Daniel Praeg

Neogene evolution of the Atlantic continental margin of NW Europe (Lofoten Islands to SW Ireland): anything but passive

A regional stratigraphic framework for the Neogene succession along and across the NW European margin is presented, based on a regional seismic and sample database. The stratigraphy provides constraints on the timing and nature of the mid- to late Cenozoic differential tectonic movements that have drivenmajor changes in sediment supply, oceanographic circulation and climate (culminating in continental glaciation). The overall context for Neogene deposition on the margin was established in the mid-Cenozoic, when rapid, km-scale differential subsidence (sagging) created the present-day deep-water basins. The Neogene is subdivided into lower (Miocene–lower Pliocene) and upper (lower Pliocene–Holocene) intervals. The lower Neogene contains evidence of early to mid-Miocene compressive tectonism, including inversion anticlines and multiple unconformities that record uplift and erosion of basin margins, as well as changes in deep-water currents. These movements culminated in a major expansion of contourite drifts in the mid-Miocene, argued to reflect enhanced deep-water exchange across the Wyville-Thomson Ridge Complex, via the Faroe Conduit. The distribution and amplitude of the intra-Miocene movements are consistent with deformation and basin margin flexure in response to enhanced intra-plate compressive stresses during a local plate reorganization (transfer of the Jan Mayen Ridge from Greenland to Europe). The upper Neogene records a seaward tilting (

Distribution and geological control of mud volcanoes and other fluid/free gas seepage features in the Mediterranean Sea and nearby Gulf of Cadiz

Existing knowledge on the distribution of mud volcanoes (MVs) and other significant fluid/free gas-venting features (mud cones, mud pies, mud-brine pools, mud carbonate cones, gas chimneys and, in some cases, pockmark fields) discovered on the seafloor of the Mediterranean Sea and in the nearby Gulf of Cadiz has been compiled using regional geophysical information (including multibeam coverage of most deepwater areas). The resulting dataset comprises both features proven from geological sampling, or in situ observations, and many previously unrecognized MVs inferred from geophysical evidence. The synthesis reveals that MVs clearly have non-random distributions that correspond to two main geodynamic settings: (1) the vast majority occur along the various tectono-sedimentary accretionary wedges of the Africa-Eurasia subduction zone, particularly in the central and eastern Mediterranean basins (external Calabrian Arc, Mediterranean Ridge, Florence Rise) but also along its westernmost boundary in the Gulf of Cadiz; (2) other MVs characterize thick depocentres along parts of the Mesozoic passive continental margins that border Africa from eastern Tunisia to the Levantine coasts, particularly off Egypt and, locally, within some areas of the western Mediterranean back-arc basins. Meaningfully accounting for MV distribution necessitates evidence of overpressured fluids and mud-rich layers. In addition, cross-correlations between MVs and other GIS-based data, such as maps of the Messinian evaporite basins and/or active (or recently active) tectonic trends, stress the importance of assessing geological control in terms of the presence, or not, of thick seals and potential conduits. It is contended that new MV discoveries may be expected in the study region, particularly along the southern Ionian Sea continental margins.

First results from shallow stratigraphic boreholes on the eastern flank of the Rockall Basin, offshore western Ireland

The results of an integrated sedimentological and seismic stratigraphical analysis of three borehole sites on the eastern flank of the Rockall Basin, offshore western Ireland are reported. Two sites were drilled on the western slope of the Porcupine High, above the North and South Brona basins (boreholes 83/20-sb01, 83/24-sb01 and 83/24-sb02), and one on the northern flank of the Porcupine High (16/28-sb01), above the Macdara Basin. The cores establish that the half-graben basins marginal to the eastern Rockall Basin contain Jurassic deposits and that they were inverted sometime in the Late Jurassic or Early Cretaceous. An angular unconformity above the Brona basins is overlain by a condensed, tripartite Cretaceous succession (‘brownsand’, ‘greensand’, chalky micrite) that records stepwise deepening, with evidence for a Cenomanian-Turonian phase of normal faulting. Above the Macdara Basin, the unconformity is overlain by a basalt that was cored at the 16/28 site and is interpreted to represent a flow of Cretaceous age derived from the Drol Igneous Centre. At all three borehole sites, Cretaceous strata are onlapped (or downlapped) by Paleocene-Eocene strata that display evidence of a minor episode of fault reactivation above the Brona basins. Cored Eocene strata vary from clastic to carbonate-prone from north to south and smectitic clays are common at the 16/28 site. Post-Mid-Eocene westward tilting of the Rockall slope rotated the Eocene stratigraphy and the underlying Cretaceous deposits (including the lava flow in the 16/28 area) at least 3° down to the west. Slope development resulted in extensional sliding and the erosion of the C30 deep-water unconformity that is onlapped by Miocene slope deposits. C30 was cored in the 83/20 area where it cuts down into Cretaceous strata and is crusted with phosphates and the Cretaceous beneath Mn-impregnated.

Submarine Mass-Movements Along the Slopes of the Active Ionian Continental Margins and Their Consequences for Marine Geohazards (Mediterranean Sea)

The Ionian margins of Calabria and Apulia (IMCA) have been affected by mass movements of varying style, scale and age. Here we present examples of seabed and subsurface features identified along more than 400 km of the IMCA from multibeam seabed imagery and subbottom profiles acquired by OGS since 2005. Four different types of mass movement phenomena are recognized with expression at seabed and in the shallow subsurface: (1) mass transport complexes (MTCs) within intra-slope basins, (2) isolated slide scars (ISS) along open slopes, (3) slope-parallel sediment undulations (SPSU) recording block-rotations linked to fluid migration, and (4) headwall and sidewall scarps (HSC) in submarine canyons. Preliminary analyses of sedimentary processes suggest that both open-slope failures capable of triggering tsunamis and retrogression of canyon headwalls within 1–3 km of the Calabrian coast represent potential geohazards for coastal populations and offshore infrastructures.

