Morpho-structural and sedimentary characterization of a new seepage feature in the Gulf of Cadiz: Henriet mud volcano

Abstract

High-resolution multibeam bathymetry, seismic reflection data, sediment core and water samples were used to first detail the morpho-sedimentary structure and seepage environment of a recently discovered mud volcano (MV) in the Atlantic Moroccan margin during the SUBVENT-2 cruise. Henriet MV is the most eastern edifice of the Gulf of Cadiz, located at 320 m water depth poorly rising 20 m high and circular in shape with a diameter of 630 m. Its summit has a pronounced crater like depression of 15 m deep, 180 m long and a maximum slope of 12º. A half moon shaped 575 m-long rim depression surrounds the edifice. The seismic character of this feature exhibits chaotic sub-bottom reflectors, small overlapped hyperbolae at the bottom of the crater and adjacent coherent reflectors bent slightly downward suggesting a possible subsidence or collapse. Parallel lateral truncated reflectors do appear in the adjacent host sediment of the outer rim depression indicating erosion possibly related to instabilities whereas the chaotic bodies correspond to mass transport deposits (MTD). Plastered and mounded contourite drifts overlap the NE flank and SW adjacent seafloor respectively, in relation to the bottom currents interaction. Buried mud flows can be seen above and below this contourite drift indicating previous mud outflow episodes. A 182 cm long core recovered over the flank consists of foraminiferal sandy mud up to the 140 cm long interpreted as hemipelagic sediments. Its last section consists of a 50 cm layer of mud breccias corresponding to a mud extrusion episode. Mud breccia layer displays low Ca/Al ratio. Henriet MV presents a high methane concentration in the water column above crater-seafloor reaching up to 118 nM. Based on these observations Henriet MV is considered an episodic mud volcano currently characterised by diffuse venting and affected by bottom currents.

Full text

VI International Symposium on Marine Sciences
Vigo (Spain), 20-22th June 2018
MORPHO-STRUCTURAL AND SEDIMENTARY
CHARACTERIZATION OF A NEW SEEPAGE FEATURE IN THE
GULF OF CADIZ: HENRIET MUD VOLCANO
O. Sánchez-Guillamón1, M.C. Fernández-Puga*2, L. Somoza3, J.T. Vázquez1, T.
Medialdea3, N. López-González1, D. Palomino1, L.M. Fernández-Salas4, R. León3
and F.J. González3
1 Instituto Español de Oceanografía, C.O. de Málaga, Fuengirola, Málaga.
osanchezguillamon@gmail.com; juantomas.vazquez@ieo.es; desiree.palomino@gmail.com;
nieves.lopez@ieo.es
2 Facultad de CC del Mar y Ambientales, Puerto Real, Cádiz. mcarmen.fernández@uca.es
3 Instituto Geológico y Minero de España, Madrid. l.somoza@igme.es; t.medialdea@igme.es;
r.leon@igme.es, fj.gonzalez@igme.es
4 Instituto Español de Oceanografía, C.O. de Cádiz, Cádiz. luismi.fernández@ieo.es
Abstract: High-resolution multibeam bathymetry, seismic reflection data, sediment core
and water samples were used to first detail the morpho-sedimentary structure and seepage
environment of a recently discovered mud volcano (MV) in the Atlantic Moroccan margin
during the SUBVENT-2 cruise. Henriet MV is the most eastern edifice of the Gulf of
Cadiz, located at 320 m water depth poorly rising 20 m high and circular in shape with a
diameter of 630 m. Its summit has a pronounced crater like depression of 15 m deep, 180
m long and a maximum slope of 12º. A half moon shaped 575 m-long rim depression
surrounds the edifice. The seismic character of this feature exhibits chaotic sub-bottom
reflectors, small overlapped hyperbolae at the bottom of the crater and adjacent coherent
reflectors bent slightly downward suggesting a possible subsidence or collapse. Parallel
lateral truncated reflectors do appear in the adjacent host sediment of the outer rim
depression indicating erosion possibly related to instabilities whereas the chaotic bodies
correspond to mass transport deposits (MTD). Plastered and mounded contourite drifts
overlap the NE flank and SW adjacent seafloor respectively, in relation to the bottom
currents interaction. Buried mud flows can be seen above and below this contourite drift
indicating previous mud outflow episodes. A 182 cm long core recovered over the flank
consists of foraminiferal sandy mud up to the 140 cm long interpreted as hemipelagic
sediments. Its last section consists of a 50 cm layer of mud breccias corresponding to a
mud extrusion episode. Mud breccia layer displays low Ca/Al ratio. Henriet MV presents a
high methane concentration in the water column above crater-seafloor reaching up to 118
nM. Based on these observations Henriet MV is considered an episodic mud volcano
currently characterised by diffuse venting and affected by bottom currents.
Key words: Mud volcano, Geomorphology, Mud breccia, Methane seepage
Acknowledgments: This work is a contribution to SUBVENT (CGL2012-39524-C02),
INPULSE (CTM2016-75129-C3-1-R), EXPLOSEA (CTM2016-75947-R) projects, Spain
MINEICO and PAIDI Group RNM328.
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