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AAPG Bulletin


AAPG Bulletin, V. 82 (1998), No. 6 (June 1998), P. 1140-1155.

Near-Bottom Sedimentation Processes Revealed by Echo-Character Mapping Studies, Northwestern Mediterranean Basin1

V. Gaullier2 and G. Bellaiche3

©Copyright 1998.  The American Association of Petroleum Geologists.  All Rights Reserved

1Manuscript received November 27, 1995; revised manuscript received April 14, 1997; final acceptance January 13, 1998.
2CEFREM, Université de Perpignan, 52 Avenue de Villoreuve, 66860, Perpignan, France.
3Géosciences-Azur (UMR 6526), Observatoire Océanologique, La Darse, B.P. 48, 06235 Villefranche-sur-Mer Cedex, France.

We are grateful to all colleagues who participated with us on the Mesea cruises and especially to G. Pautot, L. Droz, J. C. Aloisi, A. Coutelle, C. Degiovanni, J. P. Réhault, J. Deverchère, and J. R. Vanney. We wish also to acknowledge the captains and the crews of the R/V l’Atalante and R/V Jean Charcot, and all the technical teams. We are indebted to Y. Descatoire, who helped us to compile the 3.5 KHz map. Former Elected Editor K. T. Biddle and Bulletin reviewers J. E. Damuth, D. Piper, and R. G. Stanley are gratefully acknowledged in having provided constructive comments and careful reviews that helped us significantly improve the earlier manuscript. This paper forms contribution no. 003 of Géosciences-Azur (UMR 6526). 


Subbottom echoes recorded on 3.5 kHz echo sounders in the northwestern Mediterranean Basin from 1978 to 1991 can be classified into five main categories of echo character: (1) Continental shelf (CS) echo character characterizes the continental platform and areas of shallow relief and commonly is related to consolidated sediments. (2) Hyperbolic (H) echo character is recorded from areas with rough bottom morphology. Echo type H1 corresponds to steep slopes; echo type H2 reflects very coarse grained sediments deposited by energetic turbidity currents. (3) Bedded (B) echo character is widely observed throughout the study area and corresponds to terrigenous deposits distributed by turbidity currents. Echo type B1 is related to thin turbidites deposited on overbank portions of channel levees by lateral overflow of turbiditic currents moving down the channels; echo type B2 corresponds to coarser turbiditic materials. (4) Rugged (R) echo character is observed in the axes of submarine canyons and channels and characterizes hard sea floor with coarse heterogeneous turbiditic deposits subjected to energetic gravity-flow processes. (5) Transparent (T) echo character is ubiquitous in the study area and is attributed to mass-movement deposits. The widespread occurrence of those deposits testifies to the general instability of the study area. Types and distribution of echo character throughout the study area show the importance of gravity-dominated depositional processes (turbidity currents and mass wasting) in this region. This remobilization of siliciclastic sediments, induced by high sediment rates, steep slopes, and halotectonics, theoretically could create potential hydrocarbon reservoirs within the deep-water portions of the continental margin.

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