Isotopic geochemical characteristics of two types of carbonate concretions of Chang 7 member in the middle-upper Triassic Yanchang Formation, Ordos Basin, Central China

Carbonate concretion is an authigenic aggregation of minerals commonly found in sandstone and shale, especially in organic-rich shale. In the outcrop, large number of carbonate concretions developed in Chang 7 member in Tongchuan. Those concretions were studied in terms of stable isotope, clumped isotope to delineate its origin and sedimentary environment. Two types of concretions are identified. Type A concretionsmainly distribute in organic-rich shale. This concretion was divided into the inner circular layer and the outer circular layer. In the inner circular layer, the value of delta C-13 parts per thousand(VPDB) is 9.98 parts per thousand similar to 14.19 parts per thousand, and the value of delta O-18 parts per thousand(VPDB) is -13.32 parts per thousand similar to-10.07 parts per thousand. On the contrary, the delta C-13 and delta O-18 value of outer circular layer are4.74 parts per thousand similar to-0.59 parts per thousand,17.86 parts per thousand similar to-15.59 parts per thousand respectively. The formation palaeotemperature of different parts of concretion ranged from 28.6 to 76.5 degrees C. This may be due to methanogenesis arising from bacterial activities in the fermentation zone, and the fractional distillation of carbon isotope of organic matter was caused by the methane bacterial activities. The calcite and dolomite spherulites in carbonate concretions may be due to the calcilization and dolomitization of cyanobacteria in the early diagenesis. The type B concretions were distributed in the fine sandstone, silty sandstone, and silty mudstone. The internal structure of concretions is uniform. The carbon isotopic values were negative, and delta C-13 was -5.019 similar to-12.013 parts per thousand, and the delta O-18 values was -17.6 similar to-19.281 parts per thousand. The value of Sr-87/Sr-86 was in the range of 0.710072-0.710392. The temperature was established at 60.4 degrees C through clumped isotopes analysis. Based on isotopic analysis, the formation palaeotemperature at different parts of concretion was 55.4-65.6 degrees C. The origin of the concretion may be related to dehydroxylation in diagenesis, formed in the earlymiddle stage of diagenesis.