Moroccan Atlantic Margin Research Articles

Mineralogical and geochemical study of mud volcanoes in north Moroccan atlantic margin

The gulf of Cadiz is one of the most interesting areas to study mud volcanoes and structures related to cold fluid seeps since their discovery in 1999. In this study, we present results from gravity cores collected from Ginsburg and Meknes mud volcanoes and from circular structure located in the gulf of Cadiz (North Atlantic Moroccan margin) during the cruise TTR14 in 2004 on board of the R/V Logachev. The clay mineralogical analysis by XRD shows a difference in clay mineral amounts mainly in smectite between the different studied structures. Ginsburg MV shows high percentage of smectite with an average of 44% whereas Meknes MV displays illite rich clay association, smectite average percentage is about 16%. In circular structure, amount of smectite is about 13%. This variation in clay mineralogy association between the mud volcanoes suggests different nature and depth of parent layers and fluids feeding the mud volcanoes in gulf of Cadiz. Key word : Mud volcano, clay mineralogy, geochemistry, mud breccias, North Moroccan Atlantic margin.

Imaging a lithospheric detachment at the continent–ocean crustal transition off Morocco

The SISMAR seismic survey on the Moroccan Atlantic Margin recorded deep penetration images of the continent–ocean boundary. This paper focuses on the 3D observation of a landward dipping reflector, overlaid by a layered unit. The deep part of the reflector plunges towards the continent beneath thinned continental crust whereas its upper part finishes at the top of the basement in a transition zone where likely volcanics dipping reflectors are visible and where a magnetic anomaly named S1 marks the continent–ocean boundary location. As the landward dipping reflector crosses the Moho, it is interpreted as a lithospheric detachment with associated volcanic material originating from the asthenospheric mantle. Observations are used (i) to build a 2D model to explain the mechanisms of the lithospheric breaking off between Moroccan margin and its conjugate Canadian margin, the landward dipping reflector allowing mantle exhumation; (ii) to propose an ENE–WNW trend for the initial rifting extension in this part of Atlantic during late Triassic, direction compatible with that of the southern Grand Banks transform margin.

Geophysical evidence of gas hydrates in shallow submarine mud volcanoes on the Moroccan margin

Gas hydrates inside mud volcanoes have been observed in several locations but are generally found at water depths of 1000 m and deeper. We present the first observation of the base of a gas hydrate stability zone within a shallow mud volcano in the El Arraiche mud volcano field on the Moroccan Atlantic margin. The mud volcano base is located at about 475 m and is over 125 m high. On high‐resolution seismics we observed an anomalous but coherent reflection under the slopes of the mud volcano. The event was interpreted as the base of a gas hydrate stability zone because of its inverse polarity and its morphology. Far from the crater, the event is nearly parallel to the seafloor. Closer toward the crater, the event shallows. Inside the mud volcano crater, no event is observed. A stability model using thermogenic gas compositions is applied to local P‐T conditions, indicating that thermogenic gas hydrates can be stable at this depth. The high modeled heat flow in the crater of the mud volcano indicates a focused flow of warm fluids. Below the slopes of the mud volcano, the inferred heat flow is also elevated but less high. In areas of thermogenic gas production, gas hydrates can occur at shallow water depths, even in areas with high heat flow. This also suggests that dewatering of the accretionary wedge complex is mainly focused along fault surfaces and through seafloor structures, such as mud volcanoes.

Storminess control over African dust input to the Moroccan Atlantic margin (NW Africa) at the time of maxima boreal summer insolation: a record of the last 220 kyr

Precessional forcing of dust flux from Northwest Africa to the Atlantic during the last 220 kyr is recognised through changes in physical and chemical sediment properties (sediment colour reflectance, magnetic susceptibility, X-ray fluorescence element intensity) in a core from the Moroccan margin. Sediment colour reflectance, magnetic susceptibility, and Ca and Fe element intensity changes are thought to be driven by dust input from North Africa. Spectral analysis of the sediment properties records displays a dominant periodicity in the 1/23 kyr−1 frequency band that is associated with the Earth’s orbital precession. Peaks in terrigenous supply match precessional minima. This suggests a close link between maxima of boreal summer solar radiation, monsoon activity, and dust generation. Enhanced precession-driven solar radiation would have caused increased seasonal temperature contrasts, which amplified atmospheric turbulence and stimulated storminess. Such a scenario is similar to today’s summer conditions, when frequent storms cause distinct dust plumes to form in the area and transport fine terrigenous material to the adjacent North Atlantic Ocean.

