Abstract:

Based on thin section and cathodoluminescence imaging,this paper discussed the types of diagenetic facies and the characteristics of lithofacies of Ordovician carbonate rocks in Ordos Basin.In combination with carbon and oxygen stable isotopic characteristics,we also studied the geochemical characteristics of each diagenetic facies.During the early Paleozoic,the Ordovician carbonate platform was in a near-surface diagenetic tract and can be divided into four diagenetic environments and facies including normal seawater,evaporative seawater,early meteoric freshwater and hot seawater according to the diagenetic medium.The δ¹⁸O and δ¹³C of diagenetic facies of normal seawater,hot seawater and evaporative seawater are generally high,but tend to decrease along with the increasing bioclastic content.In contrast,the δ¹⁸O and δ¹³C of the early freshwater diagenetic facies,are generally low.During the late Paleozoic,the top of the Ordovician was in the epidiagenetic tract.According to the rock types of the current residual strata,the epidiagenetic tract can be divided into limestone weathering diagenetic facies in the Fengfeng Formation and dolomite weathering diagenetic facies in the Majiagou Formation.Their δ¹⁸O and δ¹³C values are significantly lower than that of the early freshwater diagenetic facies.At the end of the Late Paleozoic,the Ordovician Formation was in the moderate-deep burial diagenetic tract at a depth of 3 000-4 000 m,which can be divided into a closed system diagenetic facies and an open system diagenetic facies.The δ¹⁸O and δ¹³C values of the closed system diagenetic facies are similar with their original rocks.In contrast,the δ¹⁸O and δ¹³C values of the open system diagenetic facies are much lower than their original rocks because of the isotope fractionation effect in the hot environment.These results indicate that the petrographic characteristics and the δ¹⁸O and δ¹³C va-lues of carbonate rocks are important indicators for identification of the diagenetic environment and classification of diagenetic facies of carbonate rocks.

Key words: diagenetic facies, carbon and oxygen stable isotope, marine carbonate rock, Ordovician, Ordos Basin