Late Pleistocene-Holocene sediments on the Spanish continental shelves: Model for very high resolution sequence stratigraphy

Sequence stratigraphy analysis of high resolution seismic profiles (Geopulse, Uniboom and 3.5 kHz) of late Pleistocene-Holocene sediments has been carried out on five sectors of the Spanish continental margin. Four types of depositional settings are distinguished in these sectors: (1) low subsident ramps (Alborán Margin-Cádiz Gulf); (2) high subsident (2m/kyr) ramps (Alicante-Valencia); (3) “Ria”-type morphology on the Atlantic passive margin (Ria de Muros); and (4) fault-scarp morphology systems with subsidence (Balearic Margin). A Type 1 sequence is interpreted in all these sectors, being composed of lowstand systems tract, transgressive systems tract and highstand systems tract. This conforms to the basic concepts of sequence stratigraphy and each systems tract correlates with a particular part of the last eustatic hemicycle. Characteristic shelf features such as terraces, terraces with beach deposits and progradational sediment wedges evidence a complex stacking of lesser sub-sequences in all the systems tracts, which must be related to very-short period sea-level stillstands and fall. We propose a very high resolution sequence stratigraphy model in which the last sea-level hemicycle is punctuated by: “P” cycles (4500 years), which give rise to the neo-glacial events; “h” cycles (2200-950 years), and “c” cycles (500-50 years). These cycles interact with each other, thus establishing the placing of the high and low sea levels.

This attention to detail: (1) explains sedimentary evolution on both the shelf and upper slope during late Pleistocene-Holocene time; (2) illustrates some departures from the classical sequence stratigraphy model; and (3) also demonstrates that the late Pleistocene-Holocene eustatic curve is not one simple transgression but is modulated by three differing-period cycle groups below the Milankovitch band. Our model is delimited by fluctuating sea level during Pleistocene-Holocene times. Such features should be identifiable on any continental margin. However, localized features occur due to subsidence and continental shelf morphology which determ ine the location and depth of sedimentary bodies generated in each eustatic cycle.     

Geological characterization of the Prestige sinking area

The tanker Prestige sank off NW Iberia on the 19th November 2002. The stern and bow of the Prestige wreck are located on the southwestern edge of the Galicia Bank, at 3565 m and 3830 m water depths, respectively. This bank is a structural high controlled by major faults with predominant N-S, NNE-SSW, and NNW-SEE trends. It is characterized by moderate to low seismic activity. The faults have controlled the local depositional architecture, deforming, fracturing, relocating and distributing sediments since the Valangian (early Cretaceous). The Prestige sinking area corresponds to an asymmetric half-graben structure with a N-S trend, which conditions the present-day morphology. The faulted flank outcrops and its activity and erosion have favoured the occurrence of mass-movements (slumps, slump debris, mass-flows and turbidity currents), building valleys and depositional lobes. Nearsurface sediments comprise mostly terrigenous and biogenous turbiditic muds and sands with a minor presence of hemipelagic muds, except on the fault scarp where pelagites predominate. Potential geological hazards resulting from tectonic and sedimentary processes affect almost the entire Prestige sinking area.     

Large-scale slope failure involving Triassic and Middle Miocene salt and shale in the Gulf of Cádiz (Atlantic Iberian Margin)

Sheets of salt and ductile shale advancing beyond the thrust front of the Gibraltar Arc (Iberian–Moroccan Atlantic continental margin) triggered downslope movements of huge allochthonous masses. These allochthons represent the Cádiz Nappe, which detached from the Gibraltar Arc along low‐angle normal faults and migrated downslope from the Iberian and Moroccan continental margins towards the Atlantic Ocean. Extensional tectonics initiated upslope salt withdrawal and downslope diapirism during large‐scale westward mass wasting from the shelf and upper slope. Low‐angle salt and shale detachments bound by lateral ramps link extensional structures in the shelf to folding, thrusting and sheets of salt and shale in the Gulf of Cádiz. From backstripping analyses carried out on the depocentres of the growth‐fault‐related basins on the shelf, we infer two episodes of rapid subsidence related to extensional collapses; these were from Late Tortonian to Late Messinian (200–400 m Myr^?1) and from Early Pliocene to Late Pliocene (100–150 m Myr^?1). The extensional events that induced salt movements also affected basement deformation and were, probably, associated with the westward advance of frontal thrusts of the Gibraltar Arc as a result of the convergence between Africa and Eurasia. The complexities of salt and/or shale tectonics in the Gulf of Cádiz result from a combination of the deformations seen at convergent and passive continental margins.     

Pockmarks, collapses and blind valleys in the Gulf of Cádiz

Herein we describe a suite of fluid escape depression features, including pockmarks and collapse structures, discovered in the Gulf of Cádiz (Spain) during several recent cruises. We also establish an evolutionary model for these depressions and discuss the generation of bottom undercurrent furrows from fluid-flow structures, considering the oceanographic and tectonic framework and gas expulsion mechanisms. We describe for the first time blind valleys, which we define as giant, elongated (3 to 10 km long), collapsed and complex fault-strike features comprising mega-collapses and mega-pockmarks, generated in gas-venting areas and not associated to the collapse of mud-volcano complexes. We detected the blind valleys above diapiric structures. The collapse processes associated to blind valleys result from fluid escape through migration pathways which, in turn, are created by distension due to diapiric activity or to later tectonic reactivation of these diapirs. The evolution of these blind valleys, and their present-day morphology as furrows, derives from progressive fluid migration as well as from interaction of Mediterranean Outflow Water with the seafloor.

