MORPHOLOGY OF MICROBIAL CONSTRUCTIONS IN AN OLIGOCENE FAN-DELTA - EXAMPLE FROM THE RHINE RIFT (WESTERN-EUROPE)

At the Eocene/Oligocene boundary a first paroxysmal phase of subsidenc e created the Rhine rift graben system. A lacustrine basin with variab le salinity was formed in the graben. In the southern part of this bas in a large fan-delta developed, extending northward along the graben a xis. Microbial ecosystems settled on the fringe of the lake. In the pa ludal zone, they formed carbonate encrustations on living plants or de trital vegetal remains. Littoral carbonate facies occur as algal biohe rms an stromatolitic reefs. In the distal lacustrine zone, a discrete nodular texture within the sediment reveals the microbial origin of mi crite and gives the limestone a breccial pattern. All these facies ill ustrate the lateral zonation of the different types of constrictions f rom fluvio-deltaic to lacustrine. In certain cases, the microbial coat ing on plants permits to reconstruct seasonal rhythms and the processe s of substratum colonisation.