Mineralogy and Geochemistry of the Lower Cretaceous Jiufotang Formation, Beipiao Basin, NE China: Implications for Weathering, Provenance, and Tectonic Setting

The mineralogy and geochemistry of oil shales from the Lower Cretaceous Jiufotang Formation (K(1)jf(3)) in northeastern China were used to determine maturity, intensity of chemical weathering, recycling, provenance, and tectonic setting. The studied samples are characterized by enrichment in light rare earth elements (REEs), flat heavy REEs patterns, and negative Eu anomalies, similar to felsic components of the upper continental crust (UCC). Geochemical data (major-element discriminant functions, Co/Th, La/Sc, La/Th, Hf, Cr/V, Y/Ni, Ni/TiO2, TiO2/Zr, and Al2O3/TiO2) further confirm that the source rocks of oil shales were mainly felsic igneous rocks; in particular, they have a strong genetic relationship with the igneous rocks from the Yanliao Belt. Results for clay minerals (I/S, Illite, and kaolinite), Th/U ratio, chemical index of alteration, chemical proxy of alteration, plagioclase index of alteration, Mg/Ca, and the marked depletion in Sr,V, Ni, and Ba relative to UCC together suggest intense chemical weathering under a semihumid to humid/warm climate in the source region of high quality oil shale. Values of Th/Sc, Zr/Sc, and the index of compositional variability indicate that the oil shales source region was a recycled orogen with progressive recycling. In addition, during the deposition of high quality oil shales, tectonic activity was weak and sedimentary recycling was highly developed. A variety of geochemical parameters, when plotted on tectonic setting discrimination diagrams, identify with a high probability that the K(1)jf(3) oil shale was derived from a collision system with the characteristics of active continental margin. Provenance of the K(1)jf(3)oil shale is enriched in P, Cu, and Cd, which became sources of nutrition for the phytoplankton growing in the ancient Lake. The resulting high lake productivity, combined with the intense chemical weathering, strong sedimentary recycling, stable tectonic setting, and sedimentary regime of undercompensation, contributed to the widespread deposition of thick high quality oil shale in the Beipiao basin.

Automated summary

The mineralogy and geochemistry of Lower Cretaceous Jiufotang Formation oil shales in northeastern China indicate a source from a collision system with characteristics of active continental margin, intense chemical weathering, and sediment recycling, deposited in a stable tectonic setting.