LAS FACIES DE GRAPTOLITOS. ENSAYO SOBRE SU CLASIFICACION Y GENESIS

Abstract

On the basis of geological observations made in Lower Paleozoic series of Argentina and Bolivia, graptolitic sequences are c1assified as follows: i) flysch ; ii) euxinic and iii) derni-euxinic sequences. In the first one the occurrences of graptolites are occasional and could be absent. Very few genera and species are represented and from a biostratigraphical point of view, these sequences are discontinuous. The euxinic sequences are characterized by a rich graptolitic faunas located throughout the sedimentary column. The graptolitic faunas, rich in genera and species, are arranged in biostratigraphical zones. The demi-euxenic sequences are a combination of both f1ysch and euxinic facies vertically arranged. The graptolitic faunas are abundant at the base of the sedimentary column (euxinic facies), but diminishes upwards until its disappearence. In the upper part of the column (f1yschfacies), the graptolitic occurrences are occasional. In each case the lithological characteristics are given. On theoretical considerations, it is argued that the graptolitic faunas were not important in the planctonic communities of Ordovician and Silurian seas. It is suggested that euxinic basins acted as "biological filters" that retained the planctonic biological elements and, among them, the graptolitic faunas.