Contour current imprints and contourite drifts in the Bahamian archipelago

New data collected along the slopes of Little and Great Bahama Bank and the abyssal plain of the Bahama Escarpment provides new insights about contour current‐related erosive structures and associated deposits. The Bahamian slope shows abundant evidence of bottom current activity such as furrows, comet‐like structures, sediment waves and drifts. At a seismic scale, large erosion surfaces and main periods of drift growth resulted from current acceleration related to plate tectonic processes and progressive opening and closure of gateways and long‐term palaeoclimate evolution. At present‐day, erosion features and contourite drifts are either related to relatively shallow currents (<1000 m water depth) or to deep currents (>2500 m water depth). It appears that the carbonate nature of the drifts does not impact the drift morphology at the resolution addressed in the present study. Classical drift morphologies defined in siliciclastic environments are found, such as mounded, plastered and separated drifts. In core, contourite sequences show a bi‐gradational trend that resembles classical contourite sequences in siliciclastic deposits showing a direct relationship with a change in current velocity at the sea floor. However, in a carbonate system the peak in grain size is associated with increased winnowing rather than increased sediment supply as in siliciclastic environments. In addition, the carbonate contourite sequence is usually thinner than in siliciclastics because of lower sediment supply rates. Little Bahama Bank and Great Bahama Bank contourites contain open‐ocean input and slope‐derived debris from glacial episodes. Inner platform, platform edge and open ocean pelagic input characterize the classical periplatform ooze during interglacials. In all studied examples, the drift composition depends on the sea floor topography surrounding the drift location and the type of sediment supply. Carbonate particles are derived from either the slope or the platform in slope and toe of slope drifts, very deep contourites have distant siliciclastic sources of sediment supply. The recent discovery of the importance of a large downslope gravitary system along Bahamian slopes suggests frequent interactions between downslope and along‐slope (contour currents) processes. The interlayering of mass flow deposits and contourites at a seismic scale or the presence of surface structures associated with both contour currents and mass flow processes shows that both processes act at the same location. Finally, contour currents have an important impact on the repartition of deep‐water coral mounds. Currents can actively interact with mounds as a nutrient and oxygen supplier or have a passive interaction, with mounds solely being obstacles orienting erosion and deposition.     

中国南海西北次海盆西北陆缘洋陆过渡区深水沉积体系特征

高分辨率二维地震资料显示中国南海西北次海盆西北陆缘（水深1 000 m及以下）发育如下深水沉积体系:珠江口盆地南部隆起区缓坡带（水深约1 000~1 500 m、坡度<1.2°）出露神狐南海山,该海山附近发育“海山相关等深流沉积体系”,可能受南海中层水循环（自西向东）底流控制;神狐南海山以南水深约1 500~2 500 m的陆坡区（坡度>2°）普遍发生坡移,发育“重力流滑塌体系”和“峡谷体系”,鲜见等深流沉积;下陆坡区（水深>2 500 m,坡度稍缓<2°）滑塌现象明显减少,主要发育“峡谷体系”以及“席状等深流沉积体系”,席状等深流沉积体系可能受分散的、流速较低的南海深层水循环底流控制。地震沉积记录显示,神狐南海山附近等深流侵蚀特征最早出现于晚中新世早期,其后至现今该区较稳定发育等深流沉积/侵蚀的加积序列,说明南海西北次海盆西北陆缘的稳定底流沉积/侵蚀可追溯至晚中新世早期。     

