Platform-to-basin correlation by high-resolution sequence stratigraphy and cyclostratigraphy (Berriasian, Switzerland and France)



Based on detailed analyses of facies evolution and stacking pattern of Berriasian carbonate-dominated sections in the Swiss Jura Mountains, the Swiss Ultrahelvetic and the French Vocontian Trough, a high-resolution platform-to-basin correlation is proposed. Biostratigraphical tie points are furnished by ammonites, dinoflagellates, calpionellids, and ostracod-charophyte assemblages. The hierarchical stacking of small-scale depositional sequences reflects Milankovitch cyclicity: sequences corresponding to the 20-, 100-, and, locally, 400-ka orbital cycles can be identified. Elementary (20 ka) sequences on the platform generally consist of one bed of shallow subtidal to intertidal, high- or low-energy carbonate facies, whereas on the slope and in the basin they are commonly developed as limestone-marl couplets. These elementary sequences group into small-scale composite sequences reflecting the first orbital eccentricity cycle (100 ka), which in turn build up large-scale (3rd-order) composite sequences. One 3rd-order sequence has been analysed in detail: according to the cyclostratigraphic interpretation, it took ≃2 Myr to form, which is in accordance with the duration of the corresponding ammonite subzones. Sequence-stratigraphic and cyclostratigraphic platform-to-basin correlation shows that for about 900 ka the platform was exposed or only partly flooded, whereas on the slope and in the basin, lowstand deposits with channel fills and slumps accumulated. With rising sea level, accommodation space gradually increased on the platform and a thickening-upward sequential pattern with transgressive facies developed, while in the basin the facies still had lowstand characteristics with thick and nodular limestone beds. This situation lasted about 700 ka. The following 300 ka were characterized by sediment starvation and increased bioturbation on the platform, and by more marly, transgressive sediments on the slope and in the basin. The maximum-flooding phase is more or less isochronous on the platform and in the basin, although the surface with the best-developed maximum-flooding features may be displaced by one or two small-scale composite sequences because of superimposed high-frequency sea-level fluctuations, and/or local variations in substrate morphology and sediment distribution. Third-order highstand conditions prevailed for only about 100 ka. The combination of cyclostratigraphy and high-resolution sequence stratigraphy, constrained by good biostratigraphy, is thus a powerful tool for detailed stratigraphical correlation over long distances and from one sedimentary environment to another.