Print this pageTopographic Control on the Architecture of High-Energy, Carbonate, Incised-Valley Fills in the Miocene of Southeast France
Robert
W. Dalrymple1, Jean-Yves Reynaud2, David Besson3, Olivier Parize3,
Jean-Loup Rubino4, and Noel P. James1
1 Queen's University, Kingston, ON
2 Muséum National d'Histoire Naturelle, Paris, France
3 Ecole des Mines de Paris, Fontainebleau, France
4 Total, Pau, France
The Burdigalian succession in SE France consists of a series of incised-valley fills composed predominantly of bioclastic carbonates of cool-water affinity. Two coeval valley fills, situated 60 km apart, have been studied in detail. Both consist of high-energy, cross-bedded facies with similar faunal compositions, and both show an overwhelming dominance of up-valley transport. It is inferred that both formed in response to strong tidal currents, in an upper mesotidal to macrotidal regime. Despite these gross similarities, the architectures of the deposits are very different because of differences in the geometry of the valleys. The Uzès deposit occupies a structurally controlled basin that wraps around the nose of a plunging anticline. Flow expansion into the basin has produced a large flood-tidal delta, within which the strata show a headward decrease in energy from coarse-grained cross-bedded deposits at the entrance, to fine-grained marls in the low-energy central basin. The strata dip both up-current on the flood ramp and down-current into the central basin. Channel deposits are absent, except at the entrance. The Saumane-Venasque deposit, by contrast, occupies a narrow, steep-sided valley with no area of flow expansion. As a result, the deposit was formed by a single, sinuous channel that was approximately 25 m deep. Deposition occurred on channel-margin point bars that migrated preferentially headward, creating large-scale, obliquely headward-dipping inclined stratification. These deposits confirm the important role that valley geometry has on the nature of the infilling deposits, regardless of whether they are siliciclastic or carbonate.