Transtensive synsedimentary tectonics associated with ocean opening: the Essaouira–Agadir segment of the Moroccan Atlantic margin

The Mesozoic–Cenozoic sedimentary sequences of the Agadir–Essaouira region were studied by field observations in the onshore basin and offshore by seismic profiles and borehole data. During the Late Jurassic and the Early Cretaceous, they were frequently disturbed by repeated synsedimentary tectonic instability, extensively demonstrated by structures such as slumps, breccias, faults, etc. Regionally, these movements led to the initiation of ridges, in some cases diapiric, as well as depocentres. These are mainly N 70°E to E–W trending. Contemporaneously with a slight tectonic inversion of some of the offshore half grabens, the Late Jurassic–Cretaceous regional deformation converted the synsedimentary ridges to anticlines, and the depocentres to synclines, before the latest ‘Atlasic’ orogenic uplift, during the Cenozoic. This structural evolution distinguishes the Agadir–Essaouira segment from other parts of the Atlantic margin of Morocco. It results from an interference between the N20°E-oriented passive margin of Morocco and the N70°E-oriented Atlasic zone. The latter acted as a transform zone during Late Jurassic–Early Cretaceous times and then suffered a N–S compression during the Atlasic orogeny.

Associations d'ostracodes des marges téthysiennes et atlantiques marocaines de l'albien au turonien

Morocco situated in north-western Africa, lies at an important faunal cross point of the Atlantic and the Tethyan oceans. The marine ostracod faunas are controlled by eustatism. Atlantic influences are clear in Aptian and Albian times. In the Cenomanian, Tethys played the predominant role and towards the west tethysian faunas reach regions located some distance from the present Atlantic coast. The study of the middle Cretaceous (Albian to Turonian) ostracod associations, on tethysian and atlantic margins, allows us to establish the following facts: faunal relations between european and south-tethysian margins are moderate but constant from the Albian to the Turonian. Common species are numerous and the proximity of the european and eurasiatic plates facilitated exchanges; studies of the ostracod faunas in the north-tethysian mid-Cretaceous demonstrate endemic faunas. Relations between west-european and south-tethysian margins are almost non-existent. In the Barremian to the upper Cenomanian-lower Turonian period, faunal relations were established between the moroccan atlantic margin and the south-tethysian margin, under the influence of similar sedimentologic and climatic conditions. Faunas common to the east atlantic margin, the west atlantic margin and the south tethysian margin, are seldom seen, but seem to be present, in Albian-Turonian times, during and after the opening of the South-Atlantic Ocean and the establishment of its communication with the North-Atlantic Ocean. The synthesis of paleobiogeographic data and the dynamics of ostracod assemblages reveal, in middle Cretaceous times, a “North-African-Middle-East” paleobiogeographical province established since the lower Cretaceous and extending to the south-tethysian margin. The biogeographical homogeneity of the ostracod associations suggests similar living conditions along the whole expanse of the margin and the absence of important morphological barriers that would prevent possible migrations of benthic faunas (the spreading of these faunas was kept under control by tethysian and atlantic currents). The distribution of other paleontological groups (benthic foraminifera and rudists) bears out the existence of this province. The colonization of the North-African-Middle-East province depends on paleobiogeographic and geodynamic factors (linked with the positions of the continental margins and with the evolution of the mid-oceanic ridges), paleoceanographic factors (the positions of emerged blocks and the evolution of oceanic areas), and paleoclimatic factors (sea water temperature determining the latitudinal distribution of the ostracod genera). In the study of provincialism, the distribution of ostracods is influenced not only by local factors of depositional environments (for example; sedimentation, substratum, feeding, temperature, salinity…), but also by global events associated with the distribution of continental and oceanic masses (for example; depth of sea waters, ocean currents…).

Magnetobiostratigraphy of a Late Miocene section from the Moroccan Atlantic margin

Paleomagnetic study of a Late Miocene section from Atlantic Morocco correlated with detailed biostratigraphic data suggests an assignment of the observed polarity sequence to the uppermost part of chron 7, to entire chron 6 and to the beginning of chron 5. Comparison with time equivalent sections in the Southern Hemisphere does not suggest noticeable delay in long faunal migration (between the Northern and Southern Hemisphere) during the late Miocene. Thus the postulation of a delay in arrival of species in the Eastern Mediterranean at the base of the Messinnian has to be explained by the presence of selective barriers which could have been located either between the Mediterranean and Atlantic or between the Western and Eastern Mediterranean in the basins delineated during the final evolution of the Tethys. This study also indicates a possible application of biomagnetostratigraphy to the reconstruction of basin interconnections.