Estefanía Llave

Despite the rise in published evidence of deep‐marine bottom current processes and associated deposits there are still very few documented outcrop examples. Herein are reported results of a contourite channel system related to the late Miocene palaeo‐Mediterranean Outflow Water in the Rifian Corridor, Morocco. This work aims to unravel the sedimentary evolution and facies distribution based on the study of large morphological features related to contourite channels and their subsequent sandstone dominated infill sequences. It was found that the channel evolution and facies distribution are related to spatiotemporal changes in flow characteristics of the palaeo‐Mediterranean Outflow Water. The recognized channel facies distribution correlates well with previously established bedform stability diagrams. Erosion and upper‐stage flow regime bedforms are associated with the most vigorous bottom currents, generally related to its core. Laterally, following the decrease in flow velocity towards the adjacent drift, bedforms comprise dunes, lower‐stage plane bedforms and more heterolithic facies. Similar facies changes are also observed down‐channel, related to a decrease in flow velocities resulting from turbulent mixing of water masses, associated decreases in density gradients and the subsequent deceleration due to gravity. Results of this work have been used to propose a three‐dimensional facies model for channelized sandy contourites. This study contributes to understanding contourite systems, thus benefitting oceanographic and climatic reconstructions as well as aiding the predictability of contourite channel systems for industry geoscience applications.

The Mediterranean-Atlantic water mass exchange provides the ideal setting for deciphering the role of gateway evolution in ocean circulation. However, the dynamics of Mediterranean Outflow Water (MOW) during the closure of the Late Miocene Mediterranean-Atlantic gateways are poorly understood. Here, we define the sedimentary evolution of Neogene basins from the Gulf of Cádiz to the West Iberian margin to investigate MOW circulation during the latest Miocene. Seismic interpretation highlights a middle to upper Messinian seismic unit of transparent facies, whose base predates the onset of the Messinian salinity crisis (MSC). Its facies and distribution imply a predominantly hemipelagic environment along the Atlantic margins, suggesting an absence or intermittence of MOW preceding evaporite precipitation in the Mediterranean, simultaneous to progressive gateway restriction. The removal of MOW from the Mediterranean-Atlantic water mass exchange reorganized the Atlantic water masses and is correlated to a severe weakening of the Atlantic Meridional Overturning Circulation (AMOC) and a period of further cooling in the North Atlantic during the latest Miocene.

Despite numerous efforts to properly differentiate between contourites and other deep‐water deposits in cores and outcrops, reliable diagnostic criteria are still lacking. The co‐occurrence of downslope and along‐slope sedimentary processes makes it particularly difficult to differentiate these relatively homogeneous deposits. The main aim of this paper is to identify differences in deep‐water sediments based on Principal Component Analysis of grain size and geochemistry, sedimentary facies, and reinforced by microfacies and ichnofacies. The sediments studied were obtained from two International Ocean Drilling Program Expedition 339 sites in mounded and sheeted drifts in the Gulf of Cadiz. The statistical approach led to the discernment of hemipelagites, silty contourites, sandy contourites, bottom current reworked sands, fine‐grained turbidites and debrites over a range of depositional and physiographic elements. These elements are linked to contourite drifts, the drift‐channel transition, the contourite channel and distal upper slope. When bottom currents or gravity‐driven flows are not the dominant depositional process, marine productivity and continental input settling forms the main depositional mechanism in deep‐water environments. This is reflected by a high variability of the first principal component in hemipelagic deposits. The stacked principal component variability of these deposits evidences that the contourite drift and the adjacent contourite channel were influenced by the interrelation of hemipelagic, gravitational and bottom current induced depositional processes. This interrelation questions the paradigm that a drift is made up solely of muddy sediments. The interrelation of sedimentary processes is a consequence of the precession‐driven changes in the intensity of the Mediterranean Outflow Water related to Mediterranean climate variability, which are punctuated by millennial‐scale variability. Associated vertical and lateral shifts of the Mediterranean Outflow Water, and therefore of its interface with the East North Atlantic Central Water, controlled sediment input and favoured turbulent sediment transport in the middle slope. During the interglacial precession maxima/insolation minima, a more vigorous upper core of the Mediterranean Outflow Water and the enhanced impact of the East North Atlantic Central Water – Mediterranean Outflow Water interface allowed for the development of the sandier contourite deposits.

Statistical methods of crust surface analysis based on Digital Terrain Models (DTM) and applied to structural elements have become a useful tool in tectonic studies. This paper presents a DTM morphostructural analysis of the Galicia Margin and the Iberian and Biscay abyssal plains, which has identified about 5,800 lineaments. The highest number of lineaments are located in the Charcot and Coruña Highs, in the western sector of the Galicia Bank, in the marginal platforms zone and in the Galicia northern margin. The lineaments analysis shows that they have two dominant directions, E-W and NE-SW. In addition, two other relative maxima of NW-SE and N-S directions have been determined. The cumulative total length of the identified lineaments exceeds 45,000 km. The statistical parameters have a length mode of around 5,000 m, a length average of about 7,800 m and a log-normal distribution of the length frequency. The orientations determined from the analysis of the lineaments are a reflection of the crustal scale fault systems established from gravimetric and magnetic anomalies and multi-channel seismic profiles. These faults developed in the Galicia Margin during the Variscan and Alpine orogenies and were reactivated under the recent stress field.

