CONTRIBUTION OF QUANTITATIVE ECOLOGICAL METHODS TO THE INTERPRETATION OF STRATIGRAPHICALLY HOMOGENEOUS PRE-QUATERNARY SEDIMENTS: A PALYNOLOGICAL EXAMPLE FROM THE OLIGOCENE OF VENEZUELA

摘要

This paper deals with an Oligocene section that is stratigraphically homogeneous from both a lithological and palynological point of view. It has been impossible to subdivide it into discrete units, using either taxon-range analysis or assemblage-zone approach based on the relative abundance of palynomorphs. Furthermore, the common multivariate numerical methods used so far with success in the region (Cluster Analysis, Principal Components Analysis, etc.) gave no useful results in this case. The search for cyclicity using palynocycles and ecologs has been also unsuccessful. Instead of considering the section of low interest, an alternative, high-resolution ecological approach was attempted to extract the information contained in these sediments. Paleoecological trends were deduced from statistical methods commonly used in modern and Quaternary ecology, mainly TWINSPAN and gradient analysis, combined with diversity analysis. As a result, the fine-scale stratigraphic variability of the data could be successfully explained in terms of paleoecological succession taking place in upper delta environments, characterised by a complex mosaic vegetation including morichales, herbaceous fern swamps, and gallery forests. The succession could be reconstructed in detail, and would be of indirect stratigraphic value for high-resolution correlation. This is an example of how the search for narrow or biased objectives can hidden significant information. It is more fruitful to have a wider perspective, and to be open to any information that sediments can provide us, without a priori limitations.