Characteristics of modern carbonate contourite drifts

Carbonate environments inhabit the realm of the surface, intermediate and deep currents of the ocean circulation where they produce and continuously deliver material which is potentially deposited into contourite drifts. In the tropical realm, fine‐grained particles produced in shallow water and transported off‐bank by tidal, wind‐driven, and cascading density currents are a major source for transport and deposition by currents. Sediment production is especially high during interglacial times when sea level is high and is greatly reduced during glacial times of sea‐level lowstands. Reduced sedimentation on carbonate contourite drifts leads to early marine cementation and hardened surfaces, which are often reworked when current strength increases. As a result, reworked lithoclasts are a common component in carbonate drifts. In areas of temperate and cool water carbonates, currents are able to flow across carbonate producing areas and incorporate sediment directly to the current. The entrained skeletal carbonate particles have variable bulk density and shapes that lower the prediction of transport rates in energy‐based transport models, as well as prediction of current velocity based on grain size. All types of contourite drifts known in clastic environments are found in carbonate environments, but three additional drift types occur in carbonates because of local sources and current flow diversion in the complicated topography inherent to carbonate systems. The periplatform drift is a carbonate‐specific plastered drift that is nearly exclusively made of periplatform ooze. Its geometry is built by the interaction of along‐slope currents and downslope currents, which deliver sediment from the adjacent shallow‐water carbonate realm to the contour current via a line source. Because the periplatform drift is plastered on the slopes of the platforms it is also subject to mass gravity flow and large slope failures. At platform edges, a special type of patch drift develops. These hemiconal platform‐edge drifts also contain exclusively periplatform ooze but their geometry is controlled by the current around the corner of the platform. At the north‐western end of Little and Great Bahama Bank are platform‐edge drifts that are over 100 km long and up to 600 m thick. A special type of channel‐related drift forms when passages between carbonate buildups or channels within a platform open into deeper water. A current flowing in these channels will entrain material shed from the sediment producing areas. At the channel mouth, the sediment‐charged current deposits its sediment load into the deeper basin. With continuous flow, a submarine delta drift is built that progrades into the deep water. The strongly focused current forming the delta drift, is able to rework coarse skeletal grains and clasts, making this type of carbonate drift the coarsest drift type.     

Neogene palaeoceanographic changes recorded in a carbonate contourite drift (Santaren Channel,Bahamas)

The Santaren Drift between the Great Bahama Bank and Cay Sal Bank (Bahamas) is closely linked to the development of the Gulf Stream and its shape and geometry record the local to global oceanographic, climatic and tectonic events since the Miocene. High‐resolution multichannel seismic data from the Santaren Channel allow detailed insight into the growth phases of the contourite drift, and by using the stratigraphic information from Ocean Drilling Program Site 1006 to infer its sedimentation rates. The results bring new understanding to this region and to interpretation of carbonate drifts. The data document that the signatures of a bottom current flow in the Santaren Channel initiated about 12·3 Ma, as indicated by the first occurrence of sheeted drifts and moat development at the northern part of the Santaren Channel. Narrowing and steepening of moat flanks as well as the pronounced upslope migration of the moat reflects a sustained current acceleration of the bottom currents until 5·5 Ma, associated with a transformation into mounded elongated drifts. Between 5·5 Ma and 3·1 Ma, bottom current intensity reached its maximum probably caused by the final closure of the Central American Seaway. The last 3·1 Myr were characterized by a marked increase in volume through flow reaching a maximum during the past 900 kyr. Drift growth was driven by the combined sources of export from the shallow‐water carbonate factory and by pelagic rain. The Middle Miocene channel‐related sheeted drift of the inner Santaren Channel is characterized by low accumulation rates, but a rapid increase of accumulation rates occurred during the Early Pliocene. The contourite drift buildup was disturbed by minor erosional phases with narrow moats in the Late Pliocene due to increasing bottom‐current velocities forced by strengthened Atlantic Ocean ventilation. The Early Pleistocene was dominated by increased periplatform sedimentation and margin progradation facilitated by a reduction in along‐slope current flow speed and a concurrent widening and flattening of the moats.     

