鄂尔多斯盆地东南部马家沟组碳酸盐岩碳氧同位素特征及意义

Characteristics of carbon and oxygen isotopes of carbonate rocks in Majiagou Formation and their implication,southeastern Ordos Basin

对鄂尔多斯盆地东南部马家沟组白云岩和灰岩的碳氧同位素进行测定。结果显示,样品的碳氧同位素值分布较为离散,二者的相关性仅为0.18,氧同位素平均值为-8.34‰;对锰、锶元素含量进行测定,计算ω(Mn)/ω(Sr)平均值为1.4,表明采集样品大部分受后期成岩蚀变影响小,可较好地代表原始地层的沉积特征。马五段、马六段样品的碳氧同位素值变化差异明显:马五段白云岩δ13C在-1.33‰~1.01‰(PDB),δ18O值在-9.58‰^-6.46‰(PDB),为中低负值;马六段灰岩δ13C在-6.75‰~0.92‰(PDB),δ18O值在-11.36‰^-7.33‰(PDB),马六段的碳氧同位素值较马五段离散,且发生了明显的负漂移。Z值与碳同位素值具有较高的相关性,而与氧同位素值相关性低,仅作为该区古盐度变化的相对参考值。碳氧同位素值与Z值所反映的环境信息表明,鄂尔多斯盆地东南部马家沟组马五期为海退期,海水循环性差,气候干旱炎热,处于蒸发环境,海水盐度较高。马六期形成的灰岩由于长期裸露地表,受到大气淡水淋滤作用的影响,角砾泥晶灰岩发育,其碳氧同位素值较低。

The carbon and oxygen isotopes of dolomite and limestone in Majiagou Formation in the southeast of Ordos Basin are studied. The result shows, the carbon and oxygen isotopes distributions are relatively discrete, their correlation is only 0.18, and the average value of oxygen isotope is-8.34‰. The contents of manganese and strontium were determined, and the average value of ω （Mn）/ ω （Sr） was 1.4. It shows that the samples collected are less affected by later diagenesis alteration, and can better represent the sedimentary characteristics of the original strata. The difference of carbon and oxygen isotope values between the fifth member（M5） and the sixth member（M6） of Majiagou Formation is obvious： δ 13 C of the crystal dolomite of M5 is in-1.33%～1.01% （PDB）, δ 18 O values in-9.58%～-6.46% （PDB）, and it is medium low negative; δ 13 C of the muddy limestone of M6 is in-6.75‰～0.92‰（PDB）, δ 18 O values in-11.36‰～-7.33‰（PDB）, and it is high negative value. The carbon and oxygen isotope values of M6 are more discreter than those of M5, and there is a clear negative drift. The Z value has a high correlation with the δ（ 13 C PDB ） value, and a low correlation with the δ（ 18 O PDB ） value, which is only a relative reference value of the paleosalinity change in this area. The environmental informations indicated by the carbon oxygen isotope value and the Z value indicate. The salinity of the M5 in the southeast of Ordos Basin is relatively high, and the salinity of the M6 is lower. The period of M5 was regressive period, and the circulation of sea water was poor, climate was dry and hot, it was evaporation environment and sea water was high salinity. The limestone formed in M6 is due to long exposure to the surface, influenced by atmospheric freshwater leaching, breccia micrite limestone develops and its carbon and oxygen isotope values are relatively low.