The characteristics and formation mechanism of Ordovician dolomite in Member 6 of Majiagou Formation in South Ordos Basin

Member 6 of Majiagou Formation in south Ordos basin has been eroded in most part of the south Ordos basin,and is hence only distributed in the periphery of the basin. Therefore,less studies have been done on the origin of dolomite in member 6 of Majiagou Formation of the basin. Based on petrologic analysis,cathodoluminescence images,trace elements and carbon and oxygen isotope characteristics,the authors studied the characteristics and formation mechanism of dolomite in member 6 of Majiagou Formation in south Ordos basin. The results reveal that the dolomite of member 6 in south Ordos basin is mainly composed of finemedium crystal dolomite with cloudy cores and clear rims as well as dark red luminescence. The dolomite has relatively low Fe and Mn content (average of 447×10^-6 and 62×10^-6),relatively high K and Na content (average of 517×10^-6 and 252×10^-6) and moderate content of Sr (average of 155×10^-6). Average value of δ¹³C and δ¹⁸O is -0.617‰ and -7.6‰ respectively,moderate indicating its marine origin. The trace element content of cloudy cores is close to that of clear rims,which indicates that they were formed in similar diagenesis environments. The cloudy cores were formed in a shallow buried environment by seepage reflux dolomitization model,while the clear rims were recrystallized and adjusted in a deep buried environment.