Contourite drifts, moats and channels in the Upper Cretaceous chalk of the Danish Basin

During the Late Cretaceous, high global sea‐level meant that most of the NW European craton was flooded by the deep epeiric ‘chalk sea’. The classical paradigm for chalk deposition envisages a quiet rain of minute skeletal debris of coccolithophorid algae and other pelagic organisms deposited as horizontal, flat‐lying pelagic oozes with local redeposition by slumps, slides and debris flows along faults and other structural features. Seismic data from the Danish Basin and elsewhere necessitate a revision of this paradigm. These demonstrate that the chalk sea floor had a considerable relief, commonly of more than a hundred metres amplitude, comprising moats, drifts, mounds and channels. Seismic sections from the Kattegat sea illustrate the development in the Maastrichtian of a deep moat adjacent to a topographic ridge formed over the inverted NW–SE‐trending Sorgenfrei–Tornquist Zone. The moat was up to 120 m deeper than its SW flank which was formed by an internally complex elongate drift, up to 20 km wide with an estimated length of ca 200 km. Smaller mound‐like features, channels and clinoform beds are superimposed on the large‐scale relief. The sea floor relief is interpreted to have formed in response to persistent bottom currents, flowing parallel to bathymetric contours. The initial build‐up of the broad, gently convex‐up sheeted drift was controlled by relatively low‐velocity bottom currents. The region of highest current velocity was gradually shifted NE‐wards towards the inversion zone ridge, resulting in the formation of the deep moat flanked by the elongate drift. The current is interpreted to have flowed from the SE towards NW on the basis of the internal architecture of the elongate drift and the NW‐ward branching and decrease in moat relief. The architecture and morphology of the moat drift and other features of the chalk sea floor are in all aspects similar to contourite systems of modern continental margins. It is accordingly proposed that the fundamental physical oceanographic concept – contour currents and their resulting contourite drifts – is extended to include the deep epeiric seas which covered NW Europe during the Late Cretaceous.     

Morphological control of slope instability in contourites: a geotechnical approach

Contourite drifts are sediment bodies formed by the action of bottom currents. They are common features found on continental slopes and are often affected by slope failure. However, processes controlling slope instability in contourite depositional systems are still not well constrained, and it is not clear whether contourites have particular properties that make them more susceptible to slope failure. In this study, we compare sedimentological and geotechnical properties of contouritic and hemipelagic sediments within the Corsica Trough (northern Tyrrhenian Sea) using geophysical data sets and sediment cores in order to get a better understanding of the controlling factors of slope stability. Geomorphological and slope stability analyses reveal that differences in sediment properties have little influence on the location of submarine landslides, in comparison with the morphology of the drifts. Hence, the steep downslope flanks of plastered drift deposits are the most susceptible zones for local failure initiation. Moreover, as erosion is common at the foot of plastered drifts, undercutting is thought to contribute to the development of large-scale failure up to the point that submarine landslides are triggered.     

Evolution of contourite systems in the late Cretaceous Chalk Sea along the Tornquist Zone

Based on integration of seismic reflection and well data analysis this study examines two major contourite systems that developed during the late Cretaceous in the southern Baltic Sea. The evolution of these Chalk Sea contourite systems between the Kattegat and the southern Baltic Sea started when Turonian to Campanian inversion tectonics overprinted the rather flat sea floor of the epeiric Chalk Sea. The Tornquist Zone and adjacent smaller blocks were uplifted and formed elongated obstacles that influenced the bottom currents. As a consequence of the inversion, the sea floor west of the Tornquist Zone tilted towards the north‐east, creating an asymmetrical sub‐basin with a steep marginal slope in the north‐east and a gentle dipping slope in the south‐west. A south‐east directed contour current emerged in the Coniacian or Santonian along the south‐western basin margin, creating contourite channels and drifts. The previously studied contourite system offshore Stevns Klint is part of this system. A second, deeper and north‐west directed counter‐flow emerged along and parallel to the Tornquist Zone in the later Campanian, but was strongest in the Maastrichtian. This bottom current moderated the evolution of a drift‐moat system adjacent to the elevated Tornquist Zone. The near surface Alnarp Valley in Scania represents the Danian palaeo‐moat that linked the Pomeranian Bay with the Kattegat. The previously studied contourite system in the Kattegat represents the north‐western prolongation of this system. This study links previous observations from the Kattegat and offshore Stevns Klint to the here inferred two currents, a more shallow, south‐east directed and a deeper, north‐west directed flow.     

