Micromorphology of hydromorphic soils developed in fluvio-marine sediments during the Middle-Late Pleistocene transit in the Gulf of Cadiz (Atlantic South Spain).

Abstract

This study establishes the controlling factors in the soil development in the ancient coastal plain of the Guadalquivir River along the southern Spanish coast (Huelva) shortly before 100 ka BP. The macro and micromorphological study indicates that a sedimentation hiatus allowed soil formation (extensive clay illuviation) together with the development of widespread redoximorphic features linked to iron oxide depletion and accumulation. Groundwater fluctuations driven by the overall sea-level rise during the onset of the last interglacial period triggered the pervasive occurrence of redox processes, probably acting in the coastal zone from the whole Middle Pleistocene as witnessed in inland outcrops. The resulting redoximorphic pedofeatures show a differential development in the studied zone, which is linked mainly to the activity of the Torre del Loro Fault leading regional differential upwarping of the ancient coastal area as well as local water-escape processes in the fault zone. Petrological, mineralogical and micromorphological data strongly suggest that the occurrence of multiple inheritances of iron-rich pedofeatures from previous Plio-Pleistocene weathering profiles developed under warmer and humid climatic conditions. Several pedofeatures (corroded quartz grains and runiquartz) do not correspond to the temperate oceanic climate prevailing in the zone during soil formation, and are reworked from former stronger weathering conditions. They are included in strongly iron-impregnated opaque domains that can be interpreted as inherited iron-rich nodules or clasts already present in the host sediments. These iron-rich pedofeatures have been subject to subsequent iron mobilization giving place to the iron depletion and accumulation domains observed in the micromorphological scale. Differential iron bleaching is not always linked to the fissure and/or pore-system present in the groundmass, suggesting the occurrence of differential water and sediment vertical fluxes (micro water-escape features) within the soil enhance the redoximorphic features around the fault zone.