Structural Diagenesis in Clay Smearing Bands Developed on Plio-Pleistocene Sediments Affected by the Baza Fault (S Spain)

Abstract

This study reveals mineral and deformation processes associated with faulting of lacustrine unconsolidated sediments in the Guadix-Baza Basin (Betic Cordillera, S Spain) affected by the Baza Fault. Brittle carbonate and silt sediments develop deformation bands frequently sealed by dolomite crystallization, whereas ductile clay-rich sediments form clay smearing bands where late crystallization of gypsum can be observed. Granular flow and local cataclasis were the main deformation mechanisms in the brittle deformation bands. Flow alignment, grain-boundary sliding, and extrusion were predominant in the clay smearing bands. These water and clay-rich bands reduced shear strength of the faulting process due to their lubricating effect. Beidellitic smectite defines shear foliation of the smeared bands, but Mg-Fe, a K-rich smectite (Fe + Mg > 1 and K content up to 0.8 a.p.f.u), crystallizes in the micropores surrounding brittle clasts produced by deformation pressure shadows. These data suggest that the interaction of micromechanical events, which increased sediment porosity by the generation of pressure shadows, and the flow and concentration of saline fluids in these pores promoted structural diagenesis processes that favoured the beginning of local illitization.