Interplay between an axial channel belt,slope gullies and overbank deposition in the Puchkirchen Formation in the Molasse Basin,Austria

Deep‐water sediments in the Molasse Basin, Austria, were deposited in a narrow foreland basin dominated by a large channel belt located between the steep Alpine fold and thrust belt to the south and the gentler northern slope off the Bohemian Massif. Several gas fields occur outside the channel belt, along the outer bend of a large meander. Accumulation of these overbank sediments reflects a complicated interplay between slope accommodation and debris‐flow and turbidity‐flow interaction within the channel. The tectonically oversteepened northern slope of the basin (ca 2 to 3°) developed a regionally important erosional surface, the Northern Slope Unconformity, which can be traced seismically for >100 km in an east–west direction and >20 km from the channel to the north. The slope preserves numerous gullies sourced from the north that eroded into the channel belt. These gullies were ca 20 km long, <1 km wide and ca 200 m deep. As the channel aggraded, largely inactive and empty gullies served as entry points into the overbank area for turbidity currents within the axial channel. Subsequently, debris‐flow mounds, 7 km wide and >15 km long, plugged and forced the main channel to step abruptly ca 7 km to the south. This resulted in development of an abrupt turn in the channel pathway that propagated to the east and probably played a role in forming a sinuous channel later. As debris‐flow topography was healed, flows spread out onto narrow area between the main channel and northern slope forming a broad fine‐grained apron that serves as the main gas reservoir in this area. This model of the overbank splay formation and the resulting stratigraphic architecture within the confined basin could be applied in modern and ancient systems or for subsurface hydrocarbon reservoirs where three‐dimensional seismic‐reflection data is limited. This study elucidates the geomorphology of the oversteepened slope of the under‐riding plate and its effects on the sedimentation.     

南海北部第四系深层等深流沉积特征及类型*

以地震资料为基础, 对南海北部第四系深层等深流沉积进行了研究。南海北部水深约1200～3000m范围内发育大型长条状漂积体、限制型漂积体、陆坡席状漂积体及沉积物波。大型长条状漂积体外形为丘状, 水道在靠陆一侧发育。限制型漂积体主要沉积于地形突起之间的地势相对低洼处, 外形多平坦, 水道较为发育。陆坡席状漂积体外形为席状。沉积物波面积较大, 部分与漂积体伴生。深层等深流在自北东向南西沿南海陆架运动过程中, 在中上陆坡由于地形变化相对较大及科氏力作用影响形成螺旋型等深流, 进而产生次生环流, 形成大型长条状漂积体及限制型漂积体。在中下陆坡因地形相对平坦、开阔, 等深流为层状水流, 多形成陆坡席状漂积体。本研究不仅能提高对南海深层等深流沉积的认识, 还能为油气勘探服务。     

A Lower Ordovician Carbonate Contourite Drift on the Margin of the South China Paleocontinent at Jiuxi,Northern Hunan

Carbonate contourite drift at Jiuxi. Taoyuan, northern Hunan, was developed in a deepwater area ofnorthern Hunan on the margin of the Early Ordovician South China palcocontinent. The Lower Ordoviciansequence in the area is more than 350 m thick and contains well-developed contourites that can be groupedinto the following five types: the calcilutitic, the arenitic, the siltitic. the fine ruditic and the bioclastic. Thefirst three often constitute a complete or incomplete contourite succession. The arenitic contourite is nearlyuniformly distributed as interlayers throughout the succession, creating a monotonously rhythmic texture inthe contourite drift. The pattern of spatial distribution of the succession shows that the contourite drift is ahuge ridge-like sedimentary body extending along the trend of paleoslope. Numerous marks of flow direc-tion have pointed to an eastward paleoflow direction along the slope.     

