Carbonate Concretions in Triassic Yanchang Formation (Ordos Basin, China) as Evidence of Hydrothermal Activity

Abstract

The discovery of concretions of Chang 7 shale formations in the Ordos basin has increased interest in the exploration of lacustrine carbonate genesis in these basins. In this paper, these concretions were sampled and used in major, trace, and isotopic geochemistry tests. We used a microscope to investigate these concretions, and the results showed that the concretions consisted of carbonate rocks, the calcite was hydrothermal calcite, and obvious hydrothermal activity was present in the Yanchang period. We used seismic data to interpret the faults, and we determined that tectonic activity was relatively frequent in the middle–late Triassic period and that the faults were channels for hydrothermal upwelling. During the middle–late Triassic period, tectonic movement of the basin occurred, and synsedimentary faults developed in the Yanchang Formation. As deep hydrothermal gushers rose through faults and fractures, they carried particles upward through the deep limestone strata. When the hydrothermal gushers reached the lake bottom, the particles precipitated and eventually formed concretions via diagenesis.