Architecture and morphology of the outer segment of a mixed tide and wave-dominated-incised valley, revealed by HR seismic reflection profiling: the paleo-Charente River, France

Abstract

We present new single-channel high-resolution seismic reflection profiles, ground-truthed by vibrocore data, dedicated to the outer segment of an incised valley connected to the Charente River (French Atlantic coast). The present-day Charente is located about 50 km northward of the well-known Gironde estuary. Those river mouths are comparable in terms of marine hydrodynamic parameters, but strongly differ by their water discharge and catchment area. Seismic data are processed and interpreted to develop a three-dimensional seismic stratigraphic framework for the Charente-incised valley fill. The channel network of the drowned Charente valley is evidenced for the first time, and shows a seaward distributary pattern which is likely influenced by neotectonic control. Incision depth and width of the Charente and Gironde-incised valleys are similar, supporting the idea that correlation between valley width and modern hydrology is poor.The internal geometry of the Charente valley fill shows in seismic lines high to middle angle dipping reflectors at the base and a top of sequence with an intervening low angle seismic unit. This pattern is associated with a landward migration of the seismic unit depocentres. Sedimentary facies of the main seismic units suggests an upward and landward shift from estuarine mixed sands and muds, to estuary mouth massive sands, topped by fine marine sands. The described sedimentary and seismic characters, and the Holocene age of the valley fill near the present-day Charente river mouth, suggest that the drowned Charente valley infill mainly corresponds to a single transgressive sequence emplaced during the last sea level rise. Beyond the local interest, the dense seismic grid recorded across the Charente drowned valley gives a seismic validation for the model of large valley fill proposed by Ashley and Sheridan [Ashley, G.M., Sheridan, R.E., 1994. Depositional model for valley fills on a passive continental margin. In: R.W. Dalrymple, R.J. Boyd, B.A. Zaitlin (Eds.), Incised Valley Systems: Origin and Sedimentary Sequences. SEPM (Soc. Sediment. Geol.) Spec. Publ., Tulsa, Vol. 51, pp. 285–301.]. We propose that high–low–high internal reflection pattern succession represents the “Seismic Sandwich” in reference to the Ashley and Sheridan [Ashley, G.M., Sheridan, R.E., 1994. Depositional model for valley fills on a passive continental margin. In: R.W. Dalrymple, R.J. Boyd, B.A. Zaitlin (Eds.), Incised Valley Systems: Origin and Sedimentary Sequences. SEPM (Soc. Sediment. Geol.) Spec. Publ., Tulsa, Vol. 51, pp. 285–301.] “Sedimentary Sandwich”. Such seismic pattern is also recognized in the Gironde valley [Mar. Geol. 175 (2001) 183.].