Long-distance correlations by sequence stratigraphy and cyclostratigraphy: examples and implications (Oxfordian from the Swiss Jura, Spain, and Normandy)

Abstract

 The Oxfordian sedimentary successions studied in the Swiss Jura, in Normandy, and in the Soria and Cazorla regions of Spain display complex facies evolution and stacking patterns. Based on biostratigraphy and absolute age dating, it is suggested that the shallow-water depositional settings in the Jura, Normandy, and the Soria region as well as the deeper-water environments in the Cazorla region, recorded climatic and sea-level fluctuations in the Milankovitch frequency band. Beds and bedsets corresponding to 20-, 100-, and 400-ka cyclicities can be identified. Facies evolution inside such small-scale sequences and also in the larger sequences of million-year scale is interpreted in terms of sequence stratigraphy. Superposition of high-frequency cyclicity on a longer-term sea-level trend led to multiplication of diagnostic surfaces: sequence-boundary and maximum-flooding zones in the large-scale sequences can thus be defined. These zones are correlated between closely spaced sections, but also from the Swiss Jura to Normandy and to Spain. The narrow time lines given by Milankovitch cyclicity then allow comparison of facies evolution in the different regions on a scale of 100 ka or less. By filtering out local effects of differential subsidence and sediment supply, a long-term sea-level curve valid for the northwestern margin of the Tethys ocean can be reconstructed for the Middle to Late Oxfordian. Differential subsidence is implied from varying thicknesses of the sequences as well as from the distribution of siliciclastics which have been channelized through depressions. Tilted blocks, reduced sedimentation, or increased input of siliciclastics appearing at the same time in all study areas point to a widespread regional tectonic event. Distribution through the sequences of climate-dependent facies components such as corals, ooids, palynomorphs, and siliciclastics indicates that climate changes were dependent on atmospheric circulation patterns and thus on paleolatitude. Rainy periods and related increase of siliciclastics in the Swiss Jura were more abundant during low sea-level stands, whereas in the Soria region they coincided with sea-level highs. Through the combination of high-resolution sequence stratigraphy and cyclostratigraphy, and supported by biostratigraphy and absolute dating, it becomes possible to analyze paleoenvironmental changes in a very narrow time framework.