Broad Shelf Research Articles

Tectonostratigraphy of the exposed Silurian deposits in Arabia

Exposed Silurian deposits in Arabia are represented by the Qalibah Group, the Qusaiba and Sharawra formations. The Qusaiba Formation is composed of dark-gray claystones and siltstones. It is disconformably underlain by the Late Ordovician–Early Silurian? Uqlah Formation or unconformably underlain by the Late Ordovician Zarqa or Sarah formations. It is disconformably overlain by the Sharawra Formation. The Early Rhuddanian basal “hot shale” of the Qusaiba Formation represents the early stage of the early Silurian marine transgression over the Gondwana broad shelf. It is a regional marker used to outline the structural configuration of the area prior to the Silurian time. The Sharawra Formation is composed of siltstone and sandstone. It is unconformably overlain by the Late Silurian?–Early Devonian Tawil Formation. Silurian deposits show a pronounced thinning from 992 m in the Tabuk area in the west and are completely missing in the northern part of the Qusayba depression in the east. The thinning of the Qusaiba shale and Sharawra sandstone is interpreted as due to depositional and erosional features, respectively. Thinning and distribution of the Early Rhuddanian shale “hot shale” is depositional which is affected by preexisting Late Ordovician paleo-highs in central Arabia. Thinning of the Sharawra sandstones is erosional which is attributed to Late Silurian tectonic movements synchronous with the Acadian uplift phase of the Caledonian tectonic movements. The main structural elements in central Arabia are represented by the north–south trending and northerly plunging Hail arch and to a much lesser extent the northwest–southeast trending and southerly plunging Qusayba high.

Post-rift marine transgression of the southern Browse Basin margin: controls on hydrocarbon reservoir development and exploration potential

The formation of the Precipice Fold Belt (560 Ma) marked the last significant modification of the basement drainage pattern and catchment area supplying sediment to the southern Browse Basin. Seismic mapping of the Proterozoic basement and the results of exploration drilling confirm the extension of both the geological formations and structural features exposed on the Proterozoic Kimberley Plateau into the offshore study area. As with the present-day shelf, sedimentation during the Late Jurassic to Early Cretaceous marine transgression directly onlapped the basement in the study area and showed the influence of basement topography on stratigraphy. A marked increase in net-to-gross and reservoir sand thickness is observed moving northeast from the Leveque to the Yampi shelves. This broad change in stratigraphy can be related to specific basement-controlled morphologies. A >250 m-high escarpment formed by the Proterozoic Kimberley Plateau basement terrane bounds the Yampi Shelf to the southeast of the Palaeozoic zero-edge line. This escarpment fixed the location of shelf sediment input points forming a barrier to marine transgression, which ensured that the Yampi Shelf was relatively immune from sediment starvation during highstand systems tracts. In contrast, the broad low angle Leveque Shelf experienced large lateral shifts in shelf sediment input points during eustatic changes in sea level with sedimentation dominated by transgressive systems tracts. In addition to shelf morphology, the larger basement-controlled drainage area (80,000 km2) supplying sediment to the Yampi Shelf tended to increase sedimentation rates, leading to shelf delta progradation during highstand systems tracts. The quality and thickness of mass flow deposits seen in the Caswell Sub-basin and along the Prudhoe Terrace are the direct result of the concentration of high net-to-gross sand close to the offlap break on the Yampi Shelf. Given the extreme long term stability of the southern Browse Basin margin, recognition of the influence of basement structure on shelf sedimentation has refined the geological model for the post-rift marine transgression. This model could be used to reduce the risk on reservoir and seal distributions in the Late Jurassic to Early Cretaceous study interval. In a broader context, observations on basement-controlled reservoir and seal distribution from the Browse Basin might be usefully applied to other basins where sediment is supplied from long-lived Proterozoic margins.

Process-oriented modeling studies of the 5500-km-long boundary flow off western and southern Australia

