Petrography, geochemistry and provenance of the sediments of the Early Cretaceous Yanguoxia Formation,Lanzhou-Minhe Basin, Northwest China

摘要

Lanzhou-Minhe Basin is situated on the middle Qilian orogenic belt.Yanguoxia Formation contains abundance of maroon siltstones,mudstones and red sandstones of the lake facies.These sedimentary rocks recorded the process of the tectonic uplift of Qilian Mountains during the Early Cretaceous.We discovered plentiful dinosaur footprints,worm burrows,bird footprints,worm tracks-trails,ripple marks and cross lamination in the Yanguoxia site.Integrated petrographic studies classified sandstones of Yanguoxia Formation as feldspathic litharenite.All plots in the QFL (Q or Qt,total quartz;F,feldspar;L,lithic grains) and QmFLt (Qm,monocrystalline quartz;Lt,lithic grains plus polycrystalline quartz) diagrams fall in the recycled orogen provenance field and quartzose recycled field,respectively,implying the source occurred the tectonic activity.Furthermore,geochemical study indicates that the Yanguoxia standstone was formed in an unstable continental setting due to the northwards movement of Indian Plate triggered the collision between the Qilian fold belt and the Qinling fold belt.These sediments were derived from a mixed source and then deposited in the Lanzhou-Minhe Basin.Most of the felsic components were derived from the granitoid rocks of the Qilian Mountains due to the rapid and intense uplift during the Early Cretaceous while the mafic components were contributed by the basic and ultrabasic rocks of the rapidly rising ophiolite in the Qilian Mountain area.Bivariant log-log plot of Qp/(F+L) (Qp,polycrystalline quartz) against Q/(F+L) shows that Yanguoxia Formation was deposited in the semihumid and semi-arid.Moreover,the pollen also exhibits that the environmental condition during the deposition of Yanguoxia Formation was warm and wet,which affirm such environment was benefit to dinosaur survival.Geochemical study also infers that the Yanguoxia Formation was deposited under the oxidizing condition in a shallow marine environment.The minerals identified from the X-ray diffraction (XRD) analysis of shale and siltstone samples are 4.74％-33.53％ clays,23.45％-41.70％ carbonates and 33.99％-71.81％ quartz,respectively,which infer that depositional conditions remained uniform during the formation of shales or siltstones of Yanguoxia Formation.