Present-day Shelf Research Articles

Stratigraphic and palaeo-geomorphological evidence for the glacial-deglacial history of the last British-Irish Ice Sheet in the north-western Irish Sea

During its maximum extent around 25–23 ka BP, the British-Irish Ice Sheet extended to near the present-day shelf break with a large ice stream, the Irish Sea Ice Stream, occupied much of the present-day Celtic and Irish Seas. The different episodes of advance and retreat of the ice sheet have been extensively constrained onshore, with models based on feature characterisation and dating helping to understand the retreat pattern of the ice sheet. Offshore, glacial geomorphologies in the Irish Sea have been mostly recognised on the United Kingdom side but remains poorly constrained on the Irish side. In this paper, based on the analysis of an extensive seismic dataset, an update of the stratigraphic model of the northern Irish Sea, with new sub-units, is proposed with a mapping of newly discovered buried geomorphology. Characterisation of the geomorphology, including megascale glacial lineations, subglacial meltwater channels, moraines, and iceberg scours, has been used to extend the constraints on ice sheet retreat offshore. The transition from late glacial to marine setting has been constrained based on the mapping of marine sediment subunits and their terminations. This paper brings new constraints offshore, in a previously understudied area, and synthesises all the results into an updated conceptual model highlighting stratigraphic units succession and associated processes linked to geomorphology, within the context of a rapidly retreating ice stream.

3D sedimentary architecture showing the inception of an Ice Age

Northeast Atlantic climate shifted into the Quaternary Ice Age around 2.6 M yr ago. Until now, however, the detailed changes associated with this inception of an Ice Age have remained obscure. New high-quality three-dimensional seismic data reveal a detailed geological record of buried surfaces, landforms and sedimentary architecture over vast parts of the Norwegian North Sea. Here, we show the sequence of near-coast geological events spanning the Northeast Atlantic inception of an Ice Age. We identify the location of immediate pre-glacial fluvially derived sandy systems where rivers from the Norwegian mainland built marine deltas. The stratigraphic position of a large submarine channel, formed by enhanced meltwater from initial build-up of local glaciers, is also shown. Finally, we document the transition to full ice-sheet growth over Scandinavia from the ice sheet’s earliest position to the later pattern of debris-flow lobes reaching the present-day shelf edge.

Shelf sand supply determined by glacial-age sea-level modes, submerged coastlines and wave climate

Submerged paloeshorelines preserved on the continental shelf indicate the depths of the most frequent (modal) low sea-levels within the glacial stages of the Late Quaternary. Here we have determined the south-east Australian shelf configuration when sea level was 40 m and 60 m below present-day sea-level (depths of the most persistent paleoshorelines within the last 120 ka), and we resolve the wave climate variations influencing the sediment transport pathways over this period. We present evidence demonstrating that the combination of shelf morphological evolution, changes in sea-level and variations in wave climate is responsible for latitudinal changes in sediment transport and deposition during the interglacial states. The paleoshoreline and shelf evolution is key to understanding the distribution of present-day shelf sand deposits and the contemporary sand budget response to future wave climate changes.

Holocene source rock deposition in the Black Sea, insights from a dropcore study offshore Bulgaria

Abstract One of the main issues in source rock evaluation has always been the availability of thermally immature samples, which would represent the same source rock quality and facies as the mature source rock within the deeper parts of the basin. Forty dropcore sample locations from shallow depths beneath the present-day seafloor were selected and analysed for mineral composition and bulk geochemical parameters. The water depths of the samples range from shelfal to bathyal environments. The quartz content of the samples clearly decreases with increasing distance from sedimentary input sources (e.g. river deltas), whereas clay content increases towards the distal areas. Mass movements (e.g. slides and debris flows) along the present-day shelf are recognizable on the bathymetry, as well as in the mineral content. Bulk geochemical parameters show that currently only poor to fair gas-prone source rocks are deposited within the study area. This lack of source rock quality, as well as organic content, is attributed to the fine-grained sedimentary input from the Danube river. These fine-grained sediments decrease the organic productivity due to dulling (decrease in the thickness of the photic zone) of the water column, and dilute the currently deposited source rock with low TOC sediments. These effects decrease with distance from the Danube delta, as indicated by published data from outside the study area. Additionally mass movements along the present-day shelf rework possible source rocks. The results of this study clearly show that anoxic conditions alone are not sufficient for source rock deposition. Distance from major sedimentary input and basin geometry are of major importance, and should be considered in basin modelling.

