Late Oxfordian to Late Kimmeridgian carbonate deposits of NW Switzerland (Swiss Jura): Stratigraphical and palaeogeographical implications in the transition area between the Paris Basin and the Tethys

Abstract

Geological sections of the shallow-water, carbonate-dominated sedimentary system of the Late Jurassic Reuchenette Formation in northwestern Switzerland have been studied between the southern Jura Mountains and the Tabular Jura. The largest sections show a characteristic cyclic stacking pattern. Up to now, the age of these sediments (including the type-section) linking the Boreal and Tethyan realms, was biostratigraphically poorly constrained. In the Tabular Jura five 3rd order sequences can be assigned to the Planula- to Eudoxus-Zone (Late Oxfordian to Late Kimmeridgian) using index-fossils (ammonites and ostracodes; [Jank M., 2004, New insights into the development of the Late Jurassic Reuchenette Formation of NW Switzerland (Late Oxfordian to Late Kimmeridgian, Jura Mountains). Dissertationen aus dem Geologisch-Paläontologischen Institut der Universität Basel, 32, 121 pp.]). This time control and several new outcrops, in combination with mineralostratigraphical and lithological marker beds, allow the correlation and dating of the thickest sections of the Reuchenette Formation and thus serve to improve the previously estimated ages of their sequence boundaries. The variability of stacking pattern and facies between sections also reveals distinctive changes in facies evolution, related to Late Palaeozoic basement structures and synsedimentary subsidence. These structures acted as important controlling factors for the sediment distribution of the Reuchenette Formation besides the sea level fluctuations. The interplay of sea level changes and synsedimentary subsidence is outlined by lateral thickness variations and shifting depositional environments. A close examination of these changes also sheds much light on the nature of platform topography in the transition area between the Paris Basin in the north and the Tethys in the south, or more generally between the Boreal and Tethyan realms. During the Planula- to Divisum-time-intervals the study area was a flat platform with a more or less uniform facies distribution, which connected the above-mentioned realms. During the Divisum-to Acanthicum-time-intervals this platform changed into a pronounced basin-and-swell morpoholgy, with specific depositional environments and “separated” the Paris Basin from the Tethys.