A Cretaceous carbonate delta drift in the Montagna della Maiella,Italy

The Upper Cretaceous (Campanian–Maastrichtian) bioclastic wedge of the Orfento Formation in the Montagna della Maiella, Italy, is compared to newly discovered contourite drifts in the Maldives. Like the drift deposits in the Maldives, the Orfento Formation fills a channel and builds a Miocene delta‐shaped and mounded sedimentary body in the basin that is similar in size to the approximately 350 km² large coarse‐grained bioclastic Miocene delta drifts in the Maldives. The composition of the bioclastic wedge of the Orfento Formation is also exclusively bioclastic debris sourced from the shallow‐water areas and reworked clasts of the Orfento Formation itself. In the near mud‐free succession, age‐diagnostic fossils are sparse. The depositional textures vary from wackestone to float‐rudstone and breccia/conglomerates, but rocks with grainstone and rudstone textures are the most common facies. In the channel, lensoid convex‐upward breccias, cross‐cutting channelized beds and thick grainstone lobes with abundant scours indicate alternating erosion and deposition from a high‐energy current. In the basin, the mounded sedimentary body contains lobes with a divergent progradational geometry. The lobes are built by decametre thick composite megabeds consisting of sigmoidal clinoforms that typically have a channelized topset, a grainy foreset and a fine‐grained bottomset with abundant irregular angular clasts. Up to 30 m thick channels filled with intraformational breccias and coarse grainstones pinch out downslope between the megabeds. In the distal portion of the wedge, stacked grainstone beds with foresets and reworked intraclasts document continuous sediment reworking and migration. The bioclastic wedge of the Orfento Formation has been variously interpreted as a succession of sea‐level controlled slope deposits, a shoaling shoreface complex, or a carbonate tidal delta. Current‐controlled delta drifts in the Maldives, however, offer a new interpretation because of their similarity in architecture and composition. These similarities include: (i) a feeder channel opening into the basin; (ii) an excavation moat at the exit of the channel; (iii) an overall mounded geometry with an apex that is in shallower water depth than the source channel; (iv) progradation of stacked lobes; (v) channels that pinch out in a basinward direction; and (vi) smaller channelized intervals that are arranged in a radial pattern. As a result, the Upper Cretaceous (Campanian–Maastrichtian) bioclastic wedge of the Orfento Formation in the Montagna della Maiella, Italy, is here interpreted as a carbonate delta drift.     

等深流沉积研究进展

等深流沉积研究已有约50年的历史,其研究成果极为丰富。近10余年,随着科学技术的发展和海洋意识的提高,等深流沉积研究工作开展迅速,涌现出了大量的新成果。简要回顾了等深流沉积研究的历程,结合最新研究成果,对其进展及认识进行了总结。等深流沉积以细粒沉积为主,沉积构造及生物扰动发育,多呈细-粗-细沉积序列。其类型可分为长条形丘状漂积体、水道型漂积体、补丁型漂积体等7类。沉积模式根据地形、水动力、路径等可分为简单路径模式、复杂路径模式以及等深流与重力流交互作用模式。等深流与重力流交互作用是深水沉积研究热点之一。等深流沉积研究面临的问题及发展方向主要有三方面,即,1）完善识别标志,推广研究成果;2）综合运用多种手段和理论,探讨沉积过程与构造演化、古海洋及气候变化的耦合关系;3）加大油气勘探潜力研究力度。     

