KAOLIN MINERALOGY OF CLAYS IN PALEOSOL PROFILES ON THE LATE-MIOCENE SEDIMENTS IN PENGHU ISLANDS (PESCADORES), TAIWAN

Clay mineral compositions from 2 paleosol profiles (Chu-Wan, CW, and S hiao-Men Yu, SMY, profiles) on the late-Miocene sediments in Penghu Is lands (Pescadores), Taiwan, are characterized by random X-ray powder d iffraction (XRD) and transmission electron microscopy (TEM). By the cl ay assemblage of the paleosol profile, we want to explore the probable formation mode of the Penghu paleosols.The paleosol profiles in study are overlain by a layer of basalt flow. However, the clay mineralogy of the 2 paleosols was not altered metasomatically after burial. Resul ts show that 3 distinctive zones of different dominating kaolin-group minerals are apparent in the profiles. In descending order, they are: 1) spheroidal, hollow 7 Angstrom-halloysite, 2) platy, irregular-shape d and disordered kaolinite, and 3) platy, irregular-shaped, disordered kaolinite. The relative crystallinity of kaolin minerals of the 3 lay ers is: layer 2 > layer 3 > layer 1. On the basis of the XRD, TEM anal yses and the crystallinity calculations, the distribution of kaolin in Penghu paleosol profiles appears to be unique. Penghu paleosol profil es show systematic change in kaolin crystallinity and polymorphs with depth. Because the clay type is heterogeneous within the profile, this represents that Penghu paleosol profiles were polypedogenic. The cont act between the upper basalt and the paleosol is the erosion surface, so we do not know exactly what the thickness of the original paleosol was. The first layer (about 20 cm) of the profiles appears to be const ituents of the original paleosol. It contains high contents of pedogen ic (in situ weathering) hematites and 7 Angstrom-halloysites, which im plies that the local climate of the Penghu Islands at late Miocene was warm and humid. Intense leaching and dry/wet cycle should be the reas on for high contents of halloysite (>60%) in the Penghu paleosols. Lat erization was the probable pedogenic process for the formation of the paleosols.