Geochemical characterization of sedimentary basins by statistical analysis: The Mio-Pliocene sequences of the Vera Basin, SE Spain

Abstract The origin and history of sedimentary rocks can be deciphered from the trace elements in their clay minerals. The inheritance from weathering profiles is detected by the analogy in trace-element amounts of the μ m fractions of the sediments and the presumed parent rocks. The paleoenvironmental conditions are printed in the clay fraction by specific elements like B or by anomalous amount associations of elements not related with parent rocks. The geochemical environment during the deposit of Mio-Pliocene sediments of the Vera Basin (SE Spain) and the influence of their parent source rocks were studied by statistical analysis of the chemical composition of the μ m fraction of these sediments and the bulk samples of their surrounding parent rocks. Principal components analysis (PCA) was used to find the relationships among the chemical elements, the minerals and the parent rocks, and to select the most discriminating elements. Sr, Ni, Cr, Cu, Co, V, Sc and Zn appeared so to be representative of the clay fractions and the parent rocks, and were therefore selected to obtain information about the parent rocks of the sediments. A hierarchical ascending classification (HAC) was performed with these elements, it regroups samples with similar geochemical characteristics. Graphical comparison of mean geochemical composition among HAC groups provided a better identification of inherited similarities between some sediments and the parent rocks. Most of the clay minerals in the sediments appeared so to have been formed by weathering of the metamorphic basement. Only one laminite sample from Cuevas de Almanzora section in the north of the basin was identified as formed by weathering of volcanic rocks. Volcanic weathering fluids enriched some clays from the Coscojar, Garrapancho and Garrucha sections in Sr. Some Cu and Zn enrichments acquired during early diagenesis are also evidenced by graphic comparisons. B and Mg contents were used to indicate paleoenvironments. They distinguished two different sedimentary environments, separated by a paleogeographical high situated at the site of Garrapancho Hill. Higher B contents in the southern part of the basin indicate a more marine environment than in the north composed of several subbasins with different salinities correlated with varying inputs of fresh water. The Mg contents are always higher in the north, suggesting a more confined environment. The sediments in the south were deposited in higher-energy environments as shown by the presence of olistostromes whereas in the north marls and clayey laminites suggest a calmer and more monotonous environment.