New discovery of a mud-volcanoe field related to gas venting in the Gulf of Cadiz: Imagery of multibeam data and ultra-high resolution seismic
L. Somoza (1), V. Diaz-del-Rio, F.J. (2), F.J. Hernández-Molina,
(3), R. León (1), A. Lobato (2), F. Lobo (4), Rodero (5),
and TASYO team
1. Marine Geology Dv., ITGE Geological Survey of Spain, Rios Rosas 23, 28003 Madrid, Spain
2. Instituto Español de Oceanografia,
3. Facultad de Ciencias del Mar, Universidad de Cádiz, Polígono Río San Pedro s/n, 11510 Puerto Real (Cádiz), Spain.
4. CIACOMAR, Universidade do Algarve, Avenida das Forças Armadas s/n, 8700 Olhão, Portugal.
5. Instituto Andaluz de Ciencias de la Tierra, IACT-CSIC. Campus de . Granada, Spain
The TASYO/2000 cruise aboard of the R/V Hesperides results in the discovery of a new area, the TASYO Field, of extensive deep fluid flux through the sea floor on the slope of the Gulf of Cadiz. The area surveyed with Simrad EM-12S multibeam echosound and ultra high-resolution seismic (TOPAS parametric echosound) resulted in three unexpected results: the probable presence of a number of mud volcanoes, the presence of numerous large pockmark craters, and of the remarkable features resembling scars of a large sediment slide migrating towards the Horseshoe abyssal plain. All these findings had important implications regarding the likely presence of gas in sediments, of overpressured formations and of fluid circulation through the sediments and the sea floor. This also support by the high frequency echosound (EK500), that revealed water column targets plumes on top of mud volcanoes intepreted as active gas bubbles indicating recent activity of gas/fluid venting . This deep fluid flux is related to alongslope gravitational sliding of shale/salt deposits of the "Olistostrome", a complex tecto-sedimentary wedge formed by African-Eurasian convergence. Destabilization of gas hydrates by warming of the Mediterranean Outflow water (MOW) is another mechanism than could explain the widespread and episodic venting gas on the sea floor.
Numerous investigations worl-wide have shown that active submarine fluid discharge (particularly gases) produces specific structures on the sea floor (Ivanov et al., 1998). Muds are frequently extruded with large volumes of crude, oil and gas (both biogenic and termogenic). The most described sea-floor features related to venting of fluids, brines, crude oil, gases and fine-grained sediments are the pockmarks, brine pools, mud volcanoes and piping rills.
Fig. 1 Swath mapping showing the sea-floor morphology of the TASYO Field. The area is characterized by large craters pockmarks and elevated conical mounds morphologies Northwards, salt/shale diapiric ridges are observed controlling the Mediterranean undercurrent channel.
In the Gulf of Cadiz, evidence of gas-venting has been related to pockmarks along the shelf break of the Spanish margin (Baraza y Ercilla, 1996) and recently has been described three mud volcanoes field along the Morrrocan margin related with venting of fluids oversaturated in methane and presence of hydrates and authigenic carbonates (Gardner, 1999; Ivanov et al, 2000). In 1999, during the TTR9 cruise, were identified theYuma, Ginsburg, Kidd, Adamastor y TTR mud volcanoes (between 36º30'N-36º10'N and 6º40'W-7º00'W), and the San Petesburgh mud volcanoe (35º53.749N-07º2.44W) along the border of the Spanish-Morrocan margins ( M. Ivanov oral comm.). According to the results of gas measurements, this area is characterised by relatively high background gas content (up to 292 ml/l) in comparision with other areas of extensive mud volcanism as the Eastern Mediterranean and the Black Sea.
In this background, one of the main objetive of the TASYO/2000 cruise were surveyed the Spanish-Portuguesse continental margins in order to identify mud vulcanism, geophysical gas-ydrates evidences and other gas-related sea-floor features along the slopes of the Gulf of Cadiz.
An extent new data of more than 1200 km has been obtained during the cruise TASYO/2000 aboard of R/V Hesperides. The Simrad EM12S-120 system, a multibeam echo sounder sytem, was used for mapping sea-floor. It operates at a main frequency of 13 kHz, with 81 beams, which allow a maximum coverage angle of 120º ( about three times the depth). This system triggering with a range of pulse lentgh of 2-10 ms, reaching a resolution of 0.6 m. A preliminary processing of data has been made aboard with the Neptune software. The swath mapping provided bathymetric map contoured at an interval of 10 m also a subsidiary map showing the strength of the sea-floor backscattering of the sonar signal.