Fluid Seepage in Relation to Seabed Deformation on the Central Nile Deep-Sea Fan, Part 2: Evidence from Multibeam and Sidescan Imagery

On the central Nile deep-sea fan, stratified sediments overlying mass-transport deposits (MTDs) are deformed into slope-parallel seabed undulations associated with fluid seepage. The western part of this system, in water depths of 1,950–2,250 m, is examined using multi-frequency data from hull-mounted and deep-towed swath/profiling systems. Sub-bottom profiles show sub-vertical fluid pipes that terminate both at and below seabed, and gas signatures along fault planes bounding the undulations. Fluid seepage is recorded by high- to intermediate-backscatter patches (HBPs, IBPs) that differ in appearance on multibeam imagery (30 kHz, ≤3 m penetration) and sidescan swaths (170/190 kHz, <0.1 m penetration). Comparison of the two suggests a distinction of (a) buried carbonates (0.1–3 m), (b) broad near-seabed (<0.1 m) carbonate pavements elongate along the undulations, (c) sub-circular areas of seabed seepage up to 300 m across. Four of the latter have narrower gas flares at their edges rising 400–800 m above seabed. These results are consistent with an evolving system of narrow fluid conduits that support the growth and burial of carbonate pavements, shifting over millennial timescales along linear zones parallel to fault planes rooted in MTDs. Sediment deformation above MTDs is inferred to provide pathways for fluid escape, but migration of gas-rich fluids from depth is likely to have facilitated slope destabilisation.

Post-failure Processes on the Continental Slope of the Central Nile Deep-Sea Fan: Interactions Between Fluid Seepage, Sediment Deformation and Sediment-Wave Construction

Voluminous mass-transport deposits (MTD) have been identified on seismic profiles across the central Nile Deep-Sea Fan (NDSF). The youngest MTDs are buried under 30–100 m of well-stratified slope deposits that, in water depths of 1,800–2,600 m, are characterized by undulating reflectors correlated with slope-parallel seabed ridges and troughs. Seabed imagery shows that, in the western part of the central NDSF, short, arcuate undulations are associated with fluid venting (carbonate pavements, gas flares), while to the east, long, linear undulations have erosional furrows on their downslope flanks and fluid seeps are less common. Sub-bottom profiles suggest that the western undulations correspond to rotated fault-blocks above the buried MTDs, while those in the east are sediment waves generated by gravity flows. We suggest that fluids coming from dewatering of MTDs and/or from deeper layers generate overpressures along the boundary between MTDs and overlying fine-grained sediment, resulting in a slow downslope movement of the sediment cover and formation of tilted blocks separated by faults. Fluids can migrate to the seafloor, leading to the construction of carbonate pavements. Where the sediment cover stabilizes, sediment deposition by gravity flows may continue building sediment waves. These results suggest that complex processes may follow the emplacement of large MTDs, significantly impacting continental-slope evolution.

Reconstruction and Tsunami Modeling of a Submarine Landslide on the Ionian Margin of Calabria (Mediterranean Sea)

The Ionian margins of Calabria are affected by repeated sediment failures, recorded by slide scars at seabed and stacked slide deposits. We present a reconstruction of the geometry and dynamics of one of the largest seabed features, the Assi failure on the relatively steep slope off southern Calabria, and use it as input to numerical modeling to evaluate the potential tsunamigenic hazard. The Assi failure is up to 6 km wide and at least 18 km long, and involved the displacement of ca. 2 km3 of sediment, inferred to have taken place within the last 4,000 years in two main phases. The first and larger phase is used as input to the tsunami modeling, on the assumption that the slide moved in a single step as a coherent mass of 1.85 km3, in order to evaluate the most disruptive possible consequences. The results indicate that within 8 minutes, waves just over 1 m in height affect the southern Calabrian coast between Monasterace and Roccella Jonica, where their capacity to cause damage could be amplified in small harbours. This shows that tsunamis represent a hazard for Ionian coastal areas, and calls for accurate monitoring and further study.

Fluid Seepage in Relation to Seabed Deformation on the Central Nile Deep-Sea Fan, Part 1: Evidence from Sidescan Sonar Data

The central Nile Deep-Sea Fan contains a broad area of seabed destabilisation in association with fluid seepage: slope-parallel sediment undulations are associated with multibeam high-backscatter patches (HBPs) related to authigenic carbonates. During the 2011 APINIL campaign, a deep-towed sidescan and profiling system (SAR) was used to acquire high-resolution data along three transects across water depths of 1,700–2,650 m. Three seabed domains are distinguished, all developed within stratified sediments overlying mass-transport deposits (MTDs). Upslope of the undulations (<1,950 m), sidescan HBPs record focused fluid seepage via seabed cracks. In the western area of undulations, sidescan HBPs are distinct from intermediate-backscatter patches (IBPs) that extend up to 850 m parallel to the undulations, mainly along their downslope flanks; some contain sub-circular HBPs up to 300 m wide, three associated with smaller (<10 m) hydroacoustic gas flares. Focused fluid seeps are inferred to have shifted over time to form elongate carbonate pavements, preferentially along the footwalls of faults beneath the undulations that provide pathways for fluid flow. In contrast, in the eastern area of undulations, sidescan imagery reveal only slope-transverse furrows formed by turbulent flows, interpreted to indicate that fossil carbonates sampled during submersible operations have been exhumed by erosion.