Abstract
Newly formed smectite and palygorskite and their association are good proxies of a subtropical climate alternating dry and warm/ humid seasons during the late Cretaceous during the formation of the Chu-Syrasu basin. The association of fine-grain clays, smectite and fibres (palygorskite) and the occurrence locally of grains of albite, and natrolite, indicate they formed from water, slightly alkali-rich, and enriched in silica and magnesium. These clays may result partly from the alteration of volcanic rocks (glass) either in situ in case of volcanic emissions during sedimentation or close as smectite are euhedral and palygorskite well preserved. The flood plain may have been submitted during the hot season to drying, favouring the formation of brines which interacted with volcanic glass. Evaporation processes could have thus triggered the oversaturation with respect to smectite and palygorskite. Besides, muscovite as coarse grain particles, illite and chloritized biotites attest to a second source compatible with the coarse grain microcline and quartz, which can derive from granites. Source rocks could be, therefore, dual, with acid plutonic series (peraluminous granites probably) releasing coarse-grained detrital phyllosilicates (muscovite and biotite-chlorite) transported together with quartz and feldspars by rivers, and volcanic series, altered into newly formed clays (smectite and palygorskite).