Deep-water Basin Research Articles

Middle and Late Jurassic radiolarians from the Neotethys suture in the Eastern Alps

AbstractCherty limestones, marls and radiolarites are widespread in the Hallstatt Mélange of the Northern Calcareous Alps. The mélange was formed during the Neotethyan orogeny in a series of deep-water basins that progressively developed in front of the advancing nappe front. The low thermal overprint of these rocks favors the good preservation of radiolarians (polycystines) that have been used for dating and reconstructing the Jurassic tectonostratigraphy of the area. This paper describes rich Middle–Late Jurassic radiolarian faunas from four localities in the Hallstatt Mélange near Bad Mitterndorf in Austria. Two different successions, both spanning from the Bathonian to the Oxfordian, are dated. In the first succession, the radiolarites are intercalated between or occur as matrix in mass-flow deposits originating from the accretionary wedge. The second succession is nearly 100 m thick but is devoid of mass-flow deposits and documents a continuous radiolarite deposition in greater distance from the nappe stack. Both successions are ascribed to the Sandlingalm Basin, which evolved on a relatively distal continental margin during early stages of the orogeny.The highly diverse and well-preserved radiolarian assemblages have been used for a detailed taxonomic study. Two new families are described: Minocapsidaen. fam. andXitomitridaen. fam.; six new genera are describedDoliocapsan. gen.,Crococapsan. gen.,Parvimitrellan. gen.,Xitomitran. gen.,Campanomitran. gen., andMizukidellan. gen. In addition, one new replacement name,Takemuraella(proTriversus), is introduced, the diagnoses of 6 genera are emended, and two new species are described:Hemicryptocapsa nonagintan. sp. andMizukidella mokaensisn. sp.

A Deepwater Gasfield-Development Strategy for Trinidad and Tobago

This article, written by Special Publications Editor Adam Wilson, contains highlights of paper SPE 177102, “A Deepwater Gasfield-Development Strategy for Trinidad and Tobago,” by Ayasha Nickie, Trinidad and Tobago Ministry of Energy and Energy Affairs, prepared for the 2015 SPE Latin American and Caribbean Petroleum Engineering Conference, Quito, Ecuador, 18–20 November. The paper has not been peer reviewed. Trinidad and Tobago is the top natural-gas producer in the Caribbean, with average daily production in 2014 of 4.069 Bscf/D. With vibrant upstream, midstream, and downstream sectors, Trinidad and Tobago makes maximum use of the natural-gas value chain. Further maximization through growth in the downstream sector requires an adequate supply of natural gas in addition to the shallow-water gas fields offshore, which are currently the main supply. Because of the thriving gas sector, a large amount of infrastructure already exists that may aid with any further development. Introduction The Trinidad and Tobago Gas Development Model has shaped the growth of its natural-gas economy over the years, and, in 2014, the energy sector accounted for 45% of the country’s gross domestic product. Average production over the past 5 years was an estimated 4.2 Bcf/D, while average use for the same period was 3.9 Bcf/D, with liquefied natural gas (LNG) accounting for more than 50% of this consumption (Fig. 1). With the onset of proposed additional projects in the downstream sector, additional gas use in the near future is inevitable. Although this additional capacity can emanate from the existing proved-reserves base, another source of gas could be currently unexploited deepwater basins. In the past 5 years, nine production-sharing contracts for exploration in Trinidad and Tobago’s deepwater areas were signed. If gas is discovered, first gas can be on line within 5 to 10 years of discovery. If this occurs, additional quantities of natural gas would be available for domestic, Caribbean, and Latin American markets. While deepwater gas developments have economic challenges, existing infrastructure in Trinidad and Tobago can aid development.

Cambrian–Ordovician conodonts from slump deposits of the Argentine Precordillera: new insights into its passive margin development