While the unique character of the coastal current system off the western and southern coasts of Australia has been recognized, this vast 5500-km-long boundary flow has been studied far less than other current systems of the world. Recent observational studies from satellite altimetry and climatology are consistent with a continuous current extending from its origin at the North West Cape to the southern tip of Tasmania. To date, coastal modeling studies have focused on either the western Australian coast to Esperance or on southern Australia. There has been no process-oriented modeling study of the entire region that would allow the systematic exploration of the two independent forcing mechanisms (i.e., wind-forcing and thermohaline gradients) and their interactions that have been noted to act in a synergistic manner to maintain the longest continuous coastal current system in the world. This study uses a regional circulation model (in this case the Princeton Ocean Model (POM)) to systematically address the roles the forcing mechanisms play in generating and maintaining the major features of this continuous coastal current system. Several process-oriented experiments, arranged in order of increasing complexity, are explored. The results show that, even in the absence of bottom topography, a continuous 5500 km coastal current system can be generated by wind forcing or by thermohaline forcing. If wind forcing alone is used, coastal currents in the direction of the wind and opposing undercurrents can be generated. If thermohaline forcing alone is used, coastal currents in the opposite direction of the wind and subsurface currents similar to the Flinders Current can be generated. The addition of topography shows that topography is responsible for the currents’ shelf break locations and, for broad shelf regions, can separate the surface flow into two cores, one at the coast and one over the shelf. On the west coast, topographic beta due to the continental slope prevents currents from becoming broader and drifting offshore. The combination of wind forcing, thermohaline gradients and topography show that swift currents forced by thermohaline gradients are slowed to more realistic speeds by opposing wind and by topography. Meanders and eddies result from the opposition of surface and subsurface currents as well as from thermohaline and wind forcing. The results illustrate that the 5500-km-long current system over the shelf break can be maintained year-long due to the two independent forcing mechanisms, their interactions, and the strong trapping effect of bottom topography. The seasonal and daily wind-forcing experiments highlight both the seasonal and interannual variability of this complex current system. The Leeuwin Current along the western coast is slightly stronger in winter (July) than summer (January). There is much greater mesoscale activity in January when the opposing winds are strongest. The results also show that, although upwelling has been observed only in the summer in the Capes Current region, upwelling occurs intermittently in the 2001 winter but not in the 2001 summer. This illustrates that, depending on the strength of the forcing mechanism, such as strong equatorward winter 2001 winds, features such as upwelling on the west coast, usually thought to exist in the summer but only intermittently, can occur in different seasons. Along the southern coast, a gyre forms intermittently in the Great Australian Bight in summer, but the flow is constantly eastward across the entire shelf in winter. The production of upwelling in the Great Australian Bight during the 2000 summer but not during the 2001 summer is an indication of the importance of interannual variability. Overall, the results of this process-oriented study compare well with available observations off western and southern Australia.

Forcing and Dynamics of Seafloor-Water Column Exchange on a Broad Continental Shelf

Forcing and Dynamics of Seafloor-Water Column Exchange on a Broad Continental Shelf

Geochemistry of rare earth elements in the dissolved, acid-soluble and residual phases in surface waters of the Changjiang Estuary

The concentrations of rare earth elements in the dissolved, acid-soluble and residual phases in surface waters of the Changjiang Estuary were determined using ICP-MS. The main purposes of the study are to understand the estuarine geochemistry of rare earth elements and to explore water-particle interactions in the Changjiang estuarine mixing zone. The results show that there are two distinct processes operating on dissolved rare earth elements in the estuary: large scale removal at low salinities due to salt-induced coagulation and remarkable release at mid to high salinities. These processes result in modification of the effective river water flux and the systematical fractionation of the dissolved rare earth elements toward the East China Sea. The increase in concentration of dissolved rare earth elements in the mid to high salinity waters of the Changjiang Estuary suggests a sediment source in the mixing zone of the estuary, which is located over a shallow, broad shelf where there is extensive physical contact between bottom sediment and estuarine waters. Acid-soluble rare earth elements, the concentrations of which also dropped sharply in the low salinity region, appear to be controlled by salt-induced coagulation process and intense deposition of suspended particulate matter in the low salinity region. In the mid to high salinities, all acid-soluble rare earth element concentrations increase slightly with increasing salinity, suggesting that resuspension of sediments occurred. In contrast, the residual rare earth element concentrations are relatively constant with salinity variation in the Changjiang estuarine surface waters.

Comparison of hypoxia among four river-dominated ocean margins: The Changjiang (Yangtze), Mississippi, Pearl, and Rhône rivers

