Clay minerals and their paleoenvironmental interpretation in the Baoji loess section, Southern Loess Plateau, China

Clay minerals from the loess-paleosol section at Baoji in north-central China were studied by X-ray Diffraction. Major components, calculated by comparing major peak heights, were estimated as follows: illite (55–75%), kaolinite (10–25%), mixed layer chlorite-vermiculite (10–20%), chlorite (< 7%), and small amounts (< 5%) of smectite and mixed layer illite/smectite. An increasing trend in illite in paleosols and loess from approximately 1.2 Ma to the last glaciation may be indicating an increase in aridity. Likewise an increase in chlorite and chlorite-vermiculite near the base of the Brunhes chron to the present may indicate a decrease in weathering intensity. One of the last warm phases prior to the onset of loess deposition at 2.5 Ma is represented in the Tertiary Red Clay formation by a higher than average kaolinite content (20–25%), a relatively low illite (55–60%) content, and about 15% chlorite/chlorite-vermiculite. The kaolinite content decreases toward the boundary between the Red Clay and the overlying loess-paleosol sediments. This may indicate weaker weathering resulting from climatic deterioration near the Matuyama-Gauss boundary.