AbstractThe Los Sombreros Formation represents the western continental margin slope deposits of the Argentine Precordillera, a sub-terrane accreted to Gondwana as part of the Cuyania Terrane in early Palaeozoic times. The age of these gravity-driven deposits is controversial and, therefore, a precise biostratigraphic scheme is essential to reveal the evolution of the continental margin. New conodont samplings along with sedimentological and structural analysis carried out in the Los Sombreros Formation in the La Invernada Range provide clues to its depositional framework. The sedimentary succession is made up of dominantly calciturbidites, carbonate breccias and conglomerates, along with mudstones that represent the pelagic/hemipelagic background sedimentation. It displays hectometric to outcrop-scale slump folds with variable hinge-line orientations and pinch-and-swell structures, evidencing soft-sediment deformation, consistent with a slope to base-of-slope setting. Three limestone samples from this succession include conodonts referable to the pandemicHirsutodontus simplexSubzone of theCordylodus intermediusZone (upper Furongian, Cambrian) and from theMacerodus dianaeZone (upper Tremadocian, Ordovician), implying that a slope connected the shallow-water shelf with a deep-water (oceanic) basin at least since late Cambrian times. The conodont faunas show affinities to coeval assemblages from outer shelf and slope environments around Laurentia yet they are not conclusive to postulate a geographic origin for the Precordillera. The thermal alteration of the conodonts is consistent with sedimentary burial and nappe stacking in this sector of the Precordillera.

Composition and parageneses of massive pyrolusite from the deep-water basin of the Sea of Japan

Hard fragments (crushed only by a hammer) of manganese deposits differing strongly in appearance from the regional ferromanganese crusts were sampled from depths of 3500–3200 m during the dredging of an unnamed seamount in the Central Basin of the Sea of Japan. Their surface has a black carbonaceous coating; after its removal, the crusts become steel-gray. The specific weight of these crust fragments is 3.35 g/cm3, whereas the regional crusts have a specific weight of <2 g/cm3. X-ray diffraction analysis showed that the sampled fragments consist of pure pyrolusite. There are also fragments of crusts formed by todorokite and birnessite. All pyrolusite samples have an abnormally high content of Mn (up to 63%). The Mn/Fe ratio reaches 9016. The conclusion is drawn that the manganese crusts formed on this seamount are of hydrothermal genesis.

Hydrocarbon accumulation in the deep waters of South China Sea controlled by the tectonic cycles of marginal sea basins

AbstractThe tectonic cycle of the marginal sea basins in South China Sea (SCS) includes two cycles, i.e., the formation and contraction of Palaeo-SCS and Neo-SCS. The northern part of SCS is a rift basin on a passive continental margin, while the the Nansha Block is a drifting rift basin. The southern part is a compound compressional basin on an active continental margin; the western part is a shear-extensional basin on a transform continental margin; the eastern part is an accretionary wedge basin on a subduction continental margin. The deep-water basins are mainly distributed on the continental slope and the the Nansha Block. There are three sets of source rocks in the deep-water areas of the northern continental margin in SCS, i.e., Eocene terrestrial facies, early Oligocene transitional facies and late Oligocene marine facies. A set of Late Cretaceous-Early Oligocene terrestrial marine facies source rocks are developed in the drift-rift basin of SCS. Three sets of Oligocene, early Miocene and Mid-Miocene marine-terrestrial transitional facies source rocks are developed in the deep-water areas of both the southern and western continental margins of SCS. Four sets of reservoirs developed in the northern deep waters of SCS are dominated by deep sea fans. Two sets of reservoirs developed in the the Nansha Block are dominated by delta and biogenic reef. The southern part of SCS is dominated by deep sea fan and biogenic reef. Reservoirs of large channels and other clastic facies were developed in front of the estuaries, while biogenic reef bank was formed in the uplift zone. The hydrocarbon accumulation assemblages are mainly presented as Oligocene-Pliocene in the deep waters on the northern continental margin of SCS, Eocene-Lower Oligocene in the the Nansha Block, Oligocene-Pliocene and Oligocene-Miocene in the deep waters on the southern and western continental margin of SCS, respectively. The major hydrocarbon reservoir types in the deep waters of SCS are related to structural traps, deep water fans and biogenic reefs. The formation of basin, hydrocarbon and reservoir in the deep waters of SCS are controlled by the tectonic cycles of the marginal sea basins, revealing a great potential for hydrocarbon exploration.