We examined the occurrence of seasonal hypoxia (O 2<2 mg l −1) in the bottom waters of four river-dominated ocean margins (off the Changjiang, Mississippi, Pearl and Rhône Rivers) and compared the processes leading to the depletion of oxygen. Consumption of oxygen in bottom waters is linked to biological oxygen demand fueled by organic matter from primary production in the nutrient-rich river plume and perhaps terrigenous inputs. Hypoxia occurs when this consumption exceeds replenishment by diffusion, turbulent mixing or lateral advection of oxygenated water. The margins off the Mississippi and Changjiang are affected the most by summer hypoxia, while the margins off the Rhône and the Pearl rivers systems are less affected, although nutrient concentrations in the river water are very similar in the four systems. Spring and summer primary production is high overall for the shelves adjacent to the Mississippi, Changjiang and Pearl (1–10 g C m −2 d −1), and lower off the Rhône River (<1 g C m −2 d −1), which could be one of the reasons of the absence of hypoxia on the Rhône shelf. The residence time of the bottom water is also related to the occurrence of hypoxia, with the Mississippi margin showing a long residence time and frequent occurrences of hypoxia during summer over very large spatial scales, whereas the East China Sea (ECS)/Changjiang displays hypoxia less regularly due to a shorter residence time of the bottom water. Physical stratification plays an important role with both the Changjiang and Mississippi shelf showing strong thermohaline stratification during summer over extended periods of time, whereas summer stratification is less prominent for the Pearl and Rhône partly due to the wind effect on mixing. The shape of the shelf is the last important factor since hypoxia occurs at intermediate depths (between 5 and 50 m) on broad shelves (Gulf of Mexico and ECS). Shallow estuaries with low residence time such as the Pearl River estuary during the summer wet season when mixing and flushing are dominant features, or deeper shelves, such as the Gulf of Lion off the Rhône show little or no hypoxia.

Faulting, mass-wasting and deposition in an active dextral shear zone, the Gulf of Saros and the NE Aegean Sea, NW Turkey

Structural, mass-wasting and sedimentation processes along an active dextral shear zone beneath the Gulf of Saros and the NE Aegean Sea were investigated on the basis of new high-resolution swath bathymetric data and multi-channel seismics. A long history of dextral shearing operating since the Pliocene culminated in the formation of a NE-SW-trending, ca. 800-m-deep basin (the so-called inner basin) in this region, which is bordered by a broad shelf along its northern and eastern sides and a narrow shelf at the southern side. The western extension of the North Anatolian Fault Zone (the Ganos Fault) cuts the eastern shelf along a narrow deformation zone, and ends sharply at the toe of the slope, where the strain is taken up by two NE-SW-oriented fault zones. These two fault zones cut the basin floor along its central axis and generate a new, Riedel-type pull-apart basin (the so-called inner depression). According to the bathymetric and seismic data, these basin boundary fault zones are very recent features. The northern boundary of the inner depression is a through-going fault comprising several NE-SW- and E-W-oriented, overlapping fault segments. The southern boundary fault zone, on the other hand, consists of spectacular en-echelon fault systems aligned in NE–SW and WNW–ESE directions. These en-echelon faults accommodate both dextral and vertical motions, thereby generating block rotations along their horizontal axis. As the basin margins retreat, the basin widens continuously by mass-wasting of the slopes of the inner basin. The mass-wasting, triggered by active tectonics, occurs by intense landsliding and channel erosion. The eroded material is transported into the deep basin, where it is deposited in a series of deep-sea fans and slumps. The high sedimentation rate is reflected in an over 1,500-m-thick basin fill which has accumulated in Pliocene–Quaternary times.

The tilting continent: A new constraint on the dynamic topographic field from Australia

A pronounced latitudinal asymmetry in the present-day morphology of the Australian continental shelf is reflected in Neogene stratigraphic relationships. The northern Australian margin has a broad shelf, typically 200–500 km wide and a Neogene record of stratal onlap. Relative to the continent, sea levels are currently as high as at any stage during the Neogene. In contrast, the southern shelf is typically less than 100 km wide and shows a record of progressive offlap with Neogene palaeo-shorelines commonly many hundreds of kilometres inland, at elevations up to ∼ 250 m above present-day sea level. This continental-scale ‘reciprocal’ stratigraphy implies 250–300 m N-down, SSW-up apparent vertical motion with respect to sea level since the mid-Miocene. The apparent vertical motion can be attributed to variations in dynamic topography and the geoid along Australia's NNE plate circuit; specifically, the movement of the southern margin off the dynamic topography low, geoid low presently centred on the Australian–Antarctic discordance, and the northern margin towards a dynamic topography low, geoid high associated with the subduction realm to the north. Variations in the geoid appear to account for about 10% of the total apparent motion, depending on assumptions about how the geoid field has evolved during Australia's northward motion. This inferred Neogene, continental-scale dynamic N–S tilting rate of ∼15–20 m/myr provides a compelling new constraint on the nature of the Earth's dynamic topographic field.

How does the character of oxygen demand control the structure of hypoxia on the Texas–Louisiana continental shelf?

