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Abstract

The Lower Jurassic-Aalenian transgressive/regressive major cycle transects the western European craton from the nascent Tethyan margin, now involved in the Subalpine folding in the south, to the mid-Norway basin in the north. It includes the eastern Aquitaine, the Anglo Paris and the North Sea basins. This cycle has been named Ligurian from the main phase of the rifting that affected the southern part of the craton and led to the opening of the Ligurian part of the Tethys. The lower boundary of this major cycle is the early Cimmerian unconformity dated as latest Norian age. The upper boundary is dated as late Aalenian (mid-Graphoceras concavum ammonite Zone) and is related to the mid-Cimmerian unconformity. Peak transgression coincides with deposition of the organic-rich lower Toarcian black shales (Harpoceras falciferum ammonite Zone). Three 2nd-order transgressive/regressive facies cycles have been identified from the Tethyan margin; they record the successive steps of Ligurian rift development and subsequent phases of subsidence. However, four 2nd-order cycles are recognized in the Paris Basin and five the onshore United Kingdom basins. The number of cycles, ages of peak transgressions and main cycle boundaries vary regionally according to the local extensional tectonic development. For example, a major regressive episode is apparent at the end of the Sinemurian stage in northern areas but is not prominent in the southern areas. Instead, a regression dated as early Pliensbachian (Tragophylloceras ibex ammonite Zone) is dominant in the southern areas. Most of the 27 3rd-order depositional sequences comprising the Ligurian major cycle can be documented from the North Sea to southern Europe. Precise age dating was done using ammonites (down to the "horizon" level, where possible), from the Provence shelf in the south to the Hebrides basin in the north, allow correlation. In more northern areas where ammonites are missing, palynological determinations provided the biostratigraphical control.

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