Adolfo Maestro

Vast fields of hydrocarbon-derived carbonate chimneys related to the accretionary wedge/olistostrome of the Gulf of Cádiz

We report the first discovery and sampling of vast fields of hydrocarbon-derived carbonate chimneys along the Gulf of Cadiz continental slope, at depths between 500 and 1200 m. A large variety of carbonate chimneys (more than 200 samples) were recovered from four different areas, named the DIASOM, TASYO, GBF (Guadalquivir Basin) and ESF (East Moroccan) fields. Observations from an underwater camera revealed a spectacular high density of pipe-like chimneys, some of them longer than 2 m, lying over the sea floor, and some protruding from muddy sediment. Local fissures and alignment of isolated chimneys were also observed, suggesting that their distribution is controlled by fault planes. Chimneys collected show a wide range of morphological types (spiral, cylindrical, conical, mushroom-like and mounded) with numerous nodule protuberances and ramified fluid channelways. The chimneys are mainly composed of authigenic carbonates (ankerite, Fe-bearing dolomite and calcite) with abundance of iron oxides, forming agglomerates of pseudo-pyrite framboids. N 18 O isotopic values vary from 0.7 to 5.5x whereas N 13 C values indicate that chimney carbonates are moderately depleted in 13 C, ranging from 346x to 320x PDB, interpreted as formed from a mixture of deep thermogenic hydrocarbons and shallow biogenic methane. These vast fields of carbonate chimneys imply new considerations on the importance of hydrocarbon fluid venting in the Gulf of Cadiz and, moreover, on the active role of the olistostrome/accretionary wedge of the Gibraltar arc. B 2003 Elsevier Science B.V. All rights reserved.

Seabed morphology and hydrocarbon seepage in the Gulf of Cádiz mud volcano area: Acoustic imagery, multibeam and ultra-high resolution seismic data

Abstract Extensive mud volcanism, mud diapirism and carbonate chimneys related to hydrocarbon-rich fluid venting are observed throughout the Spanish–Portuguese margin of the Gulf of Cadiz. All the mud volcanoes and diapirs addressed in this paper lie in the region of olistostrome/accretionary complex units which were emplaced in the Late Miocene in response to NW-directed convergence between the African and Eurasian plates. The study area was investigated by multibeam echo-sounder, high and ultra-high resolution seismic profiling, dredging and coring. The structures observed on multibeam bathymetry, at water depths between 500 and 1300 m, are dominated by elongate mud ridges, mud cones, mud volcanoes and crater-like collapse structures ranging in relief from 50 to 300 m and size from 0.8 to 2 km in diameter. The main morphotectonic features, named the Guadalquivir Diapiric Ridge (GDR) and the Cadiz Diapiric Ridge (CDR), are longitudinally shaped diapirs which trend NE–SW and consist of lower–middle Miocene plastic marly clays. The GDR field and the TASYO field, which consist of mud volcanoes and extensive fluid venting related to diapiric ridge development, are described in this paper. The GDR field is characterised by numerous, single, sub-circular mud volcanoes and mud cones. The single mud volcanoes are cone-shaped features with relatively gentle slopes of 3°–6°, consisting of several generations of mud breccia deposition with indications of gas-saturation, degassing structures and the presence of H2S. The mud cones have asymmetric profiles with steep slopes of up to 25° and contain large surficial deposits of hydrocarbon-derived carbonate chimneys and slabs. The TASYO field is characterised by an extensive concentration of small, sub-circular depressions, oval and multi-cone mud volcanoes and large sediment slides. Mud volcanoes in this area are characterised by moderate slopes (8°–12°), have bathymetric relief ranging from 100 to 190 m and consist of sulphide-rich mud breccia, calcite chimneys, carbonate crusts and chemosynthetic fauna (Pogonophora tube worms). We propose that all these hydrocarbon seepage structures are related to lateral compressional stress generated at the front of the olistostromic/accretionary wedge. This stress results in the uplifting and squeezing plastic marly clay deposits. Additionally, the compressional stress at the toe of the olistostrome forms overpressurised compartments which provide avenues for hydrocarbon-enriched fluids to migrate.