A realistic hydrodynamic model of the Texas–Louisiana shelf is configured with various simple oxygen respiration models to isolate the effects of stratification and circulation on the formation and maintenance of hypoxia. Biological activity is parameterized through various forms of respiration rather than using a complex biogeochemical model. The model domain covers the region that has historically been observed to be affected by seasonal hypoxia, and is forced with observed fresh water fluxes from the Mississippi and Atchafalaya Rivers and winds. Three simple parameterizations of biological respiration are compared. Results of the numerical simulations indicate that water column respiration (dependent on the position of the two regional fresh water plumes) reproduces observed temporal and spatial structures of seasonal hypoxia in Louisiana Bight, whereas benthic respiration (dependent on local temperature and oxygen concentrations) reproduces the temporal and spatial structure of hypoxia west of Terrebonne Bay. The differences in the structure of hypoxia are related to the differences in vertical stratification east and west of Terrebonne Bay, which are controlled by the outflow characteristics from the two major river plumes. The model shows two dynamically distinct plumes. The Mississippi River plume enters the shelf near the shelf edge, forms a recirculating gyre in Louisiana Bight, and typically does not interact directly with topography. Conversely, the Atchafalaya River plume enters a broad shelf at the coast, is more diffuse, and interacts more with the shallow coastal topography. Both plumes are strongly affected by winds, and tend offshore during the mean summer conditions of upwelling winds. The principle conclusion of this study is that the biological processes responsible for producing hypoxia change from east to west, with the shelf region south of Terrebone Bay being the approximate dividing line between water column respiration (predominantly causing hypoxia to the east) and benthic respiration (causing hypoxia to the west). Also, the formation and destruction of hypoxia is a primarily vertical process; hypoxia formed in one region is not advected laterally to different shelf regions.

Trophic pathways and carbon flux patterns in the Laptev Sea

The Laptev Sea is a high-Arctic epicontinental sea north of Siberia (Russia) that is one of the least understood regions of the world’s ocean. It is characterized by a shallow and broad shelf plateau, high influx of river water, sediments and nutrients during summer, long-lasting sea-ice cover from October to May, and the formation of a narrow flaw-lead polynya off the fast-ice edge during winter. Here, we describe results of a German–Russian research project (1993-present), presenting the distribution patterns and dynamics of its marine flora and fauna, as well as pathways and processes of coupling between sea-ice, water-column and sea-floor biota. Three ecological zones are distinguished along a combined east–west and Lena-impact gradient, differing in the composition of pelagic and benthic communities. In general, high Chl a concentrations in the sediments indicate a tight coupling between sympagic and pelagic primary production and nutrient supply to the benthos throughout the entire Laptev Sea. However, there were pronounced regional differences between the ecological zones in magnitude of primary production and trophic dynamics. Primary production during the ice-free summer was highest in the estuarine zone most strongly influenced by the Lena River (210 mg C m −2 day −1). The western and northeastern Laptev Sea yielded 55 and 95 mg C m −2 day −1, respectively. Moreover, the zones differed in the partitioning of carbon flux between zooplankton and benthic food webs. In the Lena zone zooplankton carbon demand was about 31 mg C m −2 day −1 whereas in the western zone it was 21 mg C m −2 day −1 and in the eastern zone 4 mg C m −2 day −1. Total benthic carbon demand was 32 mg C m −2 day −1 for the Lena zone, 56 mg C m −2 day −1 in the western zone and 100 mg C m −2 day −1 in the northeastern zone. A carbon budget constructed for the Laptev Sea indicates that (1) a high proportion of primary production is channelled through the benthic trophic web, bypassing the pelagic trophic web, and (2) autochthonous primary production in the northeastern and western Laptev Sea might not be sufficient to fuel both pelagic and benthic secondary production and, hence, input of allochthonous organic carbon is required to balance the overall carbon demand.