Interbedded contourites, turbidites and pelagites are commonplace in many deep‐water slope environments. However, the distinction between these different facies remains a source of controversy. This detailed study of calcareous contourites and associated deep‐marine facies from an Eocene–Miocene sedimentary succession on Cyprus clearly documents the diagnostic value of microfacies in this debate. In particular, the variability of archetypical bi‐gradational contourite sequences and their internal subdivision (bedding, layering and lamination) are explored. Contourites can be distinguished from turbidites, pelagites and hemipelagites by means of carbonate microfacies in combination with bed‐scale characteristics. Particle composition provides valuable information on sediment provenance. Depositional texture, determined by the ratio between carbonate mud and bioclasts, is crucial for identifying bi‐gradational sequences in both muddy and sandy contourites, and normally‐graded sequences in turbidite beds. Equally important are the type and preservation of traction structures, as well as the temporality and impact of bioturbation. Shell fragmentation under conditions of increased hydrodynamic agitation (textural inversion) is recognized as a carbonate‐specific feature of bioclastic sandy contourites.

This work uses seismic records to document and classify contourite features around the Iberian continental margin to determine their implications for depositional systems and petroleum exploration. Contourites include depositional features (separated, sheeted, plastered and confined drifts), erosional features (abraded surfaces, channels, furrows and moats) and mixed features (contourite terraces). Drifts generally show high- to moderate-amplitude reflectors, which are cyclically intercalated with transparent layers. Transparent layers may represent finer-grained deposits, which can serve as seal rocks. High-amplitude reflectors (HARs) are likely to represent sandier layers, which could form hydrocarbon reservoirs. HARs occur on erosive features (moats and channels), and are clearly developed on contourite terraces and overflow features. Most of the contourite features described here are influenced by Mediterranean water masses throughout their Pliocene and Quaternary history. They specifically record Mediterranean Outflow Water, following its exit through the Gibraltar Strait. This work gives a detailed report on the variation of modern contourite deposits, which can help inform ancient contourite reservoir interpretation. Further research correlating 2D and 3D seismic anomalies with core and well-logging data is needed to develop better diagnostic criteria for contourites. This can help to clarify the role of contourites in petroleum systems.

Our understanding of the role of bottom currents and associated oceanographic processes (e.g., overflows, barotropic tidal currents) including intermittent processes (e.g., vertical eddies, deep sea storms, horizontal vortices, internal waves and tsunamis) is rapidly evolving. Many deep-water processes remain poorly understood due to limited direct observations, but may generate significant depositional and erosional features on both shortand long-term time scales. This paper describes these oceanographic processes and examines their potential role in the sedimentary features around the Iberian margin. The paper explores the implications of the processes studied, given their secondary role relative to other factors such as mass-transport and turbiditic processes. An integrated interpretation of these oceanographic processes requires an understanding of contourites, sea-floor features, their spatial and temporal evolution, and the near-bottom flows that form them. Given their complex, three-dimensional and temporally-variable nature, integration of these processes into sedimentary, oceanographic and climatological frameworks will require a multidisciplinary approach that includes Geology, Physical Oceanography, Paleoceanography and Benthic Biology. This approach will synthesize oceanographic data, seafloor morphology, sediments and seismic images to improve our knowledge of permanent and intermittent processes around Iberia, and evaluate their conceptual and regional role in the sedimentary evolution of the margin.

The products of bottom current circulation around the Iberian continental margin are characterised by large erosional and depositional features formed under a variety of geological and oceanographic contexts. The Iberian margins are influenced by several water masses that mainly interact along the upper and middle continental slopes, as well as along the lower slope with the abyssal plains being influenced to a lesser extent. The main depositional features occur along the Ceuta Contourite Depositional System (CDS) within the SW Alboran Sea, in the Gulf of Cadiz (the most studied so far), the western margins of the Portugal/Galician margin, the Ortegal Spur and the Le Danois Bank or “Cachucho”. Moreover, erosional contourite features have also been recently indentified, most notably terraces, abraded surfaces, channels, furrows and moats. The majority of these features are formed under the influence of the Mediterranean water masses, especially by the interaction of the Mediterranean Outflow Water (MOW) with the seafloor. The MOW is characterized as relatively warm (13°C) and with a high salinity (~ 36.5), giving it a high density relative to the surrounding water masses, hence constituting an important contribution to the global thermohaline circulation, making it one of the most studied water masses surrounding Iberia. The development of both depositional and erosional contourite features does not only depend on the bottom-current velocity but also on several other important controlling factors, including: 1) local margin morphology affected by recent tectonic activity; 2) multiple sources of sediment supply; 3) water-mass interphases interacting with the seafloor; and 4) glacioeustatic changes, especially during the Quaternary, when the increasing influence of the bottom current has been observed during the cold stages. The main objective of this special volume contribution is to provide a review and description of the regional along-slope processes and their sedimentary impact around the Iberian margin. Despite the numerous examples of bottom current processes recorded, there remains a number of challenges to understanding CDSs around the Iberian margin including: 1) evidencing their important scientific implications (stratigraphy, sedimentology, palaeoceanography and palaeoclimatology); assessing their geological hazard and their economic potential (for mineral and energy resources); and 3) using them to create conceptual models for CDS formation. There is a lack of complete knowledge about the different oceanographic processes that may drive bottom currents, and there is also the need to document the great variety of contourite features (processes and products) and facies models, along with their evolution over time and space. Therefore advances, both in new technologies and integrated studies (Geology, Physical Oceanography and Benthic/planktonic Biology), are anticipated.