Title Information

Münchner Geowissenschaftliche Abhandlungen
Reihe A: Geologie und Paläontologie

Volume 5

BERGER, Jean-Pierre: La Transgression de la Molasse Marine superieure (OMM) en Suisse occidentale 1985. [in French] – 208 pp., 94 figures, 16 plates. 29.7 x 21.0 cm. Paperback ISBN 978-3-923871-06-3 Euro 45.00 series: MGA, Reihe A: Geologie und Paläontologie order now

Abstract

The Molasse basin is located along the Alpine chain and is composed of 4 principal groups corresponding to 2 fluviolacustrine periods (Lower Freshwater Molasse, USM, and Upper Freshwater Molasse, OSM) and 2 marine invasions (Lower Marine Molasse, UMM, and Upper Marine Molasse, OMM). The second one (transgression of the OMM on the USM) is studied here with special reference to its western part. On the top of the USM, the lithology principally consists of alternating layers of marls and sandstones with rare limestone beds. The OMM, however, is almost only represented by sandstones with the exception of some rare conglomeratic and silty beds. The fossil record gave excellent biostratigraphical and paleoecological results. The mammals show a slight diachrony of the transgression (NM 2b and NM 3), and the planktonic foraminifers have probably an N 5 age.

With the aid of the fossils and sedimentology it was possible to distinguish different depositional environments which appertain to the continental, fluviatile, lacustrine, intertidal and shallow marine areas. The succession shows 3 types of transgressions:

fluvial dominated ("deltaic"), with coarse and conglomeratic sands (and very poor fauna) transgressing suddenly on the fluviatile deposits of the USM.

linear coast, with middle to coarse sandstones showing parallel stratification and containing a rich benthic fauna; the transgression is also very rapid.

tidal dominated, where the contact between marine sandstones and continental marls has been repeated and where the marine sandstones contain typical sedimentary structures ("mud draps", "herring bone" structures, "bundles"). The marine fauna is very rich and diversified and often contains some species coming from "deeper" water brought by tidal currents.

On the other hand these fauna make it possible to define the ecological conditions of the sea i.e. shallow water (0-20 m, in some cases even 100 m depth), a normal salinity (except at the estuaries), temperatures of more than 20 °C and a strong turbulence. The coast consisted of a sandy beach and dry hills alternating with swamps and humid forests.

Some typical "Rhodanian" and "Bavarian" faunas were found at the same localities, proving the twofold origin of the Molasse sea. It is now clear that the connection Rhône-Vienna (Tethys-Paratethys) was already established at the beginning of the Burdigalian. The possible modalities of this connection are represented by a paleogeographic sketch.