Crustal structure across the Colorado Basin, offshore Argentina

SUMMARY The geology of the wide shelves surrounding the South Atlantic is closely linked to the kinematics and history of the opening of the ocean. However, several wide sedimentary basins, which developed along the margins show peculiarities that are not yet understood in the context of the evolution of the South Atlantic. The Colorado Basin, a wide sedimentary basin on the broad shelf of Argentina, extends in EW direction. The basin’s evolution oblique or orthogonal to the continent‐ocean boundary indicates that it is not a product of simple progressive extension and crustal thinning. In addition a basement high, paralleling the continental margin and separating the Colorado Basin from the deep-sea basin is a common interpretation. These findings are hardly in accordance with the idea that the Colorado Basin is an extensional basin that developed in conjunction with the early E‐W opening phase of the South Atlantic in the Late Jurassic/Early Cretaceous. The composition, type, and structure of the basement, key points for the evaluation of the basins evolution, are widely speculative. In this context multichannel seismic reflection data from the Argentine Shelf and a 665-kmlong onshore‐offshore refraction profile, running across the Colorado Basin onto the coast are discussed in combination with gravity data. The stratigraphy for the sedimentary successions was adopted from the literature and the reflection seismic marker horizons formed besides the interval velocities the input for the starting model for refraction seismic traveltime modelling. The modelling strategy was an iterative procedure between refraction seismic traveltime and gravity modelling. The preparation of the density models was coarsely orientated on published velocity‐density relations. The modelling results are in favour of a continuation of the main onshore geological features beneath the sedimentary infill of the Colorado Basin. We interpret the basement along the line from west to east as offshore continuation of the Ventana Hills, the

Geomorphological Evolution of Lord Howe Island and Carbonate Production at the Latitudinal Limit to Reef Growth

Lord Howe Island is a volcanic island, rising to over 800 m, draped with Late Quaternary submarine and subaerial carbonate sediments. The island and neighbouring islets lie within a chain of seamounts and is presently at or close to the latitudinal limit to coral reef growth. Lord Howe Island and adjacent Balls Pyramid, composed of the basalts erupted around 6 million years ago, sit near the middle of broad shelves on separate peaks of one major volcanic edifice. The central part of the Lord Howe Island is covered by calcarenite that was deposited primarily as dunes (eolianite), but with isolated beach units. Uranium-series, amino acid racemisation, and thermoluminescence dating indicate that many of these were deposited during marine oxygen isotope stage 5. Eolianite units stratigraphically below the beach deposits are of penultimate interglacial, or in places perhaps older, age. Different suites of erosional landforms are associated with different lithologies. Towering plunging cliffs characterise the resistant Mount Lidgbird Basalt, in some cases fringed with large talus slopes. On less resistant lithologies or where nearshore topography means greater wave force as a result of waves breaking, there are shore platforms. Slumping cliffs abut broad erosional platforms on the poorly lithified calcarenite. A fringing reef on the western side of Lord Howe Island, the southernmost coral reef in the Pacific, is dominated by coral and coralline algae. Carbonate sediments veneering the shelf around the islands contain a more temperate biota. Located at the southern limit of reef-forming seas, but apparently having undergone erosion for much of its history outside of reef seas, Lord Howe Island provides insights into marine planation of volcanic islands close to what has been termed the Darwin Point. It represents the initial stages of fringing reef development on a volcanic island. Middleton and Elizabeth Reefs, north of Lord Howe Island, have the morphology of coral atolls and appear to be gradually subsiding. The Darwinian sequence, fringing reef to atoll, appears particularly compressed in this chain of islands. However, a fossil reef in water depths of around 30 m on the shelf around Lord Howe Island, of unknown age, implies a more complex history.

Role of OH-stretch/torsion coupling and quantum yield effects in the first OH overtone spectrum of cis-cis HOONO

A joint theoretical and experimental investigation is undertaken to study the effects of OH-stretch/HOON torsion coupling and of quantum yield on the previously reported first overtone action spectrum of cis-cis HOONO (peroxynitrous acid). The minimum energy path along the HOON dihedral angle is computed at the coupled cluster singles and doubles with perturbative triples level with correlation consistent polarized quadruple zeta basis set, at the structure optimized using the triple zeta basis set (CCSD(T)/cc-pVQZ//CCSD(T)/cc-pVTZ). The two-dimensional ab initio potential energy and dipole moment surfaces for cis-cis HOONO are calculated as functions of the HOON torsion and OH bond length about the minimum energy path at the CCSD(T)/cc-pVTZ and QCISD/AUG-cc-pVTZ (QCISD-quadratic configuration interaction with single and double excitation and AUG-augmented with diffuse functions) level of theory/basis, respectively. The OH-stretch vibration depends strongly on the torsional angle, and the torsional potential possesses a broad shelf at approximately 90 degrees , the cis-perp conformation. The calculated electronic energies and dipoles are fit to simple functional forms and absorption spectra in the region of the OH fundamental and first overtone are calculated from these surfaces. While the experimental and calculated spectra of the OH fundamental band are in good agreement, significant differences in the intensity patterns are observed between the calculated absorption spectrum and the measured action spectrum in the 2nu(OH) region. These differences are attributed to the fact that several of the experimentally accessible states do not have sufficient energy to dissociate to OH+NO(2) and therefore are not detectable in an action spectrum. Scaling of the intensities of transitions to these states, assuming D(0)=82.0 kJ/mol, is shown to produce a spectrum that is in good agreement with the measured action spectrum. Based on this agreement, we assign two of the features in the spectrum to Deltan=0 transitions (where n is the HOON torsion quantum number) that are blue shifted relative to the origin band, while the large peak near 7000 cm(-1) is assigned to a series of Deltan=+1 transitions, with predominant contributions from torsionally excited states with substantial cis-perp character. The direct absorption spectrum of cis-cis HOONO (6300-6850 cm(-1)) is recorded by cavity ringdown spectroscopy in a discharge flow cell. A single band of HOONO is observed at 6370 cm(-1) and is assigned as the origin of the first OH overtone of cis-cis HOONO. These results imply that the origin band is suppressed by over an order of magnitude in the action spectrum, due to a reduced quantum yield. The striking differences between absorption and action spectra are correctly predicted by the calculations.