Contourites: Their recognition in modern and ancient sediments

We suggest that publication of evidence that the continental rise of the western North Atlantic has been shaped by bottom currents flowing parallel to bathymetric contours (Heezen, Hollister and Ruddiman, 1966) marked the beginning of a revolution in sedimentology comparable to the turbidite revolution launched by Kuenen and Migliorini in their classical 1950 paper.Intensification of slow, thermohaline circulation on the western margins of the ocean basins leads to high-velocity, deep, boundary currents, capable of eroding, transporting and depositing fine-grained sediment. Long-period, direct current measurements suggest a complex, periodic flow for these currents, while bottom photographs indicate their influence on the sediment surface. Sediment ridges in the North Atlantic can be closely related to the deep-water circulation pattern. Other morphological features (ripples, furrows, waves), echogram characteristics, and the presence of well-developed nepheloid layers cannot be uniquely attributed to the action of bottom currents.Critical review of marine-based investigations reveals a lack of generally accepted criteria for the recognition of contourites on the basis of sediment character. We discuss the problems in establishing such criteria and recognize that: (a) a continuum may exist between dilute turbidity flows, bottom currents and hemipelagic settling; (b) interbedded turbidites, contourites and hemipelagites are common, especially in a rise environment; and (c) composition and other criteria may be only locally applicable. However, we can identify two main contourite groups, muddy contourites and sandy contourites, and have proposed new criteria for their recognition. Muddy contourites are generally bioturbated, have poorly defined bedding, and contain biogenic sand often concentrated into irregular layers. They may be texturally and compositionally distinct from interbedded turbidites, and have relatively high CaCO₃ and organic carbon contents. Sandy contourites occur as thin, bioturbated, irregular lag-deposits, or as reworked tops of sandy turbidites. In the latter case they may be clean, well sorted, parallel- or cross-laminated, but show no offshore trends or vertical structural sequence. Grain orientation shows the bottom current direction, often superimposed upon the original turbidite fabric.Reveiw of land-based work shows that there is growing recognition of the need for a new concept to complement turbidity-current theory, but that there have been relatively few claims of firm contourite identification. Recognition of ancient contourites has been based either on the application of previous sedimentological criteria, or on an interpretation of the broader environmental framework. It is suggested that it is lack of suitable criteria for the identification of contourites rather than a true scarcity of these rocks that has led to such a restricted literature.Mindful of problems created by diagenesis, tectonic activity and the limited preservation potential of many diagnostic features of marine contourites, we do suggest criteria, and a procedure, for the recognition of contourites in land-based work. Sandy contourites of the reworked-turbidite variety may be the most easily recognised; the presence of bimodal palaeocurrent directions at about 90° is an important indicator of this type.The geological significance of contourites in palaeo-oceanographical, palaeogeographical and tectonic reconstructions is emphasised, especially in relation to work on passive (Atlantic-type) continental margins. We also refer to the possible economic significance of contourites as exploration for hydrocarbons moves into deeper waters.     

The influence of turbidity currents and contour currents on the distribution of deep‐water sediment waves offshore eastern Canada

Sediment waves are commonly observed on the sea floor and often vary in morphology and geometry according to factors such as seabed slope, density and discharge of turbidity currents, and the presence of persistent contour currents. This paper documents the morphology, internal geometry and distribution of deep‐water (4000 to 5000 m) bedforms observed on the sea floor offshore eastern Canada using high‐resolution multibeam bathymetry data and seismic stratigraphy. The bedforms have wavelengths of >1 km but fundamentally vary in terms of morphology and internal stratigraphy, and are distinguished into three main types. The first type, characterized by their long‐wavelength crescentic shape, is interpreted as net‐erosional cyclic steps. These cyclic steps were formed by turbidity currents flowing through canyons and overtopping and breaching levées. The second type, characterized by their linear shape and presence on levées, is interpreted as net‐depositional cyclic steps. These upslope migrating bedforms are strongly aggradational, indicating high sediment deposition from turbidity currents. The third type, characterized by their obliqueness to canyons, is observed on an open slope and is interpreted as antidunes. These antidunes were formed by the deflection of the upper dilute, low‐density parts of turbidity currents by contour currents. The modelling of the behaviour of these different types of turbidity currents reveals that fast‐flowing flows form cyclic steps while their upper parts overspill and are entrained westward by contour currents. The interaction between turbidity currents and contour currents results in flow thickening and reduced sediment concentration, which leads to lower flow velocities. Lower velocities, in turn, allow the formation of antidunes instead of cyclic steps because the densiometric Froude number (Fr′) decreases. Therefore, this study shows that both net‐erosional and net‐depositional cyclic steps are distributed along channels where turbidity currents prevail whereas antidunes form on open slopes, in a mixed turbidite/contourite system. This study provides insights into the influence of turbidity currents versus contour currents on the morphology, geometry and distribution of bedforms in a mixed turbidite–contourite system.     