The stratigraphy and structure of the Faroese continental margin

Abstract This paper presents a summary of the stratigraphy and structure of the Faroese region. As the Faroese area is mostly covered by volcanic material, the nature of the pre-volcanic geology remains largely unproven. Seismic refraction data provide some indications of the distribution of crystalline basement, which probably comprises Archaean rocks, with the overlying cover composed predominantly of Upper Mesozoic (Cretaceous?) and Cenozoic strata. The Cenozoic succession is dominated by the syn-break-up Faroe Islands Basalt Group, which crops out on the Faroe Islands (where it is up to 6.6 km thick) and shelf areas; post-break-up sediments are preserved in the adjacent deep-water basins, including the Faroe–Shetland Basin. Seismic interpretation of the post-volcanic strata shows that almost every sub-basin in the Faroe–Shetland Basin has been affected by structural inversion, particularly during the Miocene. These effects are also observed on the Faroe Platform, the Munkagrunnur Ridge and the Fugloy Ridge, where interpretation of low-gravity anomalies suggests a large-scale fold pattern. The structure of the Iceland–Faroe Ridge, which borders the NW part of the Faroe area, remains ambiguous. The generally thick crust, together with the absence of well-defined seawards-dipping reflectors, may indicate that much of it is underlain by continental material.

An early Permian coastal flora dominated by Germaropteris martinsii from basinal sediments in the Midland Basin, West Texas

Fossils found in cores from wells in the Midland Basin of West Texas include several kinds of terrestrial plants and a variety of marine animal remains. Depositional settings ranged from basin slope to deep-water basin floor, hence the presence of land plants was unexpected. The fossil plant assemblage is depauperate, dominated by Germaropteris martinsii, a Permian-age peltasperm. Other specimens include the peltasperm Supaia, Sphenopteris germanica, axes of uncertain affinity, and incertae sedis remains presumed to be terrestrial plants. Fossil plants are found predominantly in fine-grained, siliceous mudrocks between coarser-grained calcareous floatstones and wackestones/packstones interpreted as debrites and turbidites, suggesting that the plants were carried from land by surface currents before sinking to the basin floor and being buried by slowly accumulating hemipelagic sediment. Specimens were examined from drillcores in 14 wells spanning an interval from the lower Wolfcamp through the lower Leonard. This record of G. martinsii in lower Permian Wolfcamp rocks is among the earliest occurrences of these plants, which have been found most abundantly in upper Permian strata of Western Europe.

Anomalously deep convection in the Irminger Sea during the winter of 2014–2015

The anomalous thermohaline properties of the core of the Labrador Sea water (LSW), a product of winter convection resulted from the ocean–winter interaction, which were recorded in the Irminger Sea in 2014 and 2015, are discussed. It is found that, in the summer of 2015, the LSW filled the entire deepwater basin in the south of the sea down to a depth of 1300 m over the periphery of the Irminger gyral, and down to a depth of 1800 m, in the anticyclone eddy formed in early June of 2015. The maximum density of the LSW core here was 27.75 σθ. Such deep and intense winter convection in the Irminger Sea was last recorded 20 years ago. As a result, the temperature of the LSW dropped to values observed in the years 2002 and 2007, but its average salinity remained high. The LSW that have been formed in recent years are reported to have great amplitude and nonmonotonic variability of the properties.

Heavy silicon isotopic composition of silicic acid and biogenic silica in Arctic waters over the Beaufort shelf and the Canada Basin

AbstractThe silicon isotopic composition of silicic acid (δ30Si(OH)4) and biogenic silica (δ30Si‐bSiO2) were measured for the first time in marine Arctic waters from the Mackenzie River delta to the deep Canada Basin in the late summer of 2009. In the upper 100 m of the water column, δ30Si(OH)4 signals (+1.82‰ to +3.08‰) were negatively correlated with the relative contribution of Mackenzie River water. The biogenic Si isotope fractionation factor estimated using an open system model, 30ε = −0.97 ± 0.17‰, agrees well with laboratory and global‐ocean estimates. Nevertheless, the δ30Si dynamics of this region may be better represented by closed system isotope models that yield lower values of 30ε, between −0.33‰ and −0.41‰, depending on how the contribution of sea‐ice diatoms is incorporated. In the upper 400 m, δ30Si‐bSiO2 values were among the heaviest ever measured in marine suspended bSiO2 (+2.03‰ to +3.51‰). A positive correlation between δ30Si‐bSiO2 and sea‐ice cover implies that heavy signals can result from isotopically heavy sea‐ice diatoms introduced to pelagic assemblages. Below the surface bSiO2 production zone, the δ30Si(OH)4 distribution followed that of major water masses. Vertical δ30Si(OH)4 profiles showed a minimum (average of +1.84 ± 0.10‰) in the upper halocline (125–200 m) composed of modified Pacific water and heavier average values (+2.04 ± 0.11‰) in Atlantic water (300–500 m deep). In the Canada Basin Deep Water (below 2000 m), δ30Si(OH)4 averaged +1.88 ± 0.12‰, which represents the most positive value ever measured anywhere in the deep ocean. Since most Si(OH)4 enters the Arctic from shallow depths in the Atlantic Ocean, heavy deep Arctic δ30Si(OH)4 signals likely reflect the influx of relatively heavy intermediate Atlantic waters. A box model simulation of the global marine δ30Si(OH)4 distribution successfully reproduced the observed patterns, with the δ30Si(OH)4 of the simulated deep Arctic Ocean being the heaviest of all deep‐ocean basins.

