BACKGROUNDThe black shale of the Longmaxi Formation in the Upper Yangtze region is rich in graptolites. Most of them were preserved as a carbonaceous film, and enriched in organic-rich layers. Previous research focuses mainly on the graptolite morphology and evolution process after diagenesis, but the direct evidence for the fossil formation is still lacking.

OBJECTIVESTo explore the formation mechanism of graptolite in the Longmaxi Formation and its geological significance on organic matter enrichment.

METHODSLaser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) was used for the in situ multi-element imaging of graptolites and surrounding rocks from the Longmaxi Formation of the N203 well.

RESULTSAnalysis of the distribution and enrichment degree of major ore-forming elements indicates that Mg, Al, Si, Fe were rich in the carbonaceous film surface of graptolite, with a enrichment degree ranging from 1.5 to 10. However, Sr/Ba values (1.4-2.3) of the carbonaceous film type graptolite were lower than that of the surrounding rocks (>5.0). This indicated that embedding by clay minerals was the main process of graptolite preservation. The sulfidic micro-environment caused by embedding of clay minerals benefited the pyritization of the graptolite organism. Combing with the vertical variation and correlation analysis of the cross-section graptolite ratio, organic matter, pyrite, clay mineral content and δ¹³C_org value, it can be concluded that the flourishing of the microbial mat in early stage and sulfate-reducing bacteria in later stage consumed oxygen in the pore water and caused anoxic bottom water, and should be the main reason of massive burial of graptolite and organic matter.

CONCLUSIONSThe result revealed the burial and mineralization mechanism of graptolite in the Longmaxi Formation, and also provided a new method for studying the controlling factors of organic matter enrichment and black shale formation.