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

Abstract

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.