Pedogenic and early diagenetic processes in Palaeogene alluvial fan and lacustrine deposits from the Sado Basin (S Portugal)

Abstract

The Palaeogene deposits of the Sado Basin were deposited in a continental basin that shows a typical pattern with alluvial fans system in the margins of the basin, passing towards distal areas of mudflat facies where, in some areas, the installation of shallow water bodies favoured the development of palustrine conditions. The deposits of this basin vary form coarse conglomerates and sandstones to palustrine carbonates. These sediments were affected by pedogenesis and early diagenetic processes that promoted important modifications on their primary features. These modifications have been studied by the analyses of four profiles, developed on proximal, middle and distal fan deposits and the fourth one on lacustrine deposits. The overall analyses of the sedimentological, pedogenic, diagenetic features and their relationships indicate that three main processes took place throughout the basin: soil formation, palygorskite neoformation and dolomitization. Soil formation processes led to illuviation of clays and carbonate precipitation mostly around roots. Pedogenic carbonates increase towards distal areas, whereas hydromorphic features are present throughout the basin. Palygorskite neoformation was partially diagenetic, being maximum in proximal areas and palustrine deposits. This neoformation is attributed to the percolation of alkaline Mg-rich soil and groundwaters through smectitic-rich sediments, promoting important clay transformation. Dolomitization was an early diagenetic process that occurred mainly in carbonate-rich deposits of distal and lacustrine environments, as a result of the increasing Mg/Ca ratio of the percolating groundwaters. In all these processes there has been a close spatial and temporal interplay between pedogenesis and diagenesis, driven by the chemistry of soil particles and groundwaters.