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The Berriasian Pierre-Châtel Formation in the Swiss and French Jura Mountains is dominated by shallow-marine carbonates that overlie lacustrine and marginal-marine sediments with a major transgressive surface. Detailed facies analysis of five sections allows the definition of elementary and small-scale depositional sequences, which commonly exhibit deepening–shallowing trends. Benthic foraminifera and rare ammonites on the platform, as well as a sequence-stratigraphic correlation with a well-dated deeper-water section, furnish the biostratigraphic framework. Thus, the large-scale sequence boundaries below and at the top of the Pierre-Châtel Formation can be correlated with dated boundaries in other European basins. This time constraint and the hierarchical stacking pattern on the platform as well as in the basin suggest that the sea-level fluctuations influencing the formation of the depositional sequences were controlled, at least partly, by Milankovitch cycles. The elementary sequences correspond to the 20 ky precession cycle, and the small-scale sequences to the 100 ky eccentricity cycle. Uncertainties in the definition of sequences exist if facies contrasts are too low to develop clearly marked sequence boundaries or maximum-flooding intervals. Nevertheless, a best-fit solution for the correlation of the small-scale sequences between the studied sections can be proposed. The lowermost three small-scale sequences of the Pierre-Châtel Formation are analyzed in detail. They are decompacted and correlated on the level of the elementary sequences. Within this relatively precise time frame, the flooding of the Jura platform (following the early Berriasian sea-level lowstand) can be monitored. It is seen that the transgression occurred stepwise: every 20 ky, a transgressive pulse established marine facies farther towards the platform interior. This study demonstrates that the cyclostratigraphical approach makes it possible to construct a narrow time frame, within which the rates of sedimentary, ecological, and diagenetic processes can be evaluated, phases of differential subsidence identified, and the durations of stratigraphic gaps estimated. The complex and dynamic evolution of an ancient carbonate platform can thus be studied with a time resolution of 20 to 100 ky. |
