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Environmental changes during the Tertiary: the example of palaeoweathering residues in central Spain

Abstract In central Spain three main weathering phases are found which can be put in a chronological sequence with tectonic and sedimentation phases. Weathering intensity, as recorded by micromorphological analysis, has plainly decreased during the course of the Tertiary reflecting the prevailing environmental conditions. During the Palaeogene intensive chemical activities produced, on the mineral grains of the deeply decomposed rocks, features typical of humid tropical climate. In the Miocene the Palaeogene regolith of the southern Tajo Basin was truncated and partly incorporated into sediments which comprise reworked weathering residua of this basin and of the adjoining Montes de Toledo. After sedimentation another weathering phase started. During this phase the feldspars were heavily corroded, but in contrast to the Palaeogene residues, the quartz shows hardly any features caused by etching. In the Pliocene the rañas were deposited during a second main transportation phase. Particularly in the SE of the Montes de Toledo, these sediments were subjected to intense weathering, but only where conditions were favourable. Due to sufficient soil humidity in intramontane basins, high temperatures, and presence of iron, quartz corrosion was locally possible once more during Pliocene times. The rañas of the Tajo Basin, however, were substantially less attacked.