Cretaceous and Palaeogene turbidite systems in the North Sea and Norwegian Sea Basins: source, staging area and basin physiography controls on reservoir development

The Cretaceous and Palaeogene succession in the North Sea and Norwegian Sea basins show widely variable deep-water sedimentary systems in terms of processes, facies, geometries, scale and distribution. The primary controls on the large-scale variability are considered to be source area size, basin and basin margin physiography and bathymetry, tectonic history and resulting morphology of drainage and delivery systems of sediments to deep-water areas, and the rate of sediment delivery. The North Sea and Norwegian Sea basins were comparable during the earliest Cretaceous, but thereafter developed in widely different ways as a response to proximity to oncoming North Atlantic seafloor spreading. In the North Sea Basin, the Cretaceous and Palaeogene turbidite systems were controlled by an inherited structural template from Late Jurassic rifting, and by source area size. Poorly developed or small drainage systems on the Norwegian margin and the broad Horda Platform gave little sand supply from the east to the Viking Graben area. Sand-rich systems were sourced from a relatively large hinterland and shallow marine staging area on the East Shetland Platform. North of the Horda Platform, sand supply was abundant in very discrete periods, particularly in the Early Eocene. In the Norwegian Sea basins, the Late Jurassic structural template controlled Early Cretaceous deep-water sedimentary systems in a manner similar to the North Sea Basin. Generally small and poorly developed drainage systems caused development of mud-rich systems. In contrast, in the Late Cretaceous, onset of precursor tectonic activity to sea-floor spreading led to increased sand supply from the west into the Vøring Basin. A relatively narrow palaeoshelf and a large source area contributed to forming sand-rich systems. Smaller turbidite systems developed along the Norwegian margin, which were sourced from the east from smaller drainage areas, and partially across broad shelves, such as the Trøndelag Platform. Both in the Cretaceous and Palaeogene, the sandiest systems are found only to the south and the north of the inherited structural features.

The reservoir development of the Late Maastrichtian–Early Paleocene Ormen Lange gas field, Møre Basin, Mid-Norwegian Shelf

The giant Ormen Lange Gas Field is situated in the Møre Basin, Norwegian Sea, at water depths around 1000 m. The reservoir interval consists of a lower heterolithic unit mainly of Maastrichtian age (Jorsalfare Formation) and an upper sandstone unit of Danian age (Egga Member, Våle Formation), separated by a widely distributed mudstone unit (‘Våle Tight’), and consists of both high and low density turbidites in association with fine-grained background sediments. The reservoir interval spans the Cretaceous–Tertiary boundary, without any major stratigraphic breaks. The main architectural elements of the reservoir are: channel-dominated deposits, channelized lobe deposits and non-channelized frontal splay or fan fringe deposits. The spatial distribution of these facies elements indicates a dynamic system with changing sediment supply through time. Core and log studies from the wells have been integrated with high-resolution biostratigraphy to develop a dynamic depositional model. During latest Cretaceous and Paleocene times, a series of subtle sub-basins developed along the southeastern Møre Basin margin as a result of differential subsidence above Jurassic extensional faults. The sub-basins created a stepped slope topography that influenced the flow pattern of turbidity currents. In a landward position, east of the Ormen Lange Field, the Cretaceous–Tertiary boundary is characterized by a major unconformity, implying erosion and sediment by-pass. In early Danian time, uplift and rotation of the provenance area to the east (Norwegian mainland) led to extensive erosion and redistribution of sandy sediments into the Møre Basin and caused deposition of the Egga Member. The turbidites filled the intraslope basins initially in a basinwards-stepping sense, but later sedimentation switched landward. Onshore structural, provenance and geomorphological data, combined with offshore structural, seismic and sedimentological data, yield an integrated interpretation of the deep-water depositional system in the greater Ormen Lange area. Present fjords at Møre, controlled by the Møre–Trøndelag fault zone, were valley systems that fed sands to a point source updip of the Ormen Lange area. The point source was located at the transition/relay zone between a narrow shelf area in the south and a broad shelf area in the north. In addition to being a key transition zone for both Caledonian and Jurassic structures, this zone area is also the ‘landing’ point of the major oceanic Jan Mayen Fracture Zone.

