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Three Lower Cretaceous calciturbidite bundles, well exposed in the southeastern France, are studied with a combined sedimentological and ichnological approach. They were deposited in a toe-of-slope setting in the western border of the Vocontian Trough, during the late Barremian. The bundles appear to be the result of a cyclic sedimentation, with recurrent ichnotaxa including common Thalassinoides, Planolites, Zoophycos, and less abundant Taenidium, Chondrites and Ophiomorpha. These trace fossils constitute some ichnoassemblages that follow and superimpose each other. Analysis of the tiering produced may help to estimate the time interval between two turbiditic events. Indeed, the presence of Zoophycos indicates a low frequency of the turbiditic supplies, which allowed the substrate to firm up, a necessary condition for the construction and the persistence of this spreite trace fossil. Therefore, a decreasing frequency of the turbiditic events is pointed out from the base to the top of each turbidite bundle. The trace fossil distribution may also be influenced by the grain-size. In fact, Zoophycos producers seem to have thrived in a fine-grained sediment (0.1–0.2 mm). Moreover, the organic matter exported from the platform is better preserved in these fine-grained substrates, and may be more available to opportunistic burrowers. In the studied succession, two different morphologies of Zoophycos have been observed. One of them (lobed form) probably indicates an opportunistic behaviour of an organism quickly exploiting the organic matter between two turbiditic events. By comparison, the other (unlobed form) is produced later by a more specialised organism, exploiting large amounts of organic matter during episodes of lower frequency of turbiditic supplies. The ichnofauna studied is characterized by a low diversity and this may be related to an inner submarine fan setting. |
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