Age, material source, and formation mechanism of bedding-parallel calcite beef veins: Case from the mature Eocene lacustrine shales in the Biyang Sag, Nanxiang Basin, China

The mechanisms leading to the formation of bedding-parallel calcite beef veins have been widely debated, with discussions centered on timing or burial depth vein generation, source material, driving forces for initiation and widening, growth direction mechanism. To address these issues, a comprehensive study drill core samples containing in mature Eocene lacustrine Hetaoyuan Formation Biyang Sag, Nanxiang Basin, China was undertaken through combination microstructural observation, isotopic geochronological, geochemical, fluid inclusion analyses, as well basin modeling. X-ray diffraction total organic carbon content analyses indicate that most accumulated calcite-rich laminated shales high matter contents. These yielded an absolute laser ablation?multi-collector?inductively coupled plasma?mass spectrometry U-Pb age ca. 41.02 ± 0.44 Ma, which corresponds 500?800 m. Such shallow suggests full compaction consolidation sediments would not yet achieved, is compatible following observations: (1) plastic deformation shale laminae adjacent veins, (2) temperature ?59 °C derived from fluid-inclusion microthermometry. radio-isotopic ?1?3 m.y. younger than stratigraphic host rock (ca. 43.1 Ma) but earlier model-derived oil generation 35.8 tectonic extrusion 23.0?13.0 Ma). predated bacterial sulfate reduction, evidenced by crosscutting relationships carbonate concretions, pyrite framboids, apatite pellets. A two-stage model proposed. When rich reaches methanogenic zone, overpressure triggered biogenetic gas results horizontal hydrofracturing, initiating cracks act expulsion pathways. Once all generated has migrated, opened fractures close again due overburden load. materials fed pressure solution host-rock fractions then mobilized into unhealed diffusion. Subsequently, force crystallization, antitaxial, displacive fibers commenced, contemporaneous fracture dilation, eventually veins.