深水等深积岩丘及其含油气潜能

等深积岩丘大量存在干现代海洋调查和古代地层记录的深水沉积中,其规模可与深海大型浊积扇相比拟。岩性一般由泥级、粉砂级、砂级及细砾级等深漉沉积构成,成分上可以是陆源碎屑的或是碳酸盐的,其中粉砂级和砂级等深流沉积的单层厚度在数厘米至数十厘米,厚者可达2m 左右。颗粒的分选性一般中等一较好,局部很好,原生孔隙发育,并与深水原地沉积的页岩、泥岩互层沉积,具有良好的生储盖组合特性。对比了大西洋东缘现代沉积的 Faro 等深积岩丘和中国古代奥陶纪沉积的湘北九溪等深积岩丘、甘肃平凉等深积岩丘的层序特征,提出了等深积岩丘的形成分为萌生、成型和衰退三个阶段的沉积模式。认为等深积岩丘具有潜在的油气勘探前景。     

Quaternary sedimentation,margin architecture and ocean circulation variability around the Faroe Islands,North Atlantic

The Quaternary development offshore the Faroe Islands has been studied using high-resolution seismic and core data from the R/V DANA 2000 cruise and previous cruises. Several glacial-related features and deposits are observed, all bearing witness to former extensive glaciations of the Faroe area. On the shelves, overlaying a mid-Pleistocene glacial erosional surface, glacial and glacimarine deposits form a sheet geometry interrupted by ridges of sediment that are likely to represent ice-front deposits. An iceberg turbate north of the Faroe Islands provides evidence of large-scale drift of ultra-deep draft (>600 m) icebergs in the Nordic Seas at pre-Weichselian glacial stage(s). Marginal and transverse troughs found on the eastern and western shelf are suggested to have formed during the same glacial period(s) as the iceberg turbate. Iceberg plough-marks and abundant ice rafted material of non-Faroese origin, together with the relict moraine ridges encircling the Faroe Islands at around the 100 and 200 m water depth contours, indicate that the outer shelf was probably ice free during the Weichselian ice age. On the slopes and basinal parts, the formation of fine-grained contourites was favoured during (Weichselian) glacial stages when bottom currents were reduced. Sediment overloading during these glacial stages resulted in repeated slope instability, causing mass failures of the contourite deposits.     

Evolution of a carbonate delta generated by gateway‐funnelling of episodic currents

Cool‐water carbonate sedimentation has dominated Mediterranean shelves since the Early Pliocene. Skeletal sand and gravel herein consist of remains of heterozoan organisms, which are susceptible to reworking due to weak early cementation in non‐tropical waters. This study documents the Lower Pleistocene carbonate wedge of Favignana Island (Italy), which prograded from a 5   km wide passage between two palaeo‐islands into a perpendicular, 10 to 15   km wide strait between the palaeo‐islands at one side and Sicily at the other during the Emilian highstand (1·6   Ma to 1·1   Ma). The clinoformed carbonate wedge, which is 50   m thick and 6   km long, formed by east/south‐east progradation of a platform on the submarine sill by currents that were funnelled between the two palaeo‐islands. Platform‐slope clinoforms evolved from initial aggradation (thin and low‐angle) into a progradation phase (thick and high‐angle). Both clinoform types are characterized by a bimodal facies stacking pattern defined by sedimentary structures created by: (i) subaqueous dunes associated with dilute subcritical currents; and (ii) upper‐flow‐regime bedforms associated with sediment‐laden supercritical turbidity currents. Focusing of episodic currents on the platform by funnelling between the islands controlled the downstream formation of a sediment body, here named carbonate delta. The carbonate delta interfingers with subaqueous dune deposits formed in the perpendicular strait. This study uses a reconstruction of bedform dynamics to unravel the evolution of this gateway‐related carbonate accumulation.     