Permian-Cenozoic deep-water carbonate rocks of the Southern Tethyan Domain. The case of Central Sicily

We present an integrated stratigraphy of the outcropping and buried Permian-Cenozoic deep-water carbonate successions, forming some of the tectonic units mostly buried beneath the Late Neogene sedimentary cover in the fold and thrust belt of Central Sicily. Three main successions, pertaining to the well known Lercara, Imerese and Sicanian domains, have been reconstructed on the basis of a detailed facies analysis, seismostratigraphic interpretation, bio -stratigraphy (mostly based on palynological data) and comparison between outcropping and subsurface deep-water sediments. The main results reveal a continuous sedimentation of the deep-water Southern Tethyan Sicilian succession since the Permian to Cenozoic. In detail: a ) the Permian-Middle Triassic terrigenous and carbonate deep-water successions, outcropping or buried in the Cerda, Lercara-Roccapalumba and Sosio Valley regions, are well comparable to each other and represent the common substrate of the Mesozoic-Paleogene Imerese and Sicanian carbonate successions; b ) the Mesozoic-Paleogene deep-water carbonates, when compared among them, reveal the occurrence of different sedimentary successions (Imerese and Sicanian); c ) the Oligo-Miocene foreland basin terrigenous sediments (Numidian flysch) clearly differ from the coeval foreland pelagic to open-shelf carbonates. The paleogeographic reconstruction envisages: a ) during the Permian-Triassic, a wide subsident continental rifting area, bordered by a shallow-water domain periodically supplying the basin with calciturbiditic to gravity flows sedimentation (rift stage of the Southern Tethyan margin); b ) during the Jurassic-Paleogene, two different deep-water basins developed in a context of a post-rift stage. The different sedimentation reflects the location of the Imerese and Sicanian basins, respectively, along adjacent rimmed shelf and stepped carbonate platform margins.

Geochemistry and petrogenesis of the Late Mesozoic–Early Cenozoic volcanic rocks of the Okhotsk and Japan marginal seas

The results of ICP-MS trace-element (LILE, HFSE, REE) study of the Late Mesozoic–Early Cenozoic volcanic rocks of the Okhotsk and Japan seas and geochronological K-Ar dating of the Eocene volcanic rocks are presented. Specifics of volcanism developed on submarine rises of these seas was characterized for the first time, and magma sources and geodynamic settings of the volcanic complexes predating the formation of the deep-water basins were determined. It is established that the Late Mesozoic magmas were formed in a subduction setting from spinel peridotites of suprasubduction mantle wedge, which was metasomatically reworked by aqueous fluids that were released by dehydration of sedimentary layer of subducting oceanic plate. This follows from the elevated concentrations of H2O, alkalis, potassium, LILE and LREE, and lowered HFSE (including Ta-Nb minimum) and HREE contents, at lowered Sm/Yb, Nb/Ta, Nb/Y and elevated La/Nb, Ba/La, and Zr/Y ratios. Eocene adakite-like volcanic rocks were identified for the first time in the Sea of Okhotsk. They vary from andesitic to more felsic compositions with elevated MgO (>4%) and elevated La/Yb (>14) and Sr/Y (50–60) ratios. Identification of adakite-like volcanic rocks serves as evidence in support of the transform continental-margin (or plate sliding) setting, which is characterized by breaking apart of subduction slab and formation of slab “windows” acting as pathways for the transfer of asthenospheric mantle into continental lithosphere. New geochemical data on the Late Mesozoic–Early Cenozoic volcanic rocks of the Okhotsk and Japan seas and analysis of literature data were used to distinguish two geodynamic settings within these seas: subduction and transform margin. Similar settings operated at that time in the adjacent continental- margin volcanic belts (Akinin and Miller, 2011; Martynov and Khanchuk, 2013; et al.).

