MESOZOIC

SEDIMENTOLOGY
   

Sedimentological analyses

are an important part of palaeontological excavations, and consequently, the measuring of a sedimentological log is first carried out on all excavation sites. This log attributes a number to each bed (layer) and/or dinosaur track-bearing surface (level). It defines the lithological sequence, allows to reconstruct the evolution of the palaeoenvironment in great detail, and provides important taphonomical information. Further, it is important to locate and correlate the most fossiliferous beds.
The Late Jurassic of the Ajoie region is essentially made up of shallow marine carbonates, calcareous marls or clays (in function of the degree of continental erosion), and to a minor degree calcareous sandstones. All these sediments have been deposited on the Jura carbonate platform located at the threshold between boreal depositional areas to the northwest (Paris Basin) and the Tethys sea to the south.
Macroscopic observations in the field and microscopic small-scale microfacies analysis using standard microfacies classification systems (Wilson, 1975; Flügel, 1982, 2004), define specific facies and facies zones (e.g., Ziegler, 1962; Jones & Desrochers, 1992; Dupraz, 1999, Hillgärtner, 1999; Hug, 2003). Each facies type corresponds to a characteristic depositional environment (e.g., reef, ooid bar, lagoon, tidal flat, beach, etc.), defined by different environmental factors (restriction, bathymetry, energy, salinity, oxygenation).

Fig. 1: Thin-section CTD-SCR-950.3 (uppermost part of layer 950), pack- to grainstone (Dunham, 1962), bio-, pel-, oo-micrite to –sparite (Folk, 1962), showing small ooids (1), peloids (2), intraclasts (3), organic matter (4), dolomite (5), bivalve shell fragment (6) and benthic foraminifer (7). The depositional environment can be interpreted as a mixed bioclastic-oolitic bar or lower-beach deposit in a shallow-subtidal (layer 950) to intertidal (level 1000), semi-open lagoon with intermittent, low to moderate energy. Image width is 6 mm.
Further sedimentological analyses (clay mineralogy, Rock-Eval pyrolysis, stable isotopes, organic matter (SEM),…) are performed in collaboration with Universities and laboratories in Switzerland and from abroad.