Sedimentary dynamics and climatic implications of Cretaceous loess-like red beds in the Lanzhou basin, Northwest China

The Chinese loess has been widely studied in order to reconstruct Cenozoic environmental change. Cretaceous loess-like red beds with alternating red and gray layers in the Lanzhou basin, Northwest China are well preserved and can be used for understanding the Mesozoic environmental and climatic changes. However, the red beds and gray clays are less studied and their significance is not clear. Hence, Cretaceous red beds of the basin are studied. The gray layers show horizontal lamination and are deposited in a lacustrine environment, whereas the red layers show gradational change in texture and color, and are analogous to paleosols. The red and gray clays show similar immobile trace elemental compositions and identical grain size distributions, suggesting that the sediments of the two deposits are derived from the same source and carried by identical transporting forces. The dominant 2-10 mu m fine grain size component indicates that the deposits are sorted through long-distance transportation. In this context, the magnetic properties of the red beds are mainly determined by post-depositional environmental development. Hematite is the dominant magnetic mineral and some of the gray layers contains small quantities of magnetite. Therefore, the red and gray layers develop on land and under water, respectively. The magnetic parameter IRM-100mT eliminates the effects of ferromagnetic minerals and is sensitive to hard magnetic minerals. Thus, it can be used as a proxy for paleoclimatic change. Its high values in the red layers correspond to a drier paleoclimatic environment.