Compound Incised Valley Fill and Associated Forced Regressive Deposits of a Quaternary Shelf: Western Gulf of Lion, Mediterranean France

Michel Tesson¹, Caroline Labaune¹, S. Gensous¹, Patrice Imbert², Jean-Pierre Suc³, M. Melinte-Dobrinescu⁴, Vincent Delhaye-Prat², and Olivier Parize^5
1GDARGO, Perpignan, France
²Total, Pau, France
³Lyon University, Villeurbanne, France
⁴GEOECOMAR, Bucharest, Romania
⁵AREVA NC, Paris, France

Since the advent of sequence stratigraphy, incised valleys have been considered as an exploration target of interest, based on the common perception that their infill is typically sand-dominated in a dominantly muddy background.

High-resolution seismic data acquired over the past 5 years have allowed to constrain well the network of Quaternary incised valleys in the Gulf of Lions (Western Mediterranean), in relation with the associated forced regressive and lowstand wedges. One particular compound incised valley was identified as the best spot for calibration and was drilled down to the erosional base of the valley, i.e. to a depth of 60 m. This well on the one hand allowed to precise the age dating of the various episodes of incision fill (6 episodes were identified from seismic regionally and appear to be preserved at well location), and on the other provided ground truth on the lithology of the infill.

The first finding is that the valley-fill is dominantly muddy in the well, with about 80 % of mud and silt over the lower 30 m and about only 20 % over the upper and last episode. The sequence boundaries spotted on seismic do show in the well, but are marked only by decimetric gravel lags and paleosols, except for the lowermost fluvial level (1 m thick). Putting together biostratigraphic calibration and known eustatic sea-level curves for the Pleistocene, it turns out that the observed chronology of the infill is consistent with a low and steady subsidence rate for the area, allowing only the lowest sea-level stands to have their sediment preserved from subsequent re-erosion. The dominantly muddy infill would be related to the high rate of sea-level rise during the preserved transgressive episodes. Seaward of estuary mouths sands, the forced-regressive wedges represent the depositional features generated during the composite sea-level lowering and maximum lowstand.

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