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Abstract

The Oxfordian and Kimmeridgian exposures of the Jura of northern Switzerland show a transition from a shallow-water carbonate platform to a deeper marine basin. The carbonate platform was well established in the northern part of the Jura by Middle Oxfordian time and continued to develop through Late Oxfordian and Kimmeridgian time. The platform is characterized by shallow-water carbonates, sometimes rimmed by coral reefs. The basinal sediments grade distally from marls or turbiditic carbonates into condensed, highly fossiliferous limestones or ironstones. The study is based upon 220 sedimentary logs and utilizes a very detailed chronostratigraphic framework established from a combination of ammonite biostratigraphy and clay mineralogy.

Sequence stratigraphic analysis of the succession reveals that the Oxfordian sediments as a whole represent a 2nd-order regressive-transgressive cycle, punctuated by eight 3rd-order cycles. The onset of the 2nd-order transgression is coincident with a change in the morphology of the carbonate platform from a rimmed-shelf to an epeiric carbonate platform. The lower Kimmeridgian sediments represent a minor 2nd-order regressive cycle, punctuated by three 3rd-order scale cycles. This was followed by a 2nd-order transgression, the base of which is dated as eudoxus Zone. The Oxfordian sequence boundaries are denoted O1 to O8, and the Kimmeridgian sequence boundaries are denoted K1 to K3. Relative sea-level changes are supported by the progradational, aggradational and retrogradational relationships of the facies within the sequences and by detailed study of the genesis and correlation of the key stratigraphic surfaces. Periods of relative sea-level fall are mainly characterized by progradation of the carbonates basinward and renewed influx of terrigenous siliciclastics. The start of transgression is marked by a decrease in the terrigenous influx and retrogradation of the carbonate facies across the carbonate platform. Maximum flooding is characterized by a slowdown or even termination of the growth of coral reefs on the platform, condensation of the beds and sometimes by the deposition of a condensed fossiliferous horizon in the basin.

The sequence stratigraphic method allows further refinement of correlations between the basin and carbonate platform, and provides an explanation and framework for the complexity of carbonate facies.

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