Sedimentary basin evolution, gravity flows, volcanism, and their impacts on the formation of the Lower Cretaceous oil shales in the Chaoyang Basin, northeastern China

Abstract

No detailed research has been carried out on the oil shales that are interbedded with gravity flow deposits and volcaniclastic rocks in the third Member of the Jiufotang Formation. In order to study the impact of basin evolution, gravity flows, and volcanism on oil shale formation, petrology, sedimentology, sequence stratigraphy, organic geochemistry, and elemental geochemistry were used to characterise the oil shales. Extensional tectonic subsidence is suggested as the controlling factor for basin evolution. The predominant maceral in the oil shales is sapropelinite, suggesting organic matter was derived from mainly aquatic organism (especially algae). Element indices (δU, (Cu + Mo)/Zn, Sr/Ba, Th (μg/g), Zr (μg/g), and the chemical index of alteration (CIA) show that the water conditions during deposition of the oil shales were anoxic and brackish, with little terrigenous detrital input and a cold and dry palaeoclimate. Gravity flows were accompanied by the input of terrestrial detritus and a degree of oxygenation of the bottom waters, reducing oil shale formation. Volcanic eruptions ejected intermediate-acidic ashes, not only leading to climate cooling, but also either enhancing algal blooms (by a fertilisation effect due to minor volcanic ash input) or inhibiting algal blooms (by a poisoning effect due to major volcanic ash input). There is synchronicity between the occurrence of rifting, gravity flows, volcanism, and earthquakes. The best periods for oil shale formation were during intermissions between tectonic movement, when there were few gravity flows, volcanic eruptions, or earthquakes. High bioproductivity and lake expansion along with an anoxic environment were the main controlling factors for high-quality oil shale formation. There is a series of oil shale-bearing rift basins formed in northeastern China that share a similar geological setting, so this research provides a useful example for future resource exploration of coeval oil shales in northeastern China.