Depositional processes and stratigraphic architecture within a coarse-grained rift-margin turbidite system: The Wollaston Forland Group, east Greenland

The Wollaston Forland Basin, NE Greenland, is a half-graben with a Middle Jurassic to Lower Cretaceous basin-fill. In this outcrop study we investigate the facies, architectural elements, depositional environments and sediment delivery systems of the deep marine syn-rift succession. Coarse-grained sand and gravel, as well as large boulders, were emplaced by rock-falls, debris flows and turbulent flows sourced from the immediate footwall. The bulk of these sediments were point-sourced and accumulated in a system of coalescing fans that formed a clastic wedge along the boundary fault system. In addition, this clastic wedge was supplied by a sand-rich turbidite system that is interpreted to have entered the basin axially, possibly via a prominent relay ramp within the main fault system. The proximal part of the clastic wedge consists of a steeply dipping, conformable succession of thick-bedded deposits from gravity flows that transformed down-slope from laminar to turbulent flow behaviour. Pervasive scour-and-fill features are observed at the base of the depositional slope of the clastic wedge, c. 5 km into the basin. These scour-fills are interpreted to have formed from high-density turbulent flows that were forced to decelerate and likely became subject to a hydraulic jump, forming plunge pools at the base of slope. The distal part of the wedge represents a basin plain environment and is characterised by a series of crude fining upward successions that are interpreted to reflect changes in the rate of accommodation generation and sediment supply, following from periodic increases in fault activity. This study demonstrates how rift basin physiography directly influences the behaviour of gravity flows. Conceptual models for the stratigraphic response to periodic fault activity, and the transformation and deposition of coarse-grained gravity flows in a deep water basin with strong contrasts in slope gradients, are presented and discussed.

Deep water drilling riser mechanical behavior analysis considering actual riser string configuration

A dynamic analysis model considering the actual riser string configuration is established to analyze the mechanical behavior of a drilling riser. The riser in this model is regarded as a simply supported beam located in the vertical plane and is subjected to axial and lateral ocean environment loadings. The model is solved using the finite element method. The validity of the analysis model has been proved by a similar experiment conducted in a deep water basin and ANSYS. Riser lateral displacement, bending moment, and deflection of the deep water well considering actual riser string configuration in South China Sea are simulated. The effects of top tension, drilling platform drift distance, surface current speed, wave height, wave period, wave length, wind speed and damping coefficient on riser lateral displacement, bending moment, and deflection are also discussed. Results indicate that riser string configuration has a significant effect on riser mechanical behavior, particularly in the distribution of bending moment. Riser string configuration designing according to concrete ocean environment can significantly improve the riser’s mechanical behavior. The distribution of riser lateral displacement, bending moment, and deflection increases with the increase in surface current speed and drilling platform drift distance, whereas decreases with the increasing top tension and damping coefficient. The distribution of riser lateral displacement, bending moment, and deflection significantly increases with the increase in surface current speed and large current speed may lead to large lateral displacement, bending moment, and deflection. Increasing top tension can significantly decrease riser mechanical behavior parameters and avoid large lateral displacement, bending moment, and deflection when suffering extreme ocean environment conditions in deepwater drilling field. Drilling platform should avoid a large drift distance, which may result in the deflection over the flex joint working range and the enlargement of the riser bend moment.

The Lamu Basin deepwater fold‐and‐thrust belt: An example of a margin‐scale, gravity‐driven thrust belt along the continental passive margin of East Africa

AbstractIn recent decades, advances in seismic processing and acquisition of new data sets have revealed the presence of many deepwater fold‐and‐thrust belts (DW‐FTBs), often developing along continental passive margins. These kinds of tectonic features have been intensively studied, due to their substantial interest. This work presents a regional‐scale study of the poorly explored Lamu Basin DW‐FTB, a margin‐scale, gravity‐driven system extending for more than 450 km along the continental passive margin of Kenya and southern Somalia (East Africa). A 2‐D seismic data set was analyzed, consisting of both recently acquired high‐quality data and old reprocessed seismic profiles, for the first detailed structural and stratigraphic interpretation of this DW‐FTB. The system originated over an Early to mid‐Cretaceous shale detachment due to a mainly gravity‐spreading mechanism. Analysis of synkinematic strata indicates that the DW‐FTB was active from the Late Cretaceous to the Early Miocene, but almost all of the deformation occurred before the Late Paleocene. The fold‐and‐thrust system displays a marked N‐S variation in width, the northern portion being more than 150 km wide and the southern portion only a few dozen kilometers wide; this along‐strike variation is thought to be related to the complex tectonosedimentary evolution of the continental margin at the Somalia‐Kenya boundary, also reflected in the present‐day bathymetry. Locally, a series of volcanic edifices stopped the basinward propagation of the DW‐FTB. A landward change in the dominant structural style, from asymmetric imbricate thrust sheets to pseudo‐symmetric detachment folds, is generally observed, related to the landward thickening of the detached shales.

