Sedimentología química de los sedimentos miocenos con diatomeas de la Cuenca del Guadalquivir

Abstract

The chemical data of major elements is used to study the precise lithological composition and the sedimentology of Miocene diatomaceous sediments from 24 sections of the Guadalquivir Basin. Three main episodes (Lower, Middle and Upper), defined on the basis of diatom zones have been studied. They record a highly productive coastal upwelling regime in the area during the Miocene. The wide range of variation of SiO₂ (12.1878.00), CaO (1.60-43.73) and Al₂O₃ (2.76-10.32) contents reflects the important variations of the main components: Opal A, calcite (mainly biogenic) and clay minerals-terrigenous components. Biogenic silica has been estimated by using normative calculations after determination of the mineralogical compositions of the samples by means of X-ray diffraction. Maximum and minimum contents in biogenic silica are related to the dominant diatom taxa, the structure of the rock and the geographicallocation in the basin. The ternary diagrams SiO₂ biogenic- Al₂O₃-CaO reveal that the Upper Episode has the highest amounts of clay minerals-terrigenous components. Plots of SiO₂ biogenic/CaO versus Al₂O₃show a relation between the biogenic calcite and the clay-minerals-terrigenous components. The plots of Fe₂O₃/TiO₂ versus Al₂O₃/ Al₂O₃+ Fe₂O₃ show that most of the samples may be interpreted as deposited in a continental margin enviranment. The data obtained fram the slope of linear regressions Fe (y) - Al (x) are indicative of areas with varying influence of a terrigenous source. The MnO/ Al₂O₃ ratio is used to estimate rates of sedimentation of the diatomaceous sediments. The range is between 1.7 and 180 mm/10³ years, which is consistent with the sedimentation rate values fram sediments formed in coastal upwelling regimes. The rate of sedimentation varies in relation to the structure of the rack, the dominant diatom taxa and the age of the episodes.