GEOCHEMICAL CHARACTERIZATION OF SEDIMENTARY BASINS BY STATISTICAL-ANALYSIS - THE MIO-PLIOCENE SEQUENCES OF THE VERA BASIN, SE SPAIN

The origin and history of sedimentary rocks can be deciphered from the trace elements in their clay minerals. The inheritance from weatherin g profiles is detected by the analogy in trace-element amounts of the <2-mum fractions of the sediments and the presumed parent rocks. The p aleoenvironmental conditions are printed in the clay fraction by speci fic elements like B or by anomalous amount associations of elements no t related with parent rocks. The geochemical environment during the de posit of Mio-Pliocene sediments of the Vera Basin (SE Spain) and the i nfluence of their parent source rocks were studied by statistical anal ysis of the chemical composition of the <2-mum fraction of these sedim ents and the bulk samples of their surrounding parent rocks. Principal components analysis (PCA) was used to find the relationships among th e chemical elements, the minerals and the parent rocks, and to select the most discriminating elements, Sr, Ni, Cr, Cu, Co, V, Sc and Zn app eared so to be representative of the clay fractions and the parent roc ks, 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 g eochemical characteristics. Graphical comparison of mean geochemical c omposition among HAC groups provided a better identification of inheri ted 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 Almazora section in the north of the basin was identified as formed by weathering of volcanic rocks. Volcanic weathering fluids en riched some clays from the Coscojar, Garrapancho and Garrucha sections in Sr. Some Cu and Zn enrichments acquired during early diagenesis ar e also evidenced by graphic comparisons. B and Mg contents were used t o indicate paleoenvironments. They distinguished two different sedimen tary environments, separated by a paleogeographical high situated at t he site of Garrapancho Hill. Higher B contents in the southern part of the basin indicate a more marine environment than in the north compos ed of several sub-basins with different salinities correlated with var ying inputs of fresh water. The Mg contents are always higher in the n orth, suggesting a more confined environment. The sediments in the sou th were deposited in higher-energy environments as shown by the presen ce of olistostromes whereas in the north marls and clayey laminites su ggest a calmer and more monotonous environment.