The Rockall Bank Mass Flow: Collapse of a moated contourite drift onlapping the eastern flank of Rockall Bank,west of Ireland

The Rockall Bank Mass Flow (RBMF) is a large, multi-phase submarine slope failure and mass flow complex. It is located in an area where the Feni Drift impinges upon the eastern flank of the Rockall Bank in the NE Atlantic. A 6100 km² region of slope failure scarps, extending over a wide water depth range and with individual scarps reaching up to 22 km long and 150 m high, lies upslope of a series of mass flow lobes that cover at least 18,000 km² of the base of slope and floor of the Rockall Trough. The downslope lobe complex has a negative topographic relief along much of its northern boundary, being inset below the level of the undisplaced contourite drift at the base of slope. The southern margin is topographically more subtle but is marked by the sharp termination of sediment waves outside the lobe. Within the lobe complex the southern margin of the largest lobe shows a positive relief along its southern margin. The initial failure is suggested to have occurred along coherent layer-parallel detachment surfaces at depths of up to 100 m and this promoted initial downslope block sliding which in turn transformed into debris flows which moved out into the basin. The remains of a deep erosional moat linked to the onlapping contourite complex bisects the region of failed slope, and post-failure thermohaline currents have continued to modify the mass flow in this area. Differential sedimentation and erosion associated with the moat may have promoted slope instability. Following the major failure phase, continuous readjustments of the slope occurred and resulted in small-volume turbidites found in shallow gravity cores collected on the lobes. The short term trigger for the failure remains uncertain but earthquake events associated with a deep-seated tectonic lineament to the north of the mass flow may have been important. A Late Pleistocene age for the slope failure is likely. The RBMF is unusual in that it records large-scale collapse of a contourite body that impinged on a sediment-undersupplied slope system. Unlike many other large slope failure complexes along the NE Atlantic margin, the RBMF occurs in a region where there was little overloading by glacial sediment.     

Sediment distribution on the Rockall Bank,Rockall Plateau

Rockall Bank is an unusual example of an isolated continental shelf developed on the subsided Rockall Plateau microcontinent. The sediment distribution on Rockall Bank has been investigated using topographic, seismic, sonar and photographic data. The present sedimentation regime may reflect post-Flandrian transgressive burial of the older substrate. The limited development of the shelf reflects the lack of sediment supply available on Rockall Bank which, with the exception of glacial periods, has probably been submerged for the past 37 m.y.     

Helen's Reef: a microgabbroic intrusion in the Rockall intrusive centre,Rockall Bank

Rockall Island is composed of 52 ± 9 Ma (m.y.) aegirine granite and forms part of a nearly planated intrusive complex. The nearby Helen's Reef lies within the complex but is composed of Cretaceous (81 ± 3 Ma) microgabbros. The petrography and chemistry of the microgabbros differ from Mid-Ocean Ridge tholeiites but are similar to the Tertiary gabbroic rocks of northwestern Scotland. The relationship of the Cretaceous igneous activity to the evolution of the North Atlantic Ocean is discussed briefly.     

Evidence for current-controlled morphology along the western slope of Hatton Bank (Rockall Plateau, NE Atlantic Ocean)

A multibeam bathymetric and high- (airgun and sparker) to very high-resolution (Topas) seismic study of the western slope of Hatton Bank (NE Atlantic), located between 600 m and 2,000 m water depth, has revealed a highly variable range of current-controlled morphological features. Two major seabed areas can be distinguished: (1) a non-depositional area corresponding to the top of the bank and (2) a depositional area in which the Hatton Drift has developed. Both areas are characterised by distinct morphologies associated either with rock outcrops and rocky ridges or with smooth surfaces, slides and bedforms controlled mainly by bottom currents interacting with the topography of the bank. The water depth separating the morphological areas probably coincides with the boundary of the Labrador Sea Water and the Lower Deep Water. Morphological features identified in the study area include contourite channels (moats, furrows and scours), fields of sediment waves, edges of contourite deposits, ponded deposits, scarps, gullies, ridges, depressions, slides and slide scars. These morphological features do not necessarily reflect present-day conditions but may have been associated with past current events, consistent with earlier interpretations.     

