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Séminaire


Canyons géants dans des systèmes carbonatés profonds : résultats de la mission Carambar 2 aux Bahamas (31/11/2016-02/01/2017)

Date
le 02-03-2017 à 11:00

Lieu
Salle Univers, Bâtiment B18N, OASU, Université de Bordeaux

Intervenant(s)
Thierry MULDER, Professeur, EPOC (Equipe Sédimentologie)

Résumé
New high-quality multibeam data depict the area located between Little Bahama Bank (Bahamas) and Blake Plateau (Leg 1) and the area located between the mouth of Exuma Sound and the San Salvador slope (leg 2).

The survey during Leg 1 reveals the morphology of a giant 135-km-long canyon, the Great Abaco Canyon (GAC) and its main characteristics. The canyon main axis runs parallel to the margin. The pathway shows several knickpoints and plunge pools. The most important knickpoint is underlined by an abrupt change in slope of the canyon thalweg. The last one leads to the opening towards the Blake Basin. Its morphologic head forms a vast receptacle but does not represent the main source of material at present. The material supplied through the LBB canyon systems does not reach this area which only shows lineaments related to the pathway of the Antilles current and restricted failure scars. Most of the supply comes from the canyon flanks. In the north, tributary canyons drain the contourite deposits forming large flat plateaus above the drowned carbonate platform of the Blake Plateau. In addition, these contourite plateaus are subjected to translational slides moving towards the northern edge of the canyon forming a dissymmetric debris accumulation along the toe of the north canyon edge. Another source of sediment are two large tributaries connecting the GAC directly to the LBB upper slope. Sub bottom profiles suggest the presence of a turbiditic levee on the tributary canyon sides and inferred turbiditic activity. Little Abaco Canyon (LAC) shows morphologic similarities with GAC but at a smaller size. However, the canyon seems more active in terms of sediment transport. Canyons draining the eastern part of LBB show fresh sedimentary structures (sediment waves) suggesting active sedimentary processes. These structures are made of clean sand with shallow water organisms suggesting a direct supply from the carbonate platform edge. In term of size and morphology, the GAC compares to largest canyons in silicoclastic environments. Its originality comes from the fact it is only supplied by carbonate sources.

The survey during leg 2 details the southern part of Exuma Sound, and its unchartered transition area to the deep abyssal plain of the Western North Atlantic bounded by the Bahama Escarpment (BE) between San Salvador Island and Samana Cay, referred here to the San Salvador abyssal plain (SSAP). The transition area is locally referred to as Crooked Island Passage (CIP), loosely delineated by Crooked, Long, and Conception Islands, Rum and Samana Cays. Surprisingly in such a pure carbonate landscape, the newly established map reveals the detailed and complex morphology of a giant valley formed by gravity flow processes originated in Exuma Sound, in addition to many secondary slope gullies and smaller tributaries draining the CIP upper slopes. The valley referred here as the Exuma Canyon system starts with a perched system with low sinuosity, characterized by several flow restrictions and knickpoints initiated by the presence of drowned isolated platforms and merging tributaries. The valley abruptly transforms itself into a deep incised canyon, rivaling the depth of the Colorado Grand Canyon, through two major knickpoints with outsized chutes exceeding several hundred of meters in height, a total of 1600-1800 m. The sudden transformation of the wide valley into a deep narrow canyon, occurring when the flows incised deep into an underlying lower Cretaceous drowned carbonate platform, generates a huge hydraulic jump and creates an enormous plunge pool and related deposits with mechanisms comparable to the ones operating along giant subaerial waterfalls. The high kinetic flow energy, constrained by this narrow and deeply incised canyon, formed, when it is released at its mouth in the abyssal plain, a wide deep-sea channel with well-developed levees and fan, made of coarse-grained carbonate defined layers separated by fine carbonate sediments mixed with fine siliciclastics transported along the BE by the energetic Western Boundary Undercurrent.
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