Silcrete and alunite genesis in alluvial palaeosols (late Cretaceous to early Palaeocene, Duero basin, Spain)

An extensive formation of silcretes occurs in the upper half of a late Cretaceous-to-Palaeocene Siderolithic Unit consisting of conglomerates, sandstones and sandy mudstones along the western margin of the Tertiary Duero basin (Spain). The unit is arranged in fining-upward sequences that were deposited in a fluvial braided system with floodplains subject to periodic events of exposure and drying out. The original mineralogy of the Siderolithic Unit is seen in the less altered levels, where it consists of quartz, illite–white mica, kaolinite and, at the top of the unit, K-feldspar, all coming from the palaeoalteration mantles developed on the metasediments of the Variscan Basement. The unit is characterized by a marked development of alluvial palaeosols at the uppermost part of each sequence. At field scale, the pedogenic structures and features are evident from the mottled pattern, burrowing, cracking planes and root traces; at micromorphological scale, an intense development of birefringent fabrics is seen in clays. Micas and feldspars show evidence of alteration due to hydrolysis, while the more resistant quartz grains show common corrosion gulfs and have become the only surviving elements in the most silicified beds. The neoformed minerals are related to the pedodiagenetic processes developed on the floodplains. The main neoformed phase is opal (probably amorphous silica in origin that aged to opal-CT), which ranges from local cementations in pores to an almost complete replacement of the rock, specially the finest components (matrix). Kaolinite is the most abundant neoformed clay. Locally, veinlets of alunite accompany the opalized levels.All the processes described occurred in a seasonal tropical climate. During dry episodes, strong evaporation resulted in an increase in pH, favouring the hydrolysis of the original silicates. The paucity of Mg2+ and Ca2+ in the groundwater would have favoured the increase in pH. In ensuing wetter periods, pHs fell rapidly, resulting in the precipitation of kaolinite and also favouring the precipitation of opal and alunite. The alunite could have formed either by oxidation of the iron sulphides developed in reducing areas of the flood plains or by evaporitic concentration of sulphate in Ca2+- and Mg2+-free groundwaters.