Looking for clues to paleoceanographic imprints: A diagnosis of the Gulf of Cadiz contourite depositional systems

A new morphosedimentary map of the Gulf of Cadiz is presented, showing the contourite depositional system on the gulfs middle slope. This map is constructed from a broad da- tabase provided by the Spanish Research Council and the U.S. Naval Research Laboratory. Our map shows that this contourite depositional system comprises five morphosedimentary sectors: (1) proximal scour and sand ribbons; (2) overflow sedimentary lobe; (3) channels and ridges; (4) contourite deposition; and (5) submarine canyons. The Gulf of Cadiz con- tourite depositional system stems directly from the interaction between Mediterranean Out- flow Water and the seafloor; its morphosedimentary sectors are clearly related to the sys- tematic deceleration of the Mediterranean Outflow Waters westward branches, bathymetric stress on the margin, and the Coriolis force. The slopes depositional system can be consid- ered as a mixed contourite and turbidite system, i.e., a detached combined drift and fan.

Seismic stacking pattern of the Faro-Albufeira contourite system (Gulf of Cadiz): a Quaternary record of paleoceanographic and tectonic influences

A Quaternary stratigraphic stacking pattern on the Faro-Albufeira drift system has been determined by analysing a dense network of high-resolution single-channel seismic reflection profiles. In the northern sector of the system an upslope migrating depositional sequence (elongate separated mounded drift) parallel to the margin has been observed associated with a flanking boundary channel (Alvarez Cabral moat) that depicts the zone of Mediterranean Outflow Water (MOW) acceleration and/or focussing. A consequent erosion along the right hand border and deposition on the left hand flank is produced in this sector. The sheeted aggrading drift is the basinward prolongation of the elongate separated mounded drift, and developed where the MOW is more widely spread out. The overall sheeted contourite system is separated into two sectors due to the Diego Cao deep. This is a recent erosional deep that has steep erosional walls cut into Quaternary sediments. Two major high-order depositional sequences have been recognised in the Quaternary sedimentary record, Q-I and Q-II, composed of eight minor high-order depositional sequences (from A to H). The same trend in every major and minor depositional sequence is observed, especially in the elongate mounded drift within Q-II formed of: A) Transparent units at the base; B) Smooth, parallel reflectors of moderate-high amplitude units in the upper part; and C) An erosional continuous surface of high amplitude on the top of reflective units. This cyclicity in the acoustic response most likely represents cyclic lithological changes showing coarsening- upward sequences. A total of ten minor units has been distinguished within Q-II where the more representative facies in volume are always the more reflective and are prograding upslope with respect to the transparent ones. There is an important change in the overall architectural stacking of the mounded contourite deposits from a more aggrading depositional sequence (Q-I) to a clear progradational body (Q-II). We suggest that Q-I and Q-II constitute high-order depositional sequences related to a 3rd-order cycle at 800 ky separated by the most prominent sea-level fall at the Mid Pleistocene Revolution (MPR), 900–920 ky ago. In more detail the major high-order depositional sequences (from A to H) can be associated with asymmetric 4th-order climatic and sea-level cycles. In the middle slope, the contourite system has a syn-tectonic development with diapiric intrusions and the Guadalquivir Bank uplift. This syn-tectonic evolution affected the overall southern sheeted drift from the A to F depositional sequences, but G and H are not affected. These last two depositional sequences are less affected by these structures with an aggrading stacking pattern that overlaps the older depositional sequences of the Guadalquivir Bank uplift and diapiric intrusions.

