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Organic matter accumulation mechanisms in the Lower Cretaceous Jiufotang Formation, Chaoyang Basin, NE China
thesis
posted on 2023-12-14, 04:56 authored by Li LiIn western Liaoning Province, the Lower Cretaceous Jiufotang Formation in the Chaoyang Basin is famous for the Jehol Biota and oil shale resources. Although it is an important oil shale-bearing basin, no detailed research has been carried out on organic-rich source rocks that are interbedded with gravity flow deposits and volcaniclastic rocks. The impact of basin evolution, gravity flows, and volcanism on oil shale formation is unknown. Furthermore, the impact of global volcanic activity on the lacustrine carbon cycle in the Chaoyang Basin has not yet been investigated.
This PhD uses a combination of petrology, sedimentology, sequence stratigraphy, elemental geochemistry, organic geochemistry, organic petrology, zircon U-Pb geochronology, and carbon isotope chemostratigraphy. Based on petrology, sedimentology, and sequence stratigraphic principles, the sequence stratigraphic framework of the third Member of the Jiufotang Formation was constructed. Extensional tectonic subsidence is suggested as the controlling factor for basin evolution and the trigger for earthquakes, gravity flows, and volcanic eruptions. In order to determine the characteristics and formation conditions of the lacustrine source rocks, including palaeoclimate, terrigenous detrital matter input, palaeosalinity, redox conditions, and organic matter source and bioproductivity, a range of parameters and techniques were used. These include elemental geochemistry, organic petrology, and organic geochemistry, including assessment of Rock Eval pyrolysis data, and aliphatic and aromatic hydrocarbon distributions. The oil shales were mainly deposited in a brackish and anoxic environment with salinity stratification and a low sulfur content. Gravity flows were accompanied by the input of terrestrial detritus and a degree of oxygenation in the the bottom waters, reducing oil shale formation. Volcanic eruptions ejected intermediate-acidic ashes, leading to enhancement of algal blooms (by a fertilisation effect due to minor volcanic ash input), or in some cases inhibition of algal blooms (by a poisoning effect due to major volcanic ash input). The best periods for oil shale formation were during the intermissions between tectonic movement, when there were few gravity flows, volcanic eruptions, or earthquakes. Based on U-Pb zircon geochronology and carbon isotope correlation, the lacustrine oil shale sequence in the Chaoyang Basin corresponds to the marine black shale interval within the Early Aptian Oceanic Anoxic Event 1a then refer to OAE 1a. Volcanogenic CO2 drawdown due to reduced activity of the Ontong Java Plateau and local volcanism and eruption of volcanic particles and aerosols into the stratosphere led to a trend of continuous climate cooling in this continental region. The positive δ13 Ccarb excursions in the lacustrine deposits were very likely related to the enhanced organic carbon burial rate and the reduced atmospheric CO2 concentration on a global scale. This research on the organic matter accumulation mechanisms in the Jiufotang Formation, Chaoyang Basin provides a useful example for future resource exploration of coeval oil shales in north China.
