Mud volcanism and mounds on the Moroccan margin: origin and growth

Giant submarine mud volcanoes on the Moroccan continental margin have been studied with high resolution seismics in order to analyse their origin and activity in a spatial and a temporal framework. The El Arraiche mud volcano field, discovered in 2002 during the CADIPOR cruise onboard R/V Belgica, boasts 8 mud volcanoes of which some are without equal in size and height. Orthogonal grids of narrow spacing high resolution seismic profiles over three large mud volcanoes have allowed for the first time to analyse the activity history of mud volcanoes in detail. The three grids were connected by seismic lines in order to correlate the mud flows of the different mud volcanoes.The occurrence of the mud volcanoes is related to extensional tectonics. The top of the accretionary wedge is subject to extension, resulting in large rotated blocks bound by lystric faults. The crests of the rotated blocks are expressed as anticlinal ridges at or near the seafloor. Most ridges show evidence of collapse of the ridge crest where the mud volcanoes are located, in response to the created subsurface mass deficit. This is expressed by downward bending reflections of the basement and oldest unit, sometimes accompanied by small normal faults.Regional tectonic events are controlling the long-term eruptive history of mud volcanoes, as inferred from seismic data. Based on small unconformities in the seismic profiles, a seismic stratigraphic framework was built, with 3 major units and multiple subunits. Although synchronized activity of the different mud volcanoes was not observed by analysis of extruded mudflows, units with many outflows could be discriminated from barren units. All three studied mud volcanoes activated in a short time frame, during the Pliocene, and remained active for the same period, contemporaneous to basin subsidence. A successive period of no mud volcano activity and absence of subsidence is followed by a recent pulse of high mud volcanic activity again. This suggests that regional tectonic events control the long-term eruptive history of the mud volcanoes, although the short-scale timing seems random.Two mud volcanoes boast an enigmatic but coherent reflection under its surface at a waterdepth of ~350 m. Its acoustic and morphological (3D) properties lead to the hypothesis of shallow gas hydrate presence. A stability model using thermogenic gas compositions and calculation of the thermal gradient and heat flow pattern confirmed our interpretation. The high inferred heat flow (over 1 W m-2) in the crater of the mud volcano confirms the focused flow of warm fluids. It shows that in areas of thermogenic gas production, gas hydrates can occur at very shallow water depths, even in areas with high heat flow.In the El Arraiche mud volcano field, biogenic carbonate mounds are present on the Pen Duick Escarpment, a fault-bounded cliff. In addition to all existing models for growth and origin, we want to propose a new model. A scarp or hill on the seabed subject to rather strong currents will develop zones of high pressure at the lows of the slopes and low pressure areas at or near the summit, triggering a fluid flow towards the summit of the structure. An enhanced migration of reduced chemical compounds at an uncolonized scarp or hill towards the surface could promote cementation of the sediments leading to hardground formation, suitable for subsequent settling. Fluid rise also could result in cementation of a coral rubble framework for existing mounds, leading to stabilization. It can also enhance the food supply for microbial communities.This in turn can boost species diversity, both mega- and microfauna. So, in this view, the pumping of fluids in carbonate mound systems, driven by external currents, is responsible for different processes. Numerical modelling of this process to assess the impact of pressure fields is ongoing

Integrated geophysical and petrological study of fluid expulsion features along the Moroccan Atlantic margin

In this study we integrate a geophysical – carbonate petrological data set, collected during the TTR-14 cruise (summer 2004) along the Moroccan Atlantic margin in Gulf of Cadiz (Southern area, 400m – 1000m and El Arraiche mud volcano field). This allows us to investigate the deeper structure and its control on fluid venting, to address the nature of seafloor topographical features, fluid geochemistry and venting processes. The deeper structure of the Southern area is dominated by two NW trending anticlinal acoustic basement ridges. Their northern flank and top is cut by major present-day active, normal faults, along which four dome structures and the Meknes mud volcano (mv), are concentrated. These ridges correspond to rotated, faultbounded blocks breaking up the top of the accretionary wedge. This indicates the southward prolongation of extensional tectonics and its structural control on mud volcanism, south of the El Arraiche field, which is also evidenced by the typical sandstone mud breccias recovered at the Meknes mv. Carbonate cemented mud breccia from the Meknes (type M) and the Kidd (type K) mv, and cemented sediment portions from Pen Duick Escarpment (type PD), all possess similar carbon isotopic (-19 to -29%¸ VPDB) and carbonate geochemical signatures, indicating seepage of a geochemical similar thermogenic hydrocarbon-bearing fluid source. Slightly elevated d18O values of HMC-cemented type M crusts suggest the former presence and dissociation of gas hydrates. The brecciated fabric, intraclasts and aragonite cement morphology, typical of type K crusts testify of a relative vigorous fluid ascent. HMC-calcian dolomite cemented PD crusts were likely formed under conditions of slower fluid ascent. Their actual near-seafloor occurrence, well above the base of the SRZ, is hypothesized to relate to erosion and migration of the SRZ by variations in upward hydrocarbon fluxes

Biogeochemistry of carbonate mounds from the Pen Duick escarpment in the Gulf of Cadiz