SeaMARC II study of a giant submarine slump on the northern Chile continental slope

Abstract

A giant submarine slump, encompassing a 91‐km by 26‐km block, occurring on the continental slope offshore Iquique, Chile, was identified during a SeaMARC II survey. Utilizing SeaMARC II side‐scan imagery, bathymetry, and seismic reflection data, five morphostructural zones of the slump were identified: the fissured zone, scar zone, tensional depression, central block, and front zone. The fissured zone was developed on the crown of the slump; the scar zone is characterized by scars with the crescent‐shaped slip surfaces and throws ranging from 200 m to 50 m. The tensional depression zone is marked by an area voided by mass slumping, while the central block morphology was formed by uplift. The front zone is comprised of both compressional and tensional subzones. The compressional subzone is characterized by a relative topographic low, on the middle slope, whereas the extensional subzone is characterized by a convex pattern of alternated ridges and hollows, which may represent the debris of the slump on the lower slope. The formation of the slump was strongly influenced by the subduction of the Nazca plate beneath the Chile continental margin, which resulted in the subsidence of the continental slope with a resultant increase in the slope gradient and pore‐water pressure in the sedimentary layers. Slump formation was further facilitated by the development of a complex fault system associated with the subduction and by the triggering effect of earthquakes in the area.     

Coincident sediment slump/clathrate complexes on the U.S. Atlantic continental slope

High-resolution seismic reflection data recorded on the continental slope off the east coast of the United States have revealed instances of sediment mass movement (slumps) which appear to occur above clathrate accumulations. The slumping is believed to be related to the liberation of free gas by clathrate decomposition and consequent weakening of unconsolidated sediments above the clathrate. Pleistocene sea-level lowering and/or post-Pleistocene bottom water temperature increases may have had a significant role in this process.     

The distribution and abundance of cephalopods on the continental slope of the eastern South Atlantic

During a cruise in March 1988 dedicated to investigation of the marine resources of South Africa's continental slope, 62 species in 22 families of Cephalopoda were collected from the Cape Canyon and Cape Point Valley. Multivariate analysis revealed a clear distinction between cephalopods of the upper and lower continental slope in both benthic and epibenthic habitats. Todaropsis eblanae and Todarodes angolensis were indicator species for the upper slope benthic, from 300 to 500 m, but on the lower slope, in 700–900 m of water, they were replaced by Histioteuthis miranda and Opisthoteuthis agassizii. In the epibenthic, both upper and lower slopes were dominated by Abraliopsis gilchristi and Lycoteuthis ?diadema, but other species characteristic of the lower slope were the oceanic species Mastigoteuthis hjorti, Ctenopteryx sicula and Taonius sp. A. Cranchia scabra indicated the presence of oceanic water at stations 700–900 m deep. Other species of significance in the catches were Todarodes filippovae, Histioteuthis macrohista, Rossia enigmatica and Bathypolypus valdiviae. Species affinities indicate that the cephalopod fauna of the southern African continental slope includes the following zoogeographic components: southern African endemic, circum-Subantarctic species associated with Antarctic Intermediate Water, circum-global southern tropical/subtropical, tropical Indo-Pacific and cosmopolitan tropical/subtropical.     

The effect of mass-transport deposits on the younger slope morphology,offshore Brazil

The effect of Cenozoic mass-transport deposits (MTDs) on the morphology of the Late Neogene to Quaternary seafloor is investigated using a 3D seismic volume from offshore Brazil. The studied MTD shows large remnant blocks deforming the seafloor several Ma after a principal instability event marking the base of the investigated strata. Remnant blocks formed during this latter instability event were quickly buried, with differential compaction between individual blocks and adjacent debrites triggering: a) seafloor instability on the flanks of uncompacted (remnant) blocks, b) the incision of submarine channels between seafloor highs formed by buried remnant blocks, c) local uplifted areas on the seafloor that may form potential triggers for future slope instabilities. The interpreted data show that palaeo-seafloor scarps reached more than 120 m in height, with flanking strata to remnant blocks reaching angles of 15°. Angles of this magnitude caused local collapse of seafloor strata and, in some intervals, the confinement of younger MTDs sourced from the upper slope. The statistical data presented here indicate that differential compaction over heterogeneous MTDs continued well after early burial, still deforming the seafloor c. 15 Ma after the main instability event. In addition, significant structural traps are formed by forced folds on remnant blocks that not experienced substantial compaction. Therefore, we conclude that MTDs on passive margins can control seafloor topography after early burial, at the same time contributing to the formation of significant structural traps in post-MTD successions.     

The feeding habits of slope dwelling macrourid fishes in the eastern North Pacific

The diet of slope dwelling macrourid fishes in the eastern North Pacific is poorly known. We collected several hundred stomach samples to investigate the feeding habits of Coryphaenoides acrolepis and Albatrossia pectoralis, the two dominant slope dwelling macrourids off the continental United States. Coryphaenoides acrolepis exhibited a pronounced ontogenetic shift in diet. Specimens <15 cm pre-anal fin length (PAF) consumed primarily polychaetes, amphipods, cumaceans and mysids, while larger individuals consumed increasingly larger, more pelagic prey such as fish, squid, and large crustaceans. Scavenging was also very important to specimens >15 cm with scavenged food constituting approximately 20% of the weight of total prey and occurring in approximately 20% of fish 21–29 cm. Albatrossia pectoralis consumed primarily midwater fish and squid, and we believe that it feeds in the water column. There were significant differences between the diets of A. pectoralis and C. acrolepis suggesting some degree of niche separation between macrourid species on the continental slope of the eastern North Pacific. Both species are at the top of the food web on the upper continental slope and, because of their abundance, may exert significant pressures on their prey populations.     