First 236U data from the Arctic Ocean and use of 236U/238U and 129I/236U as a new dual tracer

The first dataset of 236U/238U in the water column of the Arctic Ocean (AO) is presented and shows the widest range of ratios reported so far in the open ocean, from (5±5) to (3840±260)×10−12. Surface samples and depth profiles were collected during two GEOTRACES expeditions in 2011–2012 and analyzed for the concentrations of 236U and 129I, with the aim of investigating whether the combination of 236U/238U and 129I/236U can be used as a new oceanographic tool in the AO. Results show that the distributions of the 236U/238U and 129I/236U atomic ratios are consistent with the different water masses in the AO. High 236U/238U and 129I/236U ratios in the upper water column (>2000×10−12 and >200, respectively) illustrate the penetration of Atlantic waters (AW) into the AO. Lower values were found in Pacific waters (PW) and deep waters of the AO. Rivers seem to represent a temporally and spatially-constrained third anthropogenic source of 236U but more data are needed to confirm this. In a simple mixing model, the combination of 236U/238U and 129I/236U reveals a high contribution (>99%) of natural background waters (pre-nuclear era) in the deep and bottom waters of the Amerasian basin, indicating an apparent water mass renewal time of >1000 years. Despite the relatively high apparent age of the Amerasian Basin deep waters, this work shows the potential of using the dual-tracer approach as a new oceanographic tool in the Arctic Ocean.

Truncation Design and Model Testing of a Deepwater FPSO Mooring System

Limitation of offshore basin dimensions is a great challenge for model tests of deepwater mooring system. The mooring system cannot be modeled entirely in the basin with a reasonable model scale. A classical solution is based on hybrid model tests for the truncated mooring system. An efficient truncation method is proposed in this paper taking advantage of the mechanical characteristics of catenary mooring system. Truncation procedures are presented both in vertical and horizontal directions. A turret moored floating production storage and offloading (FPSO) is analyzed, and its mooring system is truncated from the original 914 m water depth to 736 m and 460 m, respectively. Numerical simulations are performed based on catenary theory and lumped mass model to these three systems, including the original untruncated system and two truncated systems. The static characteristics and dynamic response are investigated, and the results are compared between the truncated and untruncated system, and good agreements are obtained, verifying the preliminary truncation design. Model tests are conducted to the three mooring system configurations in the deepwater basin of the Harbin Engineering University. The static and dynamic properties are found to be mostly consistent between the untruncated system and two truncated systems, except for some discrepancy in 460 m system. It indicates that the truncation design is successful when the truncation factor is large, while difference still exists when the truncation factor is small. Numerical reconstruction to the model test in 460 m and extrapolation to 914 m are also implemented. The results are found to be consistent with those in 914 m, verifying the robustness and necessity of the hybrid model testing, especially for the mooring system with large truncation.

Sequence Development and Palaeogeography Evolution of Ewan and Oloye Fields (Middle Miocene), Northwestern Niger Delta

Sequences developed within the middle Miocene in the study area are interpreted to be controlled locally by episodic shelf instabilility, basin physiography (shelf edge), fault growth and linkage. Relative sea level changes provide the major control on sequence development and deep valley incisions. The latter have a strong geomorphological impact on the area and a strong control on sequence variability. Interpretation of GDE maps shows that sands were originally deposited in river- and mouth bars with longshore drift currents redistributing the sand parallel to the shoreline. During lowstand periods, canyons developed which served as conduits for sand to the basin (deep water sand), recognized as high amplitude seismic facies on the slope and on the basin floor. This mechanism has led to the deposition of considerable amount of sandstones in the slope and offshore environments. Idealized schematic sequence stratigraphic development models have been generated for the study area, while schematic palaeogeography and general conceptual depositional models of the study area during middle Miocene time are also presented.