Evidence for hydrothermal venting and sediment volcanism discharged after recent short-lived volcanic eruptions at Deception Island, Bransfield Strait, Antarctica

Abstract The results of a combined geophysical and geochemical research programme on Deception Island, an active volcano at 62°43′S, 60°57′W in Bransfield Strait (Antarctica), are presented. Ultrahigh-resolution acoustic data obtained with a TOPAS (TOpographic PArameter Sonar) system and multibeam bathymetry (Simrad EM1000) allow a detailed analysis of submarine vents in Port Foster, the submerged caldera of Deception Island. The data show three different types of seafloor structures: low-relief mounds, high-relief mounds (‘wasp nest’-like) and spire-like structures. We interpret these structures as products of sediment volcanism and seeps caused by heating and boiling of pore fluids in gas-charged sediments, and related to recent short-lived volcanic events, possibly those that occurred in 1967, 1969 and 1970. In addition, subsurface vertical disturbed zones, formed by increased amplitude and phase-inverse reflectors beneath the mounds, suggest the presence of fluidised and brecciated sediments within hydrofracture systems. A key finding of this study is that there appears to be a close relationship between the submarine mounds detected by our ultrahigh-resolution seismic study, geochemical haloes, fault-pathways and present-day thermal anomalies in surface waters. We suggest that seafloor hydrofracture systems and subsurface pipes can be re-used as fluid migration pathways, resulting in hydrothermal seeps and vents on the seafloor, possibly up to decades after coeval volcanic eruptions.

Geometry and structure associated to gas-charged sediments and recent growth faults in the Ebro Delta (Spain)

Abstract In the Ebro Delta, NE Spain, small- to large-scale synsedimentary deformation has been observed by means of high- and low-resolution seismics correlated with a network of wells. Lithoseismic units involved are middle–upper Pleistocene clays and gravels and Holocene deltaic sequence. Gassy sediments and large river-floor pockmarks are identified from seismic profiles along the Ebro Delta. Gas-charged sediments are identified by zones of acoustic turbidity and other acoustic anomalies on seismic profiles. Most of the gas is believed to be biogenic in origin, resulting from the decay of organic matter contained within rapidly developing shelf margin deltas. A group of large symmetric river-floor pockmarks also occurs along the inner delta plain, both on the river-floor surface and buried in the sediment column. Listric fault geometry is a typical scope-shaped plane with a steep upper surface (40–60°) that passes downward into a horizontal shear plane. The shear plane appears seismically as chaotic and convoluted high-amplitude reflectors and is correlated with well data showing layers that include gravel, sand and clays. Associated structural features include reverse drag of reflectors, antithetic faults, and fluid escapes (methane/water). The growth faulting along the Ebro Delta includes a break-away and thin, near-surface layer created by overstepping of a sandy Holocene delta slope. The over-steeping lies down-dip from growth faults along the shelf-break. The overall growth faults province appears to represent a deeper-seated general movement of the delta mass involving differential loading, overpressure of fluids, diapiric movements, and extensional tectonics that cause listric faults and associated structures at different scales. The recent growth faults are regularly spaced, with the distance between successive faults controlled by the depth to the detachment layer. Three main fault systems have been identified: A first or primary system spaced 5.2–10.5 km above a detachment layer at 55–95 m deep; a second system spaced 3.5–6 km; and a third system spaced 1.5–2.5 km above a detachment layer at 25–30 m. The slope angle calculated by empirical equations ranges between 0.5° and 1.0° for the three systems.

Recent tectonic and morphostructural evolution of Byers Peninsula (Antarctica): insight into the development of the South Shetland Islands and Bransfield Basin

