Provenance and sedimentologic variations of turbidite and slump deposits at Sites 955 and 956

Abstract

At two of the seven sites drilled during Leg 157, located south of the Canary Islands (Sites 955 and 956), more than 1000 thinly bedded to finely laminated volcaniclastic and nonvolcaniclastic silt and sand turbidite units interbedded within thick sequences of hemipelagic mud were recovered. The sediments range in age from Pleistocene to middle Miocene. Turbidite deposits composed mostly of volcaniclastic detritus are related to known periods of volcanism in the Canary Islands. The nonvolcaniclastic turbidites include various mixtures of quartz and foraminifer sand and silt derived from the continental margin of northwestern Africa. Shallow marine bioclastic deposits are thicker at Site 956 than at Site 955, suggesting that these sediments were derived from the southern Canary Islands.Although age constraints cannot be precisely defined at these sites, the general trend of sedimentation is consistent with sealevel variations being responsible for transport of the continentally derived turbidites. Quartz-rich turbidite deposits composed of well-rounded, very fine-grained sand were derived from Saharan eolian sands that migrated onto the subaerially exposed continental shelf and were shed into the deep marine environment during falls in sea level. Shallow marine bioclastic and foraminifer-rich turbidites also correspond to periods of sea-level fluctuations. However, the presence of volcaniclastic material within several bioclastic deposits indicates volcanic activity may have been responsible for some of their deposition.Large-scale submarine slumping appears to be more active during periods of intensified bottom currents and upwelling. Additionally, slumping was more frequent at Site 956 on the lower flank of Gran Canaria. Slump block ages are only slightly older than those of the surrounding sediment, and mass movement may have resulted from increased sedimentation and/or earthquakes associated with Canary Islands volcanism.

Expedition 340 summary

“…We plan to undertake a detailed lithologic, sedimentologic, and textural fabric analysis of the retrieved material at macro-and microscopic scales to investigate transport and deposition processes, the nature and magnitude of erosional processes, and interaction with the substratum (e.g., bulking) (Komorowski et al, 1991;Glicken, 1991Glicken, , 1996. These data will provide valuable insights into chronology (one or several pulses) and debris avalanche mobility, which have implications for tsunami genesis.…”

“…Utilizing a combination of detailed lithologic, sedimentologic, and textural fabric analyses, we will be able to test the hypothesis that the debris avalanche incorporated sediment eroded from the seafloor during its emplacement. We hoped to reach the bedded units below the chaotic debris avalanche unit and thus investigate the textural and structural characteristics of the base of the debris avalanche (Komorowski et al, 1991;Glicken, 1991Glicken, , 1996. Existing numerical models of debris avalanche emplacement do not sufficiently consider the interaction of the debris avalanche with the substratum and its influence on the mobility of the flow.…”

“…Existing numerical models of debris avalanche emplacement do not sufficiently consider the interaction of the debris avalanche with the substratum and its influence on the mobility of the flow. Our goal was to look for structural evidence of the development of shear zones from the base of the debris avalanche into the overlying plug and fluidization structures (clastic dikes) described in terrestrial debris avalanche deposits (e.g., Glicken, 1996;Voight et al, 2002;Gee et al, 1999;Clavero et al, 2002;Shea et al, 2008).…”

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“…Despite the importance of flank collapses and associated debris avalanches in the lifetime of arc volcanoes, mainly areas of intraplate volcanism associated with large submarine volcanic landslides, such as Hawaii (ODP Leg 136: e.g., Garcia, 1993, Garcia andMeyerhoff Hull, 1994; ODP Leg 200: e.g., Garcia et al, 2006) and the Canary Islands (ODP Leg 157: e.g., Schneider et al, 1997;Goldstrand, 1998) (Deplus et al, 2001;Le Friant et al, 2004). This can be largely explained by the fact that collapse deposits around Montserrat occur in shallower water (<1200 m) and that offshore Dominica and Martinique, the debris avalanche flew into the Grenada Basin and was emplaced on top of sedimentary units that provide good seismic reflectors.…”

“…Nonetheless, the majority of marine volcanic studies targeting volcaniclastic sediments and/or debris avalanche deposits (1) were not associated with entire volcanic Garcia, 1993;Garcia and Meyerhoff Hull, 1994; ODP Leg 200: e.g., Garcia et al, 2006; ODP Leg 157: e.g., Schneider et al, 1997;Goldstrand, 1998;Schmincke and Sumita, 1998), (2) were drilled into distal turbidites away from the proximal parts of the debris avalanche deposits, or (3) used sampling techniques with only shallow ground penetration, thus retrieving only relatively recent samples. For instance, tephrochronological studies on several volcanoes of the Lesser Antilles arc using piston cores (length = ~7 m) have extended our knowledge about the eruptive history in this area to only the last 250,000 y (Duchoiselle, 2003;Vennat, 2004;Le Friant et al, 2008;fig.…”