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Tidal Rhythmites as Possible Indicators of Very Rapid Subsidence in a Foreland Basin: An Example from the Miocene Marine Molasse Formation of the Digne Foreland Basin, SE France

¹ Département de Géologie, UMR 6143 M2C, Université de Caen, 14 000 Caen, France; CGES Sédimentologie, ENS Mines de Paris—35, rue St Honoré, 77 305 Fontainebleau, France; R.coueffe{at}brgm.fr
² Département de Géologie, UMR 6143 M2C, Université de Caen, 14 000 Caen, France
³ Département de Géologie, UMR 6143 M2C, Université de Caen, 14 000 Caen, France
⁴ CGES Sédimentologie, ENS Mines de Paris—35, rue St Honoré, 77 305 Fontainebleau, France

Tidal rhythmites can represent continuous deposition over several years at a rate of a few decimeters per year. The development of these tidal facies in certain basins remains problematic with respect to the processes responsible for the accommodation necessary for multi-year rhythmite deposition and preservation.

In the Digne foreland basin, the Miocene marine infill contains abundant tidal rhythmite successions. Some of these tidal facies cropping out on the western margin of the basin are described and analyzed in detail. Thickness measurements indicate that the rhythmite successions records four orders of tidal cycles: semidiurnal, semilunar, lunar, and semiannual. Cycle continuity suggests that sedimentation was continuous over several years, with a deposition rate that remained almost constant (4 m per year). Sedimentological evidence indicating little change in water depth suggests that the accommodation necessary for deposition was created at a rate comparable to that of rhythmite accretion.

Because the studied tidal rhythmite succession is traceable regionally, accommodation may have been regional in origin. Because synsedimentary tectonic activity is well established for the Digne Basin and sequence correlation indicates that the tidal rhythmites thicken into axes of en échelon synclines along the basin western margin, very rapid subsidence events of tectonic origin are assumed to be the key process that created such accommodation.

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