THE OCCURRENCE OF MARINE FOSSILS IN THE L ATEST CRETACEOUS OF THE CENTRAL ANDES AND ITS PALEOGEOGRAPHIC IMPLICATION

The Maastrichtian deposits (late Campanian? to early Danian?) of the c entral Andes comprise three transgressive-regressive sequences. In Bol ivia (El Molino Formation) they have yielded diverse terrestrial, brac kish-water or marine fossils, which make it possible to define the dep ositional environments at their exact stratigraphic position. Marine-i nfluenced transgressive facies are by far the best developed in the fi rst sequence. Among the vertebrates, five groups of fishes are restric ted, with some rare exceptions, to marine environments. Some other fis h families, genera or species, are known to occur both in freshwater, brackish and marine environments and therefore provide no precise envi ronmental indications. Among the invertebrates, echinoids, serpulids, foraminifera and some pelecypods and gastropods attest to a marine-inf luenced environment, which may however have experienced periodic salin ity variations. The other pelecypods and gastropods generally indicate freshwater environments. The ostracods, often ubiquitous, only provid e paleoecologic indications when they constitute oligospecific shell b eds, suggesting abnormally low salinity values. Calcispheres and dinof lagellates indicate a stronger marine influence. Charophytes, especial ly when abundant, suggest a lacustrine environment. Because of their s tratigraphic and paleogeographic positions, stromatolites may indicate intertidal to supratidal environments. The fossils most likely of mar ine origin generally occur in the transgressive lower parts of the thr ee sequences, where marine oolites and glauconite are also observed. T he often endemic nature of the marine assemblages, the common occurren ce of dwarfism and the lack of true nektonic forms suggest that most p art of the basin was permanently rather shallow and subjected to impor tant, frequent and/or local, salinity variations. The freshwater fishe s appear to be reworked and dissociated in the marine levels, but are better preserved in the mainly continental upper parts of the sequence s. Marine fossils are rare in the upper sequence. These data contradic t with interpretations which suggest that the central Andean Senonian deposits are exclusively terrestrial. Given other paleogeographic cons traints, we propose that Maastrichtian sedimentation took place in a w ide, elongated, subsident basin, forming the eastern foreland of the p aleo-Andes and showing a ''cul-de-sac'' shape in the south. This restr icted basin was periodically connected with the open sea through prese nt-day Venezuela. The salinity of its southern part, situated far from the basin mouth, was shallow and affected by fresh water from inflowi ng rivers.