Abstract

Early Cretaceous condensed successions containing both phosphorite and glauconite occur in South-Eastern France. During the late phase of rifting this area was part of the Helvetic shelf; shallow water and the dominant westward current system allowed the formation of a large upwelling-affected area along the northern margin of Tethys. This peculiar situation generated the condition for condensed sedimentation. A stratigraphical analysis has been performed in order to define the internal organization of the succession and the evolution of the sedimentary environment. Hauterivian deposits directly overlay Upper Valanginian limestones. The succession can be divided into four similar units, showing a repetitive internal structure. Each unit is formed by a hardground layer (polyphasic, phosphate-rich, and bioclastic) followed by a detritic bed. formed through the erosion and winnowing of the hardground itself and by the re-deposition into a more proximal environment; reworked clasts are welded by a glauconite-rich matrix. The top of each unit is formed by a mainly glauconitic rock (glauconite-rich sandstone) followed by open shelf limestones. The investigated succession is sealed by an Upper Cretaceous turbiditic unit. The genesis of the condensed beds can be related to an inner shelf margin origin, close to the slope. The presence of phosphate and glauconite suggests a peculiar environment, including the occurrence of an upwelling current affecting normal marine condition with the establishment of a minimum oxygen zone; the persistance of this caused reduced deposition during transitory periods of the Early Cretaceous, here reported as "condensation events". The lithological variety within each condensation event (of Valanginian to Albian age) is interpreted as the result of eustatic changes upon the same upwelling-affected environment. Each phosphate hardground associated with a highstand phase, is followed by early regression deposits and by glauconite in all the above described units. Hardground levels have been compared to similar horizons belonging to the Eastern Swiss Helvetic shelf, and to the Ligurian Brianconnais. Analogous ages and depositional similarities have been found for these layers in different domains, suggesting that the condensed deposition is a large scale event. Phosphate deposition developed during precise periods along the northern Tethys margin for hundreds of kilometers as far as Brianconnais, showing similar depositional characters.

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