doi:10.16186/j.cnki.1673-9787.2023030038

Received:2023/03/20

Revised:2023/04/18

Published:2024/01/25

Origin of calcareous concretions in marine sedimentary rocks

LIU Chao, LIANG Tian, LIU Xiaoxiao, AN Haihua, CHEN Hao, LI Xin

School of Resources and Environment，Henan Polytechnic University，Jiaozuo 454000，Henan，China

Abstract: Objective As a common sedimentary structure in deep-time marine strata，calcareous concretions， which is mainly composed of authigenic carbonate mineral aggregates，have become an important archive for studying sedimentary environment，paleoclimate，and sedimentary pore fluid evolution due to their special growth conditions.In order to deeply understand the genesis of different calcareous concretions and highlight their geological significance， Methods this paper reviews the macroscopic morphology，micro-fabrics，and isotopic characteristics of different calcareous concentrations，and elaborates the timing and mode of concretionary growth，and the related biogeochemical processes during their formation. Results Calcareous concentrations can be formed in the quasi-syndepositional stage，early or late diagenetic stage，with concentric or pervasive growth mode.Its formation mechanism mainly have three ways：（1）In anaerobic environment，organic matters in the sediment are oxidized by providing electron donors for sulfate-reducing bacteria，which undergo oxidation and generate bicarbonate ions，leading to an increase in alkalinity in pore water and the formation of calcareous concentrations；（2）Methane in cold-seep fluids upward to the seafloor is oxidized via consortia of methanotrophic archaea and sulfate-reducing bacteria，increasing carbonate alkalinity in sediment porewaters，and promoting in-situ carbonate precipitation；and（3）In deep burial settings，over-pressured porewaters in the sediment trigger the formation of late digenetic carbonates in fractures，which are characterized by the development of "beef" and "cone in cone" structures. Conclusion  In recent years，microbial processes involved in the formation of calcareous concentrations have been extensively studied，but more high resolution in-situ analysis is needed to clarify how different biogeochemical processes（e.g.，organic mineralization，sulfate reduction and methane anaerobic oxidation coupling） control on calcareous concretion growth，and to further indicate its genetic mechanism.

Key words:calcareous concretion;isotopic characteristic;anaerobic oxidation of methane; biogeochemical  process;diagenesis