Manganese and iron distributions off central California influenced by upwelling and shelf width

In July 2002, a combination of underway mapping and discrete profiles revealed significant along-shore variability in the concentrations of manganese and iron in the vicinity of Monterey Bay, California. Both metals had lower concentrations in surface waters south of Monterey Bay, where the shelf is about 2.5 km wide, than north of Monterey Bay, where the shelf is about 10 km wide. During non-upwelling conditions over the northern broad shelf, dissolvable iron concentrations measured underway in surface waters reached 3.5 nmol L −1 and dissolved manganese reached 25 nmol L −1. In contrast, during non-upwelling conditions over the southern narrow shelf, dissolvable iron concentrations in surface waters were less than 1 nmol L −1 and dissolved manganese concentrations were less than 5 nmol L −1. A pair of vertical profiles at 1000 m water depth collected during an upwelling event showed dissolved manganese concentrations of 10 decreasing to 2 nmol L −1, and dissolvable iron concentrations of 12–20 nmol L −1 in the upper 100 m in the north, compared to 3.5–2 nmol L −1 Mn and ∼0.6 nmol L −1 Fe in the upper 100 m in the south, suggesting the effect of shelf width influences the chemistry of waters beyond the shelf. These observations are consistent with current understanding of the mechanism of iron supply to coastal upwelling systems: Iron from shelf sediments, predominantly associated with particles greater than 20 μm, is brought to the surface during upwelling conditions. We hypothesize that manganese oxides are brought to the surface with upwelling and are then reduced to dissolved manganese, perhaps by photoreduction, following a lag after upwelling. Greater phytoplankton biomass, primary productivity, and nutrient drawdown were observed over the broad shelf, consistent with the greater supply of iron. Incubation experiments conducted 20 km offshore in both regions, during a period of wind relaxation, confirm the potential of these sites to become limited by iron. There was no additional growth response when copper, manganese or cobalt was added in addition to iron. The growth response of surface water incubated with bottom sediment (4 nmol L −1 dissolvable Fe) was slightly greater than in control incubations, but less than in the presence of 4 nmol L −1 dissolved iron. This may indicate that dissolvable iron is not as bioavailable as dissolved iron, although the influence of additional inhibitory elements in the sediment cannot be ruled out.

Massive siliciclastic discharge to slopes of the Great Barrier Reef Platform during sea-level transgression: constraints from sediment cores between 15°S and 16°S latitude and possible explanations

The northeast Australian continental margin, including the Great Barrier Reef (GBR) Platform, provides an outstanding modern analogue for understanding sedimentation in tropical mixed siliciclastic/carbonate depositional systems. Previous studies of cores from this margin have suggested that, in contrast to widely accepted models, maximum siliciclastic fluxes to slopes over the last 30 ky occurred when sea level transgressed the shelf ca. 11–7 ka rather than when sea level was at a lowstand before 18 ka. However, climate and bathymetry can affect regional sediment deposition significantly, and the cores examined to date have come from the southern Queensland Trough where the margin has a wet tropical climate and a broad shelf (>50 km) with gradual slopes. In this study, we examine the abundance and accumulation of siliciclastic material in four cores from lower slopes of the northern Queensland Trough where rivers drain a drier region and debouche onto a narrow shelf with steep, incised slopes. Similar to previously examined cores, all cores contain a dark greyish brown horizon within the upper few meters. This horizon represents a low in carbonate content and corresponding high in siliciclastic material, primarily clay, and oxygen isotope stratigraphy shows that it was deposited during late transgression, nominally between 11 and 7 ka. Moreover, mass accumulation rate calculations demonstrate that this horizon reflects a significant rise in siliciclastic fluxes. When combined with the previous work to the south, it appears that slopes all along a large portion of the northeast Australian continental margin received greatly enhanced inputs of siliciclastic material during late transgression, independent of proximal climate and bathymetry. Two general explanations are offered for this observation: (1) climate change induced highly variable riverine discharge over the past 30 ky with a prominent maximum coincident with late transgression, or (2) the shelf stores large quantities of siliciclastic sediment during lowstand and releases this material to the slope during late transgression. In either case, siliciclastic accumulation on the northeast Australia continental margin behaves completely opposite to predictions from conventional models.

