Chronostratigraphic distribution and genesis of palygorskite in Tertiary sediments of the Isfahan region, central Iran

Abstract

AbstractNo comprehensive study has yet been conducted to determine the chronostratigraphic distribution of palygorskite in the Tertiary sediments of Iran. Thirty sediment samples of different Tertiary epochs were taken, based on the field observations and geological maps. The clay fraction of samples was then investigated by X-ray diffraction (XRD), transmission and scanning electron microscopy (TEM and SEM), and inductively coupled plasma mass spectrometry (ICP-MS). Results showed that sediments of the Miocene and Pliocene had large amounts of palygorskite whereas no trace of this mineral was found in the sediments from the Palaeocene, Eocene and Oligocene. Geochemical analyses revealed that sediments younger than the Oligocene had greater amounts of soluble Mg and H4SiO4and a higher pH than those of the Palaeocene and Eocene. The stability diagram of the smectite-palygorskite system suggests that smectite is unstable and transforms to palygorskite in Neogene sediments. The SEM micrographs showed palygorskite as interwoven fibrous mats, coatings, pore-fillings and pore-bridging material in Neogene sediments. This textural evidence suggests a direct chemical precipitation of palygorskite by dissolution of silicates under the alkaline conditions. The results also suggest that geochemical conditions in the Early Tertiary era, represented by deep-sea conditions in central Iran, were not apparently favourable for palygoskite formation until the Late Oligocene.