Rhythmic lacustrine sequences with silcretes from the Madrid Basin, Spain: Geochemical trends

The Madrid Basin is a complex intracratonic basin located at the center of the Iberian Peninsula, Spain. The basin was filled by a thick succession of continental sediments of Tertiary age. During the Miocene, the area was a closed basin where a succession of alluvial and lake complexes developed. Three main stratigraphic units have been distinguished (Lower, Intermediate and Upper). In the western part of the Madrid Basin, the arkosic alluviums of the Intermediate Unit grade laterally into marginal lake facies with Mg-clays (sepiolite and magnesian smectites). Well-developed rhythmic sequences located in the transition between the arkosic alluvial fans and the marginal lake facies are present in the vicinity of Madrid. This paper deals with a petrological and geochemical study of four rhythmic sequences included in arkosic sediments. Each sequence is constituted of a lower part of mudstone (unit A) and an upper part of argillaceous opal (unit B). Unit A slows massive structure and dark to brown color. Unit B ranges from a clear brown to a cream colour and presents different structures (globular microstructures, siliceous-rich micronodules and bioturbation). The transition between units A and B is not sharp, but with transitional facies having a laminated structure, white micronodules and white lenticular milimetric levels.