Byers Peninsula forms the western extremity of the Livingston Island (Antarctica) in the continental South Shetland Block. This tectonic block is bounded by the South Shetland Trench to the north, the Bransfield back-arc basin to the south, and extends to the South Scotia Ridge on the east. Westwards it is connected to the Antarctic Plate by a broad deformation zone located at the southern end of the Hero Fracture Zone. In Byers Peninsula we analyzed more than 1,200 lineaments, and 359 fault planes from 16 sites, both in sedimentary and intrusive igneous rocks. Statistical analysis of lineaments and mesoscopic fractures, with a length varying between 31 and 1,555 m, shows a NW-SE maximum trend, with two NE-SW and ENE-WSW secondary maximums. Fault orientation analysis shows similar trends suggesting that most of the lineaments correspond to fractures. Due to the absence of striated faults and the lack of kinematic evidence on the regime in most of the analyzed faults we have used the Search Grid paleostress determination method. The results obtained allow us to improve and complete the data on the recent evolution of the South Shetland Block. In this complex geodynamic setting, Byers Peninsula has been subjected to NNW-SSE to NNE-SSW extension related to Bransfield Basin opening and NE-SW and NW-SE local compressions respectively associated to Scotia-Antarctic plate convergence and the South Shetland Trench subduction.

Geomorphological mapping in the Antarctic Peninsula region applying single and multipolarization RADARSAT-2 data

RADARSAT-2 data in the ultra-fine and fine quad polarization mode was evaluated to identify morpho-structural, periglacial, and other geomorphological features within the Antarctic Peninsula region. The study was carried out within Byers Peninsula, the largest ice-free area on the South Shetland Islands. A methodology was developed to integrate and compare data from different sources to optimize the selection of criteria and techniques to obtain information from RADARSAT-2 relevant to the geomorphology of the study area. Ultra-fine data were focused on the extraction of linear features and fine structures; whereas, fine quad polarized data were implemented to identify scattering mechanisms for different surface covers. It was possible to extract a greater number of lineaments and structures with the ultra-fine data. A supervised classification was carried out using the fine quad polarized data, where a careful selection of training and validation sites was needed. The overall classification accuracy was 75% with a Kappa coefficient of 0.71. Surface cover features with low entropy scattering mechanisms were well identified with a maximum of 86% accuracy. The accuracy for higher entropy volume and multiple scattering mechanisms were only slightly lower and are considered especially important for the type of geomorphological features studied on Byers Peninsula.

Tectonics and Geomorphology of Elephant Island, South Shetland Islands

New tectonic and geomorphological data from Elephant Island have been obtained during a cooperative fieldwork campaign carried out in the 2002–2003 season by Brazilian and Spanish groups. The main phases of ductile deformation affecting the high-pressure metamorphic rocks of Elephant Island, D1, D2 and D3, were studied in more detail along a N-S profile in the western sector of the island. D2 was subdivided in two subphases, D2a and D2b. The brittle and brittle-ductile deformation postdating the three main phases was studied systematically for the first time in the island. The measurement of the orientation of more than 200 faults with their kinematics pointed out that normal faults indicating an extensional setting predominate in the southern sector of the island. By contrast, in the northern part of the studied profile, major faults are predominantly reverse, although normal faults are also present, indicating the overprinting of at least two deformation stages. A new 1:50000 topographic map of the island, with 20 m contour interval, was prepared being included in this paper a shadow map from a 3D digital model based on it. Geomorphological features of marine and glacial origin were identified and mapped. The most significant of these are the raised marine platforms up to 150 m a.s.l. at Cape Lindsey area and lower platforms, Holocene raised beaches and moraines at Stinker Point area.

Elephant Island Recent Tectonics in the Framework of the Scotia-Antarctic-South Shetland Block Triple Junction (NE Antarctic Peninsula)

Elephant Island is an outcrop of the South Shetland Block, and is located at the southeastern prolongation of the Shackleton Fracture Zone. The South Shetland Block constitutes a continental fragment of the southern branch of the Scotia Arc and has been separated since the Pliocene from the Antarctic Peninsula by the opening of the Bransfield Strait and the transtensional fault zone that extends along the South Scotia Ridge. The northern boundary of the block is determined by subduction at the South Shetland Trench and its eastwards prolongation. The Shackleton Fracture Zone extends up to the South Shetland Block and constitutes the westernmost segment of the Scotia-Antarctic plate boundary. In this tectonic context, Elephant Island permits study of the rocks and the structures that resulted from deformation of the South Shetland Block near the triple junction with the Antarctic and Scotia Plates.