Geotechnical characterization of slump scars,eroded slopes,and correlated deposits on continental margins: Gulf of Lions,Western Mediterranean sea

Abstract

Vertical variations of geotechnical properties in the uppermost sediment layers characterize the main sedimentary processes acting on the construction and destruction of progressive‐type continental slopes. In the Gulf of Lions, the original thicknesses and distribution of the uppermost sedimentary layers of the continental slope and rise, which consist of Holocene muds overlying Pleistocene muds, have been greatly modified by erosion and several kinds of slope failure processes. Each process is typified through sets of geotechnical properties measured in the eroded or slumped sections and in the associated sediment accumulations.

In slump scars, the water‐rich Holocene muds lie on fine, overconsolidated, Pleistocene muds with high plasticity and low shear strength. In bottom current‐eroded slopes, where modern sedimentation is extremely reduced, the Pleistocene muds frequently outcrop and may sometimes be overlain by a very thin layer of Holocene muds. The Pleistocene muds of eroded slopes are overconsolidated and more silty and less plastic than the Pleistocene muds from slopes affected by slope failure, their shear strength being 10 times greater.

Deposits at the toe of slumps are very often formed by several superposed three‐layer units (triplets of interstratified Holocene, transitional, and Pleistocene layers) issued from retrogressive slumping occurring in the slump scars above their head area. The main body of each layer is then relatively undisturbed, showing the usual burial geotechnical gradients due to overburden pressure (i.e., decrease of water content and increase of unit weight and shear strength). At the toe of bottom current‐eroded slopes, a thick and homogeneous layer of Holocene muds overlies the Pleistocene muds; this Holocene layer has unappreciable burial depth gradient of its geotechnical parameters because of a high rate of modem and continuous deposition.     

Late Cenozoic sea-level changes and the onset of glaciation: impact on continental slope progradation off eastern Canada

Late Cenozoic sedimentation from four varied sites on the continental slopes off southeastern Canada has been analysed using high-resolution airgun multichannel seismic profiles, supplemented with some single channel data. Biostratigraphic ties are available to exploratory wells at three of the sites. Uniform, slow accumulation of hemipelagic sediments was locally terminated by the late Miocene sea-level lowering, which is also reflected in changes in foraminiferan faunas on the continental shelf. Data are very limited for the early Pliocene but suggest a return to slow hemipelagic sedimentation. At the beginning of the late Pliocene, there was a change in sedimentation style marked by a several-fold increase in accumulation rates and cutting of slope valleys. This late Pliocene cutting of slope valleys corresponds to the onset of late Cenozoic growth of the Laurentian Fan and the initiation of turbidite sedimentation on the Sohm Abyssal Plain. Although it corresponds to a time of sea-level lowering, the contrast with the late Miocene lowstand indicates that there must also have been a change in sediment delivery to the coastline, perhaps as a result of increased rainfall or development of valley glaciers. High sedimentation rates continued into the early Pleistocene, but the extent of slope dissection by gullies increased. Gully-cutting episodes alternated with sediment-draping episodes. Throughout the southeastern Canadian continental margin, there was a change in sedimentation style in the middle Pleistocene that resulted from extensive ice sheets crossing the continental shelf and delivering coarse sediment directly to the continental slope.     

南海北部陵水陆坡重力流沉积调查与分析

深水沉积环境复杂、浅层沉积物土质差异明显、重力流沉积多样,对深水海洋工程水下设施的设计、施工和运营安全提出了新的挑战。通过二维/三维数字地震等方式能够识别出数百米的滑坡体,但是对于海洋工程上所关注的浅层海底的重力流沉积体系,其分析能力有限。以南海北部陵水区块为例,通过船载多波束的后向散射成果、自主水下航行器（AUV）搭载的浅地层剖面资料,结合重力活塞取样器（JPC）取样和碳14测年（AMS14C）等资料综合分析,对深水海底浅层重力流沉积的形成和分布模式进行探讨。研究表明,船载多波束后向散射图能够较为准确地识别区域性重力流沉积区域,以及表层沉积物的变化,AUV搭载的浅地层剖面能够精确地识别地层的纵向差异,JPC取样能够获取重力流沉积样品及其物理力学参数,以上资料的综合分析,是准确识别、研究和认识现代重力流沉积体系的重要方法。研究区内最近的2期重力流沉积分别发生于5.5 kaBP左右和45 kaBP前,重力流的发生一般都是由上陆坡区海底峡谷的活动引起的。最近的一期重力流事件中,先后发生了浊流沉积和块状搬运体（MTDs）沉积,MTDs沉积过程中会对下伏地层产生明显的冲蚀现象。在重力流沉积区进行水下结构物设计和施工时,应特别关注重力流沉积引起的地形坡度变化,以及地层中土质成分的差异,可能对水下结构物的安装和维护产生的影响。     

