Lithostratigraphy of a Cenozoic intra continental basin (Guadiana Basin, SW Spain) by using clay mineral assemblages and geophysical properties

Abstract

In the Iberian Peninsula, Alpine tectonics built several Cenozoic basins that were mainly filled with continental deposits. The Guadiana Basin is located halfway between two of the most important Cenozoic basins of the SW of Spain, the Tajo and the Guadalquivir basins. In this paper, both geophysical and mineralogical studies have been carried out in order to establish the lithostratigraphy of the Guadiana basin. A combination of electric resistivity properties and variations in clay mineral assemblage content of the basin infill was used to identify several depositional sequences within the Cenozoic deposits of the basin. Three depositional sequences, DS1, DS2 and DS3 characterized by differentiated clay content and mineral assemblages were distinguished. The lower depositional sequence, DS1, consists of reddish siltstones and clays that scarcely crop out in the basin. Clay content of the <2 μm fraction is dominated by illite averaging 60% of the total clay content, with kaolinite and a few amount of smectite. DS2 deposits lie unconformably over DS1 and the Variscan basement and consist of a siliciclastic succession where carbonate units occur at bottom and at the top of the depositional sequence. Smectite dominates throughout the entire succession except in the basal carbonate unit where up to 70% of palygorskite is found. In the younger sequence DS3, kaolinite is the dominant clay mineral. Electrical resistivity imaging helped in the physical characterization and horizontal and vertical distribution of each unit and provided good insight on the structure of the Guadiana Basin, allowing to infer the basin structure due to high resistivity contrast between the basement and the sedimentary rocks. In addition, in areas where the lower sedimentary units do not crop out, and borehole information is not available, the geophysical response of the resistivity units imaged can be used to infer the corresponding depositional sequence.

The joint interpretation of the clay mineral assemblages and geophysical properties of the sedimentary units showed to be a useful tool to gather a reliable identification of depositional sequences in basins characterized by scarcity of good outcrops, lack of borehole information and similarity of sedimentary rocks. This is very important to properly correlate the different sedimentary units mapped as well as to infer the basin structure and its evolution through time.