Controlling factors on clay mineral assemblages: insights from facies analysis of Pliocene to Pleistocene coastal margin deposits, Western Portugal

Abstract

The clay fractions in the Pliocene to Pleistocene coastal margin record are usually dominated by kaolinite and illite, with lesser amounts of vermiculite, 10-14 mixed layer clays and smectite. The high clay mineral crystallinity, the mineralogical relations to facies and depositional setting and some horizontal variations along coeval deposits suggest that clay assemblages are mainly detrital. Illite is more important in inner shelf deposits and particularly in alluvial deposits from eastern locations. The high illite content in eastern alluvial deposits is explained by the input from the neighbouring Iberian Variscan Massif that is rich in mica. The high kaolinite content in prograding sand and gravel coastal plain is partially explained by the availability of this mineral in the drainage areas. Given the arkosic nature of some of these deposits, post-depositional feldspars weathering would also contribute to an increase in kaolinite content. Vermiculite is particularly important close to the surface and to unconformities, in horizons influenced by pedogenetic processes that have more aluminous illite with relatively low crystallinity. In organic rich mud sediments low pH conditions favour post-depositional transformation of illite to vermiculite and mixed layer clays.