The upper sediment colum and sea floor were studied with high reolution methods. A Parasound echosounder TOPAS (Topographic Parametric Sound) , which is a sub-bottom profiles was used. It works with CHIRP wavelet, operating at two simoultaneous primary frequencies of 15kHz and 18 kHz. Through the parametric effect in the water , a secondary frequency is produced in the range of 0.5 kHz-5 kHz. An penetration of up to 100 m has obtained with a resolution of 1-0.5 m. Analogue plots are available for preliminary interpretation onboard (Figs. 2 and 3). Digital data were recorded using Delph2 Triton software and preserved on magnetic disks so that details can be studied with post-processing techniques. A fishering-type echosound Simrad EK500 operating at a frequency 38 kHz (Splitbeam) was also used for observed water column targets plumes related to active gas venting. Depth-Temperature measures of the water column were made with XBT sounds with a precisition of ?0.15ºC. This allow to construct the phase diagram of the methane hydrate stability for present conditions of mass water.
SEA FLOOR OBSERVATIONS
The SIMRAD EM12 swath mapping survey of the Gulf of Cadiz resulted in three unexpected results: the probable presence of a number of mud volcanoes, the presence of numerous large pockmark craters, and of the remarkable features resembling scars of a large sediment slide migrating towards the Horseshoe abyssal plain. All these findings had important implications regarding the likely presence of in sediments, of overpressured formations and of fluid circulation through the sediments and the sea floor.
The TASYO field extend within 36º15´N-7º15´W and 35º 55´N-7º 30´ (see figure 1), characterized by an irregular sea floor formed by the presence of numeorus crater-like pockmarks and dome features. Deeps range from 750 meters to 1050 meters. Deeper batymetries corresponds to large crater pockmarks whereas shallower are top of mud volcanoes and dome structures. The TASYO field is surrounded to the north by a main channel of the Mediterranean undecurrent (MOW Mediterranean Outflow Water). Several channels are also observed related to downslope-arcuated features that resemble scars of slides.
Ultra-high resolution seismic shows detailed strucuture of mud volcanoes. The mud volcanoes can be divided into two types according their dimensions and morphology. The first type has a seabed diameter of more than 5 km, building over a plateau that reaches a height of 80 m, has a complex top with a conical structure and relatively steep slopes with a relief of 15 m over the plateau. The plateau is surronded by steep slope depressions that are clearly reflected on the multibeam mosaic (Fig. 1). These depressions are interpreted as large depressions formed by gas and brines seepages (brine pools ?). Evidence of activity of gas seepages in the mud volcanoe area is shown by water column targets plumes (gas bubbles) observed on the EK500 echosound. The thickness of this type of mud volcanoe evidenced by transparent acoustic facies range from 15 meter on the border to 45 m on the axis. On the axis, clearly can be observed feeder channels that deform the underlying sediments rising up from 1000 meters till the most prominent conical structure.
The second type (Fig 3) has smaller dimensions, a single mud volcanoe with a diameter of 400 meter surrounded by a partially collapsed rim with steep slopes and a diameter of 1500 meters. The axis of this type of mud volcanoe show a conical transparent facies deforming the underlying sediments.
Crater-like depressions are most probably formed by gas eruption whereas mud volcanoes are building up by viscous deeper fluids. Observations on the high-resolution stratigraphy of the mud volcanoes support a pattern of episodic venting.
The main mechanism to explain the observed widespread and episodic venting gas on the sea-floor subsurface, of overpressured formations and of fluid circulation through the sediments and the sea floor are:
(a) Contractional faults regulated by salt/shale adjustment. (Somoza et al. 1999). Therefore, on the upper slope, contractional faults provide avenues of vertical transport for overpressured compartments which provide the driving force for fluid and gas expulsion (Lowrie et al. 1999, Fernández-Puga et al. 1999).
(b) Destabilization of gas hydrates by warming of the Mediterranean Outflow water (MOW) (Somoza et al. 2000) and/or by mega-slides detached on gas-hydrate stability zone (GHZ) According to depth-temperature distribution of the sea water masses in this area and the with geothermal gradients of 2.4 ºC per 100m, the GHZ begin at 680 m on Atlantic water mass and at 850 m when MOW is present. Thus, the influence of MOW reduce drastically the existence of the gas hydrates.
Both mechanism are probably modulated by sea-level changes in
response to lowstand sedimentary loading and changes in current strenght
and patterns of the Mediterranean outflow water.
The TASYO team include: M.C. Fernandez-Puga, M. Garcia-Garcia, E. Llave, A. Maestro, M.A. Perucha, J.T. Vazquez.
This work has been carried out within the activities of the "TASYO" project (CICYT MAR98-0209) in the framework of the scientific cooperation between Spain and Portugal. Thanks to the crew of the R/V “Hesperides”. Special thanks to Pablo Rodríguez (UGBO) and Francisco Gonzalez and Ricardo Gomez (IEO), electronic technicians for their supporting aboard.
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