Intensive kaolinization during a lateritic weathering event in South-West Spain

Abstract

Abstract This study documents the mineralogical and geochemical record of a lateritic weathering event during the Pliocene in South-west Spain. The paleoweathering profile derived from arkosic sands and comprises a white sandy clayey saprolite, a red mottled clay zone overlain by a thick soft layer, and a ferruginous pisolitic hardcap partially dismantled by erosive processes. Kaolinite, quartz and degraded K-feldspars are the major minerals forming the saprolite, along with Al-goethite (mottled zone) as well as gibbsite and boehmite (soft layer), while the pisolitic duricrust is essentially composed of quartz grains embedded in a matrix of Fe oxy-hydroxides (hematite, goethite, and/or maghemite). Besides quartz, the phases most resistant to alteration were a variety of heavy minerals (mostly titanium oxides) that are present as minor impurities in the residual kaolin deposit. The dominant weathering process of kaolinization is geochemically characterized by striking losses of Na, K, Ca, Mg, and large-ion lithophile elements (Rb, Sr, Cs, Ba, and U), because of the almost entirely breakdown of primary minerals (mainly feldspars and 2:1 phyllosilicates). The Ti-normalized distribution patterns of trace elements also reveals remobilization and redistribution processes within the profile, involving high field strength elements classically regarded as immobile, like Zr and Hf. Iron showed a contrasted behaviour, with depletion in the saprolite zone and strong accumulation in the pisolitic duricrust. Various transition trace metals (Cr, V, Sc, Th, Nb, and Ta) remained relatively immobile during the weathering, and were residually concentrated in the duricrust linked to the Fe oxy-hydroxides. The geochemical and mineralogical evolution of the paleosol studied is consistent with a laterization process evolved under humid subtropical conditions, and driven by morphodynamic and hydrodynamic factors (gently sloping coastal area with fluctuating water table and rapid water percolation) favourable for the development of a deeply kaolinized paleosurface.