In the Gulf of Cadiz, carbonate mounds build by cold-water corals were recently discovered on the Renard Ridge, a zone of active fluid flow and mud volcanism. Their sizes vary from 25 to more than 60 m high, at a depth of 520 m and they are aligned along the ridge axis. These mounds, located in the close vicinity of fluid flow markers such as carbonate crusts and mud volcanoes, provided a novel opportunity to study a possible fluid flow control on the mound processes and distribution. Previous geochemical studies on the southernmost mound of the ridge indeed showed that this mound was located on focused fluid flow compared to surrounding sediments, and we observed typical profiles of methane migration and anoxic oxidation (AOM) at 3,8 m below the sea floor within the mound. Such AOM occurrence imprinted a characteristic d13C signature (down to –21,9 %¸ Vs. PDB) and significantly contributes to the overall carbonate budget of the mound.During the recent R/V Maria S. Merian cruise (April-June 2006), we sampled by mean of a gravity corer six new structures likely to be cold-water carbonate mounds, along the Pen Duick escarpment and the Renard Ridge. Our aim was to determine if the geochemical profiles observed in the first mound could be generalized to all the mounds in this area.Each core yielded a full sequence of cold-water corals down to about 5 meters below the sea floor. Hence, the numerous knoll-like structures revealed by high-resolution bathymetry along the ridge are indeed carbonate mounds build by cold-water corals and the entire Ridge has been massively colonized by corals. No live reef-forming coral could be recovered from the cores, nor observed by towed video instruments. Then, fluid migration seems to be a common feature all along the ridge. However, important discrepancies were observed: methane concentrations are higher and sulfate gradients steeper on both side of the ridge, whereas the central part of the ridge seems less active in term of fluid migration. In this case, the sulfate to methane transition zone could not be reached using conventional gravity corer. In order to obtain the full biogeochemical picture of these mounds, the use of a long piston corer, or drilling devices, will be required.The reasons of the formation of massive reefs in this area are still unknown and are probably linked to locally enhanced hydrologic conditions. However, it is possible that cold-water coral could have benefited from the hard substrate and the topographic elevations provided by fluid related structures such as carbonate crusts, chimneys and clasts, as observed in several other locations in the Gulf of Cadiz

Mud extrusion dynamics constrained from 3D seismics in the Mercator Mud Volcano. El Arraiche mud volcano field, Gulf of Cadiz

Located on the western Moroccan continental shelf of the Gulf of Cadiz, the Mercator Mud Volcano (MMV) is one of a total of eight mud volcanoes which compose the El Arraiche mud volcano field. We collected a high-resolution P-cable 3D seismic cube during the Charles Darwin cruise 178 in April 2006, covering an area of 25 km2. The data image the upper 500-1000 m of the MMV. El Arraiche mud volcano field is located in the top of the Tortonian accretionary wedge in the Gulf of Cadiz, between 200 and 700 m water deep. Despite of the general compressive trend of the Gulf of Cadiz due to the westward movement of the Gibraltar arc, the local regimen of the western Moroccan margin is extensional in the study area. The MMV is a 2.5 km diameter positive conical structure at 350 m water deep that rises from the flank of a salt diapir. The high-resolution 3D cube shows the main internal structure in the southern flank of an anticline and a secondary structure southwest of it. Parallel and continuous reflections onlapping the anticline structure define the seismic character outside the mud volcano. The body of the main structure shows the typical "Christmas tree" features related to mud flow deposits. The preliminary interpretation of the 3D seismic cube shows four main mud flows southwestward oriented from the main structure and interfingered into the hemipelagic regional sedimentation. From deeper to shallower, the flows are located approximately at 0.870 s, 0.838 s, 0.774 s, and 0.685 s travel time, respectively. The extrusions correlate with the main seismic sequences observed in the surrounding hemipelagic deposits. The maximum run-out distance for the mud flows is approximately 1 km southwestward from the main structure, which corresponds to the third youngest mud flow described. The secondary "Christmas tree" structure penetrates the hemipelagic sediments almost to the seabed. Its seismic character is defined by low amplitude and chaotic signal. Several mud flows are interfingered with the surrounding sediments and, in some cases, overlap the mud flows from the main structure but they are less extensive and thinner but more frequent than those from the main structure. The MMV is an active mud volcano and depends on complex fluid and mud dynamics. The existence of a secondary and apparently "abandoned" structure indicates the variation of mud pathways during the evolution of its plumbing system

Overflow Tests on Grass-Covered Embankments at the Living Lab Hedwige-Prosperpolder: An Overview

In regions with a temperate climate, a well-maintained grass sod on a clay layer is considered a reliable protection for dams and dikes. In the Living Lab Hedwige-Prosperpolder, on the left bank of the Scheldt river straddling the border between Belgium and the Netherlands, a series of 27 overflow tests with a purpose-built overflow generator has been executed to determine the strength of the protective layer against erosion at various conditions. The goal of this paper is to inform on the executed test program and the initial results. From the results, it was concluded that in general, a high-quality grass cover on the landside dike slope can withstand high overflow discharges well for 12 to 30 h, without severe erosion damage. Anomalies, such as the presence of animal burrows, reed vegetation, and already present deformations can strongly reduce the resistance of the cover layer and may lead to failure within a couple of hours

Externally driven subsurface fluid pumping and consequences

Thibodeaux & Boyle (1987) showed by analog modeling that convective transport in sediments can be generated by the presence of bedforms (e.g. sand ripples). This process is known to be of great importance for the biogeochemistry of the subsurface realm in many settings (e.g. rivers, shelf sediments,...).A multiphysics model was developed to numerically explore this process in deep-sea environments. The model integrates a stream over different seafloor obstacles, pressure effects at the sediment surface and the there from resulting subsurface fluid flow. Additionally, the geochemical consequences in settings where anaerobic methane oxidation is an important process, are simulated through a simple second order kinetics model.Through this model, we simulated two submarine settings: deep-sea carbonate mounds and seafloor pockmarks. The model is further evaluated by comparing natural examples of these features with the model results