red colour under cathodoluminecence. The analytical data of 67 samples shows the ordering of dolomites averages 0.85,and the molar concentration of CaCO₃ averages 50.65%;δ ¹³ C averages 0.6‰(PDB),δ¹⁸ O averages -7.4‰(PDB).The characteristics of cathodoluminecence and X-ray indicate approximate ideal dolomites are formed in stable diagenetic environment of slow conclear and crystalization.The homogeneous temperature of 21 primary inclusions in the host crystals is from 104℃ to 355℃(uncorrected by pressure),and the temperature of 104℃ is the lowest one.According to the earth temperature gradient of 3.98℃/100m in this area,the maximum burial depth of dolomitization is at least 2 600 m. Besides,the composition of inclusions has two parts-organic and inorganic and there is a certain of CH 4 both in gas phase and in liquid phase. All characteristics above prove that the huge thick dolostone body is formed in deep burial dolomitic environment.

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Reference (9)

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Mechanism of Deep Burial Dolomitization of Massive Dolostones in the Middle Maj iagou Group of the Ordovician, Ordos Basin

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