The eastern slope of the southern Adriatic basin: a case study of submarine landslide characterization and tsunamigenic potential assessment

Marine Geophysical Research - The southern Adriatic basin is the current foredeep of the Albanide fold-and-thrust belt that runs along the eastern boundary of the Adriatic basin and partly owes its...     

Geotechnical properties of shelf and slope deposits off the Rhône Delta

Some geotechnical parameters, such as water content, bulk density, shear strength, and Atterberg limits, result directly from sedimentation mechanisms. The results of geotechnical tests for each physiographic zone of the Rhône Delta system—continental shelf, continental slope, and upper rise (base of slope)—are presented in this paper. These geotechnical parameters are correlated with the hydrodynamic, morphologic, and structural environmental conditions. They are also used to explain and forecast the instability of superficial deposits.     

Rockall and the limits of national jurisdiction of the UK: Part 2

Part 1 of this article, published in the July issue of Marine Policy, contained the first in a four-section examination of the Rockall question. Section I presented a survey of the law governing the delimitation of the continental shelf, the EEZ, and exclusive fishing zones, thus providing a background for sections II–IV, which constitute Part 2. In this Part, the author examines the geography and geology of the Rockall sector and the claims made by the UK, Ireland and Denmark, before drawing conclusions from the whole article.     

Post-Palaeozoic stratigraphy of the eastern Agulhas Bank,South African continental margin

The Agulhas Bank, which forms the continental margin on the southern tip of Africa, consists of a thick Meso-Cainozoic sedimentary sequence (up to 6.2 km) resting on and behind pre-Mesozoic continental acoustic basement. The stratigraphy of this sequence is outlined and its history and facies variations mentioned where they are known. Refraction seismic velocity and bottom sample data indicate a basic three-fold subdivision of the Mesozoic sequence, which can be correlated with the onshore succession in the Algoa Basin. It is separated by a major hiatus from the Cainozoic sediments, which consist of a Palaeogene and Neogene sequence subdivided by another well-defined level of erosion. Various formations within the Cainozoic are defined and named. An outline of the bathymetry of the eastern Agulhas Bank is also given.     

Submarine landslide structures in neopleistocene deposits on the western slope of the Derbent basin of the Caspian Sea

In the autumn of 2004, during cruise 19 of R/V Rift, an “SES-2000 standard” super high-resolution narrow-beam parametric profiler was applied for studies of the fine structure of the uppermost sediments for the first time in the Caspian Sea. In this paper, we discuss the latitudinal seismoacoustic profile across the western slope of the Derbent Basin in the region of the Russian part of the Yalama-Samur hydrocarbon field. In the upper part of the slope, in the Neopleistocene deposits, a series of overthrust covers with a step-wise structure of displacement surfaces (duplexes) was revealed. They seem to have been formed by gravity processes. We also suggest the existence of a “pushed wedge” type of structure of landslide origin formed by the Bakinian-Khvalynian deposits intruding into the overlying Holocene sediments. Sufficiently distinct signs of disintegration of the bottom sediments and landslide-slumping processes were distinguished within the Eopleictocene deposits on the steepest lower part of the slope. The data of seismoacoustic profiling indicate that submarine landslide processes proceeded on the western slope of the Derbent Basin during the Neopleistocene-Holocene, which may have retained their activity up to present. These facts should be considered when designing engineering constructions over the Yalama-Samur structure.     

东营凹陷南坡东段油气成藏模式及勘探潜力

东营凹陷油气资源丰富,但油气分布差异大。东营凹陷南坡东段成藏及油气分布特征研究结果表明,研究区内凸起带、斜坡带和缓坡带的沉积环境和构造条件差异大,油气成藏模式不同。凸起带以不整合遮挡油藏为主,斜坡带以构造油气藏为主并含少量小规模地层超覆油藏,缓坡带以岩性油气藏为主。研究区内断层对油气运移、聚集和油气藏分布都有明显的控制作用。在断层发育区,通过断层、不整合和砂体的三维输导体系,油气可以发生长距离运移,形成构造和不整合油气藏;在断层稀少区,油气以近源运移为主,形成岩性油气藏。初步勘探开发结果显示,岩性油气藏的地层压力高、产量好、前景大,成藏模式为油田老区的“增储上产”指明了方向。