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:   Significance Calcareous concretions, composed of authigenic carbonate mineral aggregates, are common sedimentary structures in deep-time marine strata. Due to their unique growth conditions, they have become critical archives for studying sedimentary environments, paleoclimate, and the evolution of sedimentary pore fluids. To gain a deeper understanding of the genesis of different calcareous concretions and highlight their geological significance. Progress This paper reviews the macroscopic morphology, micro-fabrics, and isotopic characteristics of different types of calcareous concretions, and provides an overview of their diagenetic timing, growth modes, and the associated biogeochemical processes. Conclusions and Prospects Calcareous concretions can form during the quasi-syndepositional stage, early diagenetic stage, or late diagenetic stage, and exhibit concentric or pervasive growth modes. The formation mechanisms primarily involve three pathways: (1) In an anaerobic environment, organic matter in the sediment serves as an electron donor for sulfate-reducing bacteria, leading to oxidation, generation of bicarbonate ions, and an increase in alkalinity in the pore water, which promotes the formation of calcareous concretions; (2) Methane in cold-seep fluids, ascending to the seafloor, undergoes anaerobic oxidation via consortia of methanotrophic archaea and sulfate-reducing bacteria, increasing carbonate alkalinity in sediment pore waters and facilitating in-situ carbonate precipitation; (3) In deep burial environments, over-pressured pore fluids trigger the formation of late digenetic carbonates in fractures, characterized by the development of “beef” and “cone-in-cone” structures. Recent studies have provided a deeper understanding of the microbial processes involved in the formation of calcareous concretions. However, more high-resolution in-situ analyses are needed to clarify how different biogeochemical processes (e.g., organic mineralization, sulfate reduction, and methane anaerobic oxidation coupling) control concretion growth and to further explore their formation mechanisms.

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