Barra Fan: a major glacial depocentre on the western continental margin of the British Isles

The Barra Fan is a large prograding sedimentary wedge of Pliocene–Pleistocene age that has built out into the deep-water basin of the Rockall Trough west of Britain (Holmes et al. 1998). Together with the Donegal Fan, considered to be part of the same fan complex, it covers an area of about 7000 km2 (Armishaw et al. 2000) and locally approaches 700 m in thickness. The bulk of the sediments are thought to be glacially derived from the British Ice Sheet, with ice flowing from northern Ireland and western Scotland converging to reach the shelf edge between 56° N and 57° N via several cross-shelf troughs, most recently at c. 29–27 ka cal BP (Dunlop et al. 2010). Located on the western British continental margin between the Hebrides Shelf and the Rockall Trough, a large sedimentary depocentre with outward-bulging contours is found in water depths between c. 200 m at the shelf edge and c. 2500 m in Rockall Trough (Fig. 1a, b). Gradients commonly reach 7.5° on the upper slope, reducing to 1.5° on the …

Tidal Mixing in the South China Sea: An Estimate Based on the Internal Tide Energetics

AbstractBy taking into account the contributions of both locally and remotely generated internal tides, the tidal mixing in the Luzon Strait (LS) and the South China Sea (SCS) is investigated through internal-tide simulation and energetics analysis. A three-dimensional nonhydrostatic high-resolution model driven by four primary tidal constituents (M2, S2, K1, and O1) is used for the internal-tide simulation. The baroclinic energy budget analysis reveals that the internal tides radiated from the LS are the dominant energy source for the tidal dissipation in the SCS. In the LS, the estimated depth-integrated turbulent kinetic energy dissipation exceeds O(1) W m−2 atop the two subsurface ridges, with a dissipation rate of &gt;O(10−7) W kg−1 and diapycnal diffusivity of ~O(10−2) m2 s−1. In the SCS, the most intense turbulence occurs in the deep-water basin with a dissipation rate of O(10−8–10−6) W kg−1 and diapycnal diffusivity of O(10−3–10−1) m2 s−1 within the ~2000-m water column above the seafloor as well as in the shelfbreak region with a dissipation rate of O(10−7–10−6) W kg−1 and diapycnal diffusivity of O(10−4–10−3) m2 s−1. These estimated values are consistent with observations reported in previous studies and are at least one order of magnitude larger than those based solely on locally generated internal tides.

A new concept for glacial geological investigations of surges, based on High-Arctic examples (Svalbard)

Svalbard is a key area for the investigation of glacial surges, and almost two centuries worth of field observations exists from this region. Studies have shown that the course of a surge and the associated formation of landforms are strongly influenced by basinal factors, and that the broad range of variables involved can hamper interpretations and comparisons. Based on a review of surges in Svalbard, a new concept for glacial geological investigations has been developed that combines ice-flows, ice-front movements, and morphostratigraphy. The concept is comprised of the following four elements: 1) classification based on the configuration and characteristics of the receiving basin, 2) division of the surge cycle into six stages, 3) guidelines for morphological mapping, and 4) use of an allostratigraphic approach for interpreting ice-front movements. In this context, delineation of the active phase is critical, which include the history of terminus movements, and four main categories of receiving basins are recognized. These are (A) terrestrial basins with deformable substrates, (B) terrestrial basins with poorly deformable substrates, (C) shallow water basins, and (D) deep water basins. The ice-front movement history is reconstructed by coupling information from the proglacial moraines (syn-surge), the supraglacial moraines (post-surge), and the associated traces of meltwater to the surge stages (I–VI). This approach has revealed a critical relationship between the termination of the active phase and three morphological elements, namely, the maximum ice-front position, the maximum moraine extent and the youngest proglacial moraine, which are unique for each of the basins A–D. The concept is thus a novel and more precise approach for mapping the active phase and the active phase duration, as shown by the ∼12-year long surge of Fridtjovbreen, where stage I was 30 months (inception), stage II was 54 months (ice-front advance), stage III was 12 months (stillstand), and stage IV was 48 months (retreat during active flow). The glacier has been in quiescent phase (stages V/VI) since 2002.