Towards a Plio-Pleistocene chronostratigraphy in Eastern Betic Basins (SE Spain)

Abstract

The evolution of Neogene and Quaternary littoral basins in the Eastern Betic Cordillera is largely related to tectonic activity along the Eastern Betic sinistral shear zone.

Detailed mapping of sedimentary units in these basins, together with sedimentological and paleomagnetic analysis lead to the proposal of a new chronostratigraphie framework for Pliocene and Quaternary deposits.

This chronostratigraphie setting rejects the synchronous character of the “Pliocene Unite” previously referred to as: “P.I” (grey-blue marls), “P.H” (yellow calcarenites), and “Р.Ш” (variegated silts and clays). Instead, tectonics would have controlled the paleogeographic evolution of the Eastern Betic realms, causing the lithofacies to occur repeated in space and time. The Plio-Pleistocene boundary in these basins is not accompanied by changes in geodynamic behaviour or climatic conditions. According to the paleomagnetic data, these changes occurred at different times during the Pleistocene in the different studied basins.     

Reconstructions of the Mediterranean Outflow Water during the quaternary based on the study of changes in buried mounded drift stacking pattern in the Gulf of Cadiz

Contourite deposits in the central sector of the middle slope of the Gulf of Cadiz have been studied using a comprehensive acoustic, seismic and core database. Buried, mounded, elongated and separated drifts developed under the influence of the lower core of the Mediterranean Outflow Water are preserved in the sedimentary record. These are characterised by depositional features in an area where strong tectonic and erosive processes are now dominant. The general stacking pattern of the depositional system is mainly influenced by climatic changes through the Quaternary, whereas changes in the depositional style observed in two, buried, mounded drifts, the Guadalquivir and Huelva Drifts, are evidence of a tectonic control. In the western Guadalquivir Drift, the onset of the sheeted drift construction (aggrading QII unit) above a mounded drift (prograding QI unit) resulted from a new Lower Mediterranean Core Water hydrodynamic regime. This change is correlated with a tectonic event coeval with the Mid Pleistocene Revolution (MPR) discontinuity that produced new irregularities of the seafloor during the Mid- to Late-Pleistocene. Changes in the Huelva Drift from a mounded to a sheeted drift geometry during the Late-Pleistocene, and from a prograding drift (QI and most part of QII) to an aggrading one (upper seismic unit of QII), highlight a new change in oceanographic conditions. This depositional and then oceanographic change is associated with a tectonic event, coeval with the Marine Isotope Stage (MIS) 6 discontinuity, in which a redistribution of the diapiric ridges led to the development of new local gateways, three principal branches of the Mediterranean Lower Core Water, and associated contourite channels. As a result, these buried contourite drifts hold a key palaeoceanographic record of the evolution of Mediterranean Lower Core Water, influenced by both neotectonic activity and climatic changes during the Quaternary. This study is an example of how contourite deposits and erosive elements in the marine environment can provide evidence for the reconstruction of palaeoceanographic and recent tectonic changes.     

Tectonic,volcanic, and hydrothermal event sequence on Deception Island (Antarctica)

On the basis of the correlation between seismic tectonostratigraphic units, volcanism, and geochemical distribution patterns of sea-floor sediments on Deception Island (Antarctica), a regional tectonic extension is suggested to explain the sequence of volcanic and hydrothermal events. The syncollapse caldera episode is composed of five tectonostratigraphic units separated by progressively tilted and bent unconformities. Each unit shows a sequence of Quaternary volcanic cones and mound structures that display high contents of Fe, Mn, and Zn. As-rich active fumaroles and hot-spring areas are associated with the postcollapse episode and linked to normal faulting and present volcanism.     

Gas-related morphologies and diapirism in the Gulf of Cádiz

Recently developed high-resolution profiling (multibeam, sonar) and surface sampling were used to map seafloor morphology of the Gulf of Cádiz middle continental slope. Multichannel seismic profiling has made it possible to elucidate the geologic origin of these features as well as the main triggering mechanisms of gas-related morphologies, principally mud volcanoes, carbonate mud mounds, pockmarks and slides. Throughout the entire Gulf of Cádiz, from the continental slope to the shelf and even on land, a close correlation between morphology and gas mobility and associated diapirism can be observed. The middle slope area is strongly deformed by several diapiric ridges, named Guadalquivir, Cádiz and Doñana. Most of the diapirs identified in this study are related to the Allochthonous Unit of the Gulf of Cádiz, a chaotic body emplaced during the Tortonian, containing salt and shale nappes affected by later gravitational extension collapse and reactivated compression thrusts. It can be proposed that diapirism and related tectonics provided gas migration pathways.     

New discoveries of mud volcanoes on the Moroccan Atlantic continental margin (Gulf of Cádiz): morpho-structural characterization

During the MVSEIS-08 cruise of 2008, ten new mud volcanoes (MVs) were discovered on the offshore Moroccan continental margin (Gulf of Cádiz) at water depths between 750 and 1,600?m, using multibeam bathymetry, backscatter imagery, high-resolution seismic and gravity core data. Mud breccias were recovered in all cases, attesting to the nature of extrusion of these cones. The mud volcanoes are located in two fields: the MVSEIS, Moundforce, Pixie, Las Negras, Madrid, Guadix, Almanzor and El Cid MVs in the western Moroccan field, where mud volcanoes have long been suspected but to date not identified, and the Boabdil and Al Gacel MVs in the middle Moroccan field. Three main morphologies were observed: asymmetric, sub-circular and flat-topped cone-shaped types, this being the first report of asymmetric morphologies in the Gulf of Cádiz. Based on morpho-structural analysis, the features are interpreted to result from (1) repeated constructive (expulsion of fluid mud mixtures) and destructive (gravity-induced collapse and submarine landsliding) episodes and (2) interaction with bottom currents.