Subsidence and tectonic controls on glacially influenced continental margins: examples from the Gulf of Alaska and the western Scotian Shelf and Slope

The glacial record on continental shelves and slopes at mid- to high latitudes is dominated by marine strata, but little is known regarding the large-scale architecture of such deposits. Models showing the gross architecture and facies successions of Late Cenozoic deposits in the Gulf of Alaska and on the western portion of the Nova Scotian Shelf and Slope of southeastern Canada have been developed to guide the interpretation of ancient glacially influenced marine sequences and to illustrate the influences of their tectonic setting. The processes of deposition (including ice rafting, suspension rain-out, debris flows, and turbidity currents) are the same on both the Gulf of Alaska and the Scotian Shelf and Slope, yet the large-scale depositional architecture of glacially influenced marine deposits on both margins differs because sediment preservation is strongly influenced by the impact of tectonics on relative sea-level changes. Eustatic sea level was lower during major glaciations, enabling ice sheets to advance across the Scotian Shelf, whereas rapid ongoing subsidence in the forearc basin of the Gulf of Alaska restricted glacial advances to shallow, nearshore areas until the forearc basin filled with sediments to create a broad shelf. Both shelf and slope deposits are preserved in the Gulf of Alaska, which has developed by both progradation and aggradation, whereas slope deposits are selectively preserved on the Nova Scotia continental margin, which has developed by propagation.

Influence of shelf topography upon along shelf flow and across shelf exchange in the region of the Ebro Delta

The influence of along shelf topography upon shelf edge flow, and cross shelf exchange in the Ebro Region of the Catalan shelf, is studied using a non-linear hydrodynamic model. This region is of particular interest as it is an area of transition from a narrow to a broad shelf. Calculations are performed using both idealized and physically realistic topography, for a number of inflows at the shelf edge on the narrow shelf. Calculations using idealized topography show that the position of the inflow relative to the shelf edge, and the horizontal shear within the inflow are the dominant factors influencing the along shelf flow and the formation of eddies on the narrow shelf. For a wide jet, or a narrow jet inflow in shallow water, the along shelf flow is more likely to be steered along the topography with minimum cross shelf exchange of water. However, in the case of a narrow jet discharged on the deep water side of the shelf edge, small-scale eddies (having a length scale comparable with the width of the narrow shelf and period corresponding to the first shelf wave) are formed on the narrow shelf and instabilities occur in the shelf edge flow. The intensity of the instability in the inflow is related to the degree of across shelf shear in the inflow current. In essence the onset of the instability in the along shelf flow is governed by barotropic stability theory. Hence, it is controlled by the potential vorticity gradient across the shelf edge at the location of the inflow. Similar effects are found in the model when realistic topography is used, although in this case some eddies appear on the wide shelf associated with topographic features. The influence of bottom friction and the form and location of the inflow upon the dynamics of the flow in the region is investigated using an empirical orthogonal function (EOF) approach. This shows that in the case of weak barotropic instability a large proportion of the energy is contained within the first two modes. Coefficients of these modes have a period close to the resonant period of the shelf wave of order 48 h with a spatial pattern reflecting that of the shelf wave. On the wide shelf, a larger scale circulation is present with a period of the order of 5 days. In the case of strong barotropic instability, the EOF analysis shows that the shelf modes and associated eddies grow with time and non-linear interaction can take place due to non-linearities in the model. In this case higher modes are important and the coefficients of the EOFs show the presence of a range of periods associated with some non-linear effects. Calculations clearly show that the magnitude of the circulations associated with these flows, and the intensity of meanders of the along shelf flow are critically dependent upon the inflow conditions and bottom friction.

A “broad‐shelf effect” upon postglacial relative sea level history

Recently published analyses of postglacial relative sea level (RSL) histories from coastal locations which are proximate to broad continental shelves that were significantly exposed land at Last Glacial Maximum, have demonstrated that the Holocene RSL variations at such sites are not adequately explained unless particular care is taken to treat the influence of shelf inundation. This is herein shown to require a local adjustment to the water load based upon a mass conservation constraint. Previously revealed misfits of theory to data from such regions therefore do not signify a requirement for lateral heterogeneity in model viscoelastic structure. The quality of the revision to the theory required to account for this “broad shelf effect” is illustrated through analysis of RSL histories from the east coast of the South American continent and from Northwestern Europe.