Abstract:

Huadian basin, which is located within Dunhua-Mishan fault zone, is well known as a Paleogene oil shale-bearing fault-basin in China. Previous studies mostly focus on the sedimentary characteristics of oil shale, but there was little concern on provenance features and tectonic settings. The geochemical analysis based on the mudstone samples from the Oil Shale Member of Huadian Formation shows that the SiO₂ content is respectively low, The K₂O/Na₂O ratio is relatively high(＞1). The parameter of (MgO+Fe₂O^T₃) ranges from 3.91% to 11.66% and the Al₂O₃/（Na₂O+K₂O）ratio ranges from 3.77% to 6.29%, all of which demonstrate that the mudstones contain a certain amount of femic constituents and a considerable quantity of stable components. The PAAS-standardized distribution curves of trace elements show variable enrichment and depletion trends among different samples, significant variations in the REE contents, enrichment in light REE and negative anomalies of Eu. These features suggest that the material sources of the Oil Shale Member came from several provenances. The mudstone weathering and alteration indexes range from 74 to 82 and the Th/U ratio mainly ranges from 4.65 to 6.07, both of which suggest that provenances had underwent moderate weathering processes. The Th/Sc and Zr/Sc ratios illustrate that these mudstones were mainly controlled by source rocks. On the basis of source rocks, discrimination diagrams of the tectonic setting and previous research material, we conclude that the provenances of the Oil Shale Member of Huadian Formation in Huadian Basin were mainly Hercynian and Yanshanian granites, and also involve some andesitic to rhyolitic effusive rocks. Source rocks were formed in the continental margin orogenic belt and belonged to continental island arc volcanic series.

Key words: oil shale, geochemistry, provenance, tectonic setting, Paleogene, Huadian basin