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Abstract

During the early Toarcian, black-shale deposition was widespread, and several different models have been proposed to explain environmental conditions and controlling factors. Multidisciplinary investigations combining microfacies analysis, geochemical parameters, and paleoecological data reveal that sea-level variation was the main forcing factor for facies distribution within the Central European Basin. This interpretation is supported by the comparison of several Lower Toarcian sections from Europe.

The results show that a Pliensbachian to early Toarcian regression caused the enclosure of the SW German basin, inducing stagnant conditions. Deposition of organic-matter-rich sediments started in the central part of the basin while contemporaneous sediments in basin-margin areas were affected by reworking. The subsequent slow transgression was of minor extent and led to long-term stagnation, which led to anoxic conditions in the benthic environment. Maximum oxygen depletion existed during the exaratum Subzone times and is indicated by largest fecal pellet sizes, a distinct type of lamination, highest content of organic carbon and sulfur, and a lack of paleocurrents and benthic macrofauna. Enhanced water circulation and improved living conditions in the benthic environment could not have been established until a further sea-level rise. Consequently, the bituminous mudstones of the European Epicontinental Sea were not deposited during a Liassic sea-level maximum.

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