As. Bellon et al., GEOCHEMICAL CHARACTERIZATION OF SEDIMENTARY BASINS BY STATISTICAL-ANALYSIS - THE MIO-PLIOCENE SEQUENCES OF THE VERA BASIN, SE SPAIN, Chemical geology, 116(3-4), 1994, pp. 229-243
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.