Post‐Variscan exhumation of the Central Anti‐Atlas (Morocco) constrained by zircon and apatite fission‐track thermochronology

The origin of the Anti‐Atlas relief is one of the currently debated issues of Moroccan geology. To constrain the post‐Variscan evolution of the Central Anti‐Atlas, we collected nine samples from the Precambrian basement of the Bou Azzer‐El Graara inlier for zircon and apatite fission‐track thermochronology. Zircon ages cluster between 340 ± 20 and 306 ± 20 Ma, whereas apatite ages range from 171 ± 7 Ma to 133 ± 5 Ma. Zircon ages reflect the thermal effect of the Variscan orogeny (tectonic thickening of the ca. 7 km‐thick Paleozoic series), likely enhanced by fluid advection. Apatite ages record a complex Mesozoic–Cenozoic exhumation history. Track length modelling yields evidence that, (i) the Precambrian basement was still buried at ca. 5 km depth by Permian times, (ii) the Central Anti‐Atlas was subjected to (erosional) exhumation during the Triassic‐Early Cretaceous, then buried beneath ca. 1.5 km‐thick Cretaceous‐Paleogene deposits, (iii) final exhumation took place during the Neogene, contemporaneously with that of the High Atlas.     

How turbidity current frequency and character varies down a fjord‐delta system: Combining direct monitoring,deposits and seismic data

Submarine turbidity currents are one of the most important processes for moving sediment across our planet; they are hazardous to offshore infrastructure, deposit petroleum reservoirs worldwide, and may record tsunamigenic landslides. However, there are few studies that have monitored these submarine flows in action, and even fewer studies that have combined direct monitoring with longer‐term records from core and seismic data of deposits. This article provides one of the most complete studies yet of a turbidity current system. The aim here is to understand what controls changes in flow frequency and character along the turbidite system. The study area is a 12 km long delta‐fed fjord (Howe Sound) in British Columbia, Canada. Over 100 often powerful (up to 2 to 3 m sec^?1) events occur each year in the highly‐active proximal channels, which extend for 1 to 2 km from the delta lip. About half of these events reach the lobes at the channel mouths. However, flow frequency decreases rapidly once these initially sand‐rich flows become unconfined, and only one to five flows run out across the mid‐slope each year. Many of these sand‐rich, channelized, delta‐sourced flows therefore dissipated over a few hundred metres, once unconfined, rather than eroding and igniting. Upflow migrating bedforms indicate that supercritical flow dominated in the proximal channels and lobes, and also across the unconfined mid‐slope. These supercritical flows deposited thick sand beds in proximal channels and lobes, but thinner and finer beds on the unconfined mid‐slope. The distal flat basin records far larger volume and more hazardous events that have a recurrence interval of ca 100 years. This study shows how sand‐rich delta‐fed flows dissipate rapidly once they become unconfined, that supercritical flows dominate in both confined and unconfined settings, and how a second type of more hazardous, and much less frequent event is linked to a different scale of margin failure.     

Three‐dimensional seismic characterization of a complex sediment drift in the South China Sea: Evidence for unsteady flow regime

This study describes a previously unobserved reflection seismic configuration comprising a honeycomb planform and a repeated erosion/infill cross‐section, based on high‐resolution three‐dimensional/two‐dimensional seismic data and bathymetric data. The honeycomb structures cover an area of more than 5000 km² and are developed within the Late Miocene to recent deep‐water sediments of the north‐western South China Sea. Linear erosional troughs up to 10 km long and 1 km wide are widely developed in this area, are intimately related to the particular seismic configuration and interpreted to represent a new type of sediment drift that is caused by unsteady bottom current regimes operating since the Late Miocene. The unsteady bottom current regimes are suggested to be triggered by irregular seabed morphologies. Considerable sea‐floor topography was generated as a direct result of tectonic movements in the area since the Late Miocene, and this topography then influenced the pathways of strong bottom currents. This study highlights that: (i) an unsteady bottom current regime can be laterally extensive and persist for millions of years; (ii) structurally controlled sea‐floor relief plays an important role in controlling the depositional pattern; and (iii) the bottom currents were active since the Late Miocene, flowing from the south‐east through the Xisha–Guangle Gateway and crossing the honeycomb structure zone. This study documents a new style of drift and will help to improve current knowledge of palaeoceanography and understanding of the South China Sea deep‐water circulation which is at present still poorly understood.