A discussion on the genesis of carbonate concretions in the Chang 7 Member of the Middle-Upper Triassic Yanchang Formation of Ordos basin

There are a large number of spherical, ellipsoidal and oblate carbonate concretions in the sequence of fine sandstone, siltstone, argillaceous siltstone and silty mudstone in the Chang 7 Member of the Middle Triassic Yanchang Formation in the Tongchuan area, south of Ordos Basin. Macroscopic description was carried out for concretions in several sections in the Tongchuan area. On the basis of thin section observation, rock and mineral composition, stable isotopes, cluster isotopes and micro-area isotopes were analyzed. In particular, one concretion was selected for detailed anatomical studies. The results showed that the concretion was composed mainly of calcite and a little dolomite. The inner structure is different, which is divided into two circles on the cross section of the sphere. The carbon isotope value was positive and decreased from the inner layer to the outer layer. The variation trend of oxygen isotope is consistent with that of carbon isotope, and the variation range of the concretion formation palaeo-temperature range was 28.6~76.5℃. The formation temperature of cluster isotope in the middle of parts was 42.9℃. It is believed that such nodules are formed by the early fermentation zone bacteria participating in methanogenesis, and the activity of methanogens causes the organic carbon isotope fractionation, and the calcite and dolomite spherules in carbonate nodules may be the result of calcite or dolomitization of cyanobacteria cells. According to the burial history, it is believed that the nodules were formed in the early stage of diagenesis and grew superimposed in the later stage of diagenesis, with obvious concentric ring structure. © 2021, Science Press. All right reserved.