Éditeur(s) : Elsevier
Résumé : The Danube Canyon is a large shelf-indenting canyon that has developed seaward of the late Pleistocene paleo-Danube valley. Mechanisms of canyon evolution and factors that controlled it are revealed by analyzing the morphology and the sedimentary structure of the canyon, as well as the main features of the continental margin around the canyon. This is based on investigation by swath bathymetry in the canyon area combined with different types of seismic data. The canyon is a major erosional trough with a flat bottom cut by an entrenched axial thalweg. The thalweg path varies from highly meandering to fairly straight in relation to the local gradient. Segments of the canyon are characterized by specific morphology, orientation and gradient along the axial thalweg. We interpret these segments in terms of canyon maturity. The sedimentary structure of the canyon documents an older phase of erosion followed by partial infilling, and thus attests for repeated cycles of canyon development. Canyon morphology is interpreted as a result of erosive sediment flows along the entrenched axial thalweg that caused downcutting into the canyon bottom and instability of the canyon walls, and hence enlargement of the canyon and expansion by headward erosion. During the last lowstand level of the Black Sea the canyon was located in an area of high sediment supply close to the paleo-Danube River mouths. This is indicated by buried fluvial channels on the shelf and by a wave-cut terrace associated with a water level situated about - 90 m below the present level. We infer that erosive flows in the canyon resulted from hyperpycnal currents at the river mouths, probably favored by the low salinity environment that characterized the Black Sea during lowstand times. Other mechanisms could have contributed to trigger sediment failure along the canyon, such as instability related to the presence of shallow gas, or the effect of a deep fault.
Marine Geology (0025-3227) (Elsevier), 2004-05 , Vol. 206 , N. 1-4 , P. 249-265

Droits : 2004 Elsevier B.V. All rights reserved
http://archimer.ifremer.fr/doc/2004/publication-476.pdf
DOI:10.1016/j.margeo.2004.03.003
http://archimer.ifremer.fr/doc/00000/476/

Éditeur(s) : Elsevier
Résumé : A previously unknown pattern of multiple bottom-simulating reflections (BSRs) occurs on high-resolution reflection seismic data in the Danube deep-sea fan, associated with acoustic features indicating free gas. Our study provides evidence that this pattern is developed in relation with the architecture of distinct channel-levee systems of the Danube fan. Channel-levee systems hosting multiple BSRs act as relatively sealed gas-bearing systems whose top is situated above the base of the gas hydrate stability zone (BGHSZ). Inside these systems, free gas accumulates below the BGHSZ under a combined lithological, structural and stratigraphical control. The uppermost BSR marks the current equilibrium BGHSZ, for a gas composition of more than 99% methane. Model-derived depths of the BGHSZ for different gas compositions and pressure-temperature conditions show that multiple BSRs would correspond to the BGHSZ either for (1) layers of gas hydrates with high contents of heavy hydrocarbons or hydrogen sulphide, or (2) stable climatic episodes with temperatures between glacial values and the present-day conditions. As the gas hydrate compositions required by hypothesis (I) are in sharp contradiction with the general background of the gas composition in the study area, we suggest that multiple BSRs are most probably relics of former positions of the BGHSZ, corresponding to successive steps of climate warming. In this case, they can provide sea-bottom paleotemperature values for these episodes, and hence they are potential new proxies for deciphering past climate conditions.
Marine Geology (0025-3227) (Elsevier), 2006-03 , Vol. 227 , N. 3-4 , P. 163-176

Droits : 2006 Elsevier B.V. All rights reserved
http://archimer.ifremer.fr/doc/2006/publication-1210.pdf
DOI:10.1016/j.margeo.2005.12.006
http://archimer.ifremer.fr/doc/00000/1210/