Los depósitos aluviales del paleógeno basal en el sector suroriental de la Cuenca del Duero (provincia de Segovia): evolución y minerales de la arcilla característicos

Abstract

This paper deals with the detritic facies which represents the prearcosic cycle and the lowermost part of the arcosic cycle (Paleogene in age) outcropping in the southeastern margin of the Duero Basin. On the basis of geological mapping and lithological correlation, four lithostratigraphic units, which also presents different clay minerals associations, have been distinguished: Unit T.1.1., composed by clays and quartzose sands, in which the c1ay mineral association is illite + kaolinite; unit T.1.2., of quartzose sands with ferruginous paleosoils, with smectite as the main clay mineral; unit T.2.1., of polimictic conglomerates, arcosic sands and clays, with frequent calcrete profiles and palygorskite is the most abundant mineral; unit T.2.2., composed by arcosic sands with duricrust levels resembling those of the previous unit, and with smectite and illite as the main clay minerals. The first and second units are transitional and they keep a genetic relationship corresponding lo a «First Depositional Sequence», which is large scale coarsening upwards and represents a prograding evolution from a distal braided system (T.1.1.) to a proximal one (T.1.2.) of humid alluvial fans. Units T.2.1. and T.2.2. belong to a «Second Depositional Sequence», which is large scale fining and reflects the transition from proximal (T.2.1.) to medial (T.2.2.) parts of arid to semiarid alluvial fans. From the mineralogical and sedimentological study it may be concluded that, during the deposits of these materials, an important climatic change took place. During the sedimentation of T.1.1., climate was warm and wet producing a clay mineral association (illite + kaolinite) tipical of well drained areas. Unit T.1.2. represents a more arid climate with seasonal fluctuations and long dry periods, yielding smectite as the main clay mineral. Nevertheless, the more drastic change towards arid conditions took place during the sedimentation of the «Second Depositional Sequence», dominated by arid or semiarid alluvial fans. For this reason, this episode presents an important development of duricrusts and palygorskite is the characteristic clay mineral, probably related with the edafic processes found in the frequent calcrete profiles appearing in these units.