The Formation Mechanism and Geological Significance of Graptolite from the Longmaxi Formation: Constraints from <i>in situ</i> Multi-element Imaging Analysis

Cheng-lin ZHU; Hua-jian WANG; Yun-tao YE; Xiao-mei WANG; Jia-xuan HUANG; Yu-mei ZHU; Rui-dong YANG

doi:10.15898/j.cnki.11-2131/td.201810110113

Rock and Mineral Analysis > 2019 > 38(3): 245-259. > DOI: 10.15898/j.cnki.11-2131/td.201810110113

Cheng-lin ZHU, Hua-jian WANG, Yun-tao YE, Xiao-mei WANG, Jia-xuan HUANG, Yu-mei ZHU, Rui-dong YANG. The Formation Mechanism and Geological Significance of Graptolite from the Longmaxi Formation: Constraints from in situ Multi-element Imaging Analysis[J]. Rock and Mineral Analysis, 2019, 38(3): 245-259. DOI: 10.15898/j.cnki.11-2131/td.201810110113

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The Formation Mechanism and Geological Significance of Graptolite from the Longmaxi Formation: Constraints from in situ Multi-element Imaging Analysis

1.
College of Resources and Environmental Engineering, Guizhou University, Guiyang 550025, China
2.
Key Laboratory of Petroleum Geochemistry, Research Institute of Petroleum Exploration and Development, Beijing 100083, China

More Information

Received Date: October 10, 2018
Revised Date: March 07, 2019
Accepted Date: April 08, 2019
Published Date: April 30, 2019

Abstract

HIGHLIGHTS
(1) The multi-element distribution of graptolite and surrounding rock was in situ described in micrometer to centimeter scale.

(2) Mineralization of organic matter with clay minerals benefit the preservation of biological structures.

(3) Anoxic bottom water caused by flourishing of microbial mat and sulfate-reducing bacteria was the main reason for massive burial of graptolite and organic matter.

ABSTRACT

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.
- Longmaxi Formation,
- graptolite,
- multi-element imaging,
- Laser Ablation-Inductively Coupled Plasma-Mass

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