New insights into the development of the Late Jurassic Reuchenette Formation of NW Switzerland (late Oxfordian to late Kimmeridgian, Jura Mountains)

In the Ajoie-Region, seven in situ collected species of ammonites helped to establish a new biostratigraphical an dlithological frame for the platform sediments of the Reuchenette Formation, i.e. eighteen closely spaced sections have been spliced by means of lithological, sedimentological, microfacial data and index-fossils (ammonites). Three marker beds achieved the exact lithological correlations between the outcrops corroborated by the vertical faciese changes.
Based on the biostratigraphical data five 3rd order sedimentary sequences could be asigned to the Late Oxfordian to Late Kimmeridgian time interval. The sequence boundaries lie within the Planula-, Platynota-, Divisum-, Acanthicum- and Eudoxus-Zone. The upper three 3rd order sequences correspond to the Boreal sequences Kim3 to Kim5 of Hardenbol et al. (1998). The deduced "large-scale" sea level trend matches those from other Euoprean regions (Spain, Russia).
At platform scale, this time control and further outcrops south of the Ajoie-Region, in combination with mineralostratigraphical and lithological marker beds, allowed the correlation and dating of the thickest sections - including the type-section - of the Reuchenette Formation and thus serve to oimprove the previously estimated ages of their sequence boundaries (see above).
The variability of stacking pattern and facies between the sections also reveals distinct changes in facies evolution occurring across Late Palaeozoic basement structures and suggest synsedimentary differential subsidence. These sturctures acted as important controlling factors for the distribution of the sediments of the Reuchenette Formation besides the sea level fluctuations. The interplay of sea level changes and synsedimentary differential subsidence is outlined by lateral thickness variations and conspicuous laterally changing 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 and the Tethys. During the Planula- to Divisum-Zone time interval 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-Zone time interval this platform changed into a pronounced basin-and-swell morphology with specific depositional environments and "separated" the Paris Basin from the Tethys. Dinosaurs might have used this boundary to traverse between the Central Massif and the London-Brabant Massif during sea level lowstands.