Interaction of down-slope and along-slope processes off Capo Vaticano (southern Tyrrhenian Sea, Italy), with particular reference to contourite-related landslides

The Late Quaternary architecture of the upper slope-shelf sector of the Calabro-Tyrrhenian continental margin off Capo Vaticano was reconstructed to decipher the interaction of along-slope and down-slope sedimentary processes, based on the analysis of very high-resolution single-channel seismic profiles and multibeam bathymetry. Both along-slope bottom currents and downslope gravity processes influenced the stratigraphic architecture of the margin during the last eustatic cycle, locally forming a mixed turbidite and contourite system. Along-slope processes formed elongated drifts located on the upper continental slope and outer shelf, while gravity-driven down-slope processes formed mass-transport deposits and turbidite systems with erosive channels, locally indenting the present-day shelf. Several slide events affected the elongated drifts and failure processes appear to be dominated by translational sliding that affect the upper 10–20m of the stratigraphic record, with glide plains developed within contourite deposits. The most striking feature is the Capo Vaticano slide complex, which displays a large spatial coverage (area of about 18km2), between ca −190m and −500m and is composed of several intersecting slide scars and overlapping deposits. These characteristics are peculiar for the Tyrrhenian continental margins, where slide events developed in open-slope areas are usually less complex and smaller in size. This would indicate that high sedimentation rates and occurrence of contourite deposits can represent a preconditioning factor for medium-large scale slope instability on the Tyrrhenian margins.

Crustal structure off Kong Oscar Fjord, East Greenland: Evidence for focused melt supply along the Jan Mayen Fracture Zone

The complex structure of the North-East Greenland margin is the consequence of two rifting events: the initial separation of Greenland and Scandinavia around 56Ma, and the breaking off of the Jan Mayen microcontinent from Greenland around 33Ma likely driven by the arrival of the Iceland Plume beneath the east coast of Greenland. The boundary between these two rifting events is the Jan Mayen Fracture Zone. While seismic lines north of this pronounced topographic structure document the presence of a high velocity lower crust, existing data south of the fracture zone show no indications that the lower crust consists of a large, high velocity body. This gap is closed by a 500km long seismic transect, which starts in Kong Oscar Fjord and extends out beyond the Kolbeinsey Ridge. It provides a detailed view of the crustal structure just south of the Jan Mayen Fracture Zone. The transect includes parts of the Caledonian crust of Greenland, crosses the present-day shelf and the oceanic crust formed since the separation of the Jan Mayen microcontinent. The seismic refraction line shows a constant ~9km thick oceanic crust between the extended continental margin and the mid-ocean ridge. Evidence for an only 3km thick high-velocity lower crustal body is observed within the continent-ocean transition zone. While such high velocity crust is ubiquitous beneath the shelf to the north of the fracture zone, to the south it only occurs beneath Mesozoic basins that have been attributed to extensional collapse of the Caledonian orogeny. Inline with earlier interpretations of high velocities in the lower crust of extended continental margins, we suggest that Kong Oscar Fjord is underlain by the products of excess magma production that were focused along the Jan Mayen Fracture Zone during the breakup of the Jan Mayen microcontinent from Greenland.

Geology and tectonics of the northeast Russian Arctic region, based on seismic data

The structure of the sedimentary cover and acoustic basement in the northeastern Russian Arctic region is analyzed. Beneath the western continuation of the North Chukchi trough and Vil’kitskii trough, a Late Caledonian (Ellesmere) folded and metamorphozed basement is discovered. It is supposed that Caledonides continue further into the Podvodnikov Basin until the Geofizikov branch. A large magnetic anomaly in the Central Arctic zone has been verified by seismostratigraphic data: the acoustic basement beneath the Mendeleev (and partially Alpha) Ridge is overlain by trapps. Wave field analysis showed that the acoustic basement of the Lomonosov Ridge has folded structure, whereas beneath the Mendeleev Ridge, the sporadic presence of a weakly folded stratum of Paleozoic platform deposits is interpreted. It is supposed that the Caledonian and Late Cimmerian fold belts in the periphery of the Arctida paleocontinent appeared as a result of collision between arctic continental masses and southern ones. After Miocene extension and block displacements identified from appearance of horsts, grabens, and transverse rises both on the shelf and in the ocean, a general subsidence took place and the present-day shelf, slope, and the deepwater part of the Arctic Ocean formed.

Chapter 8 Petroleum Geology of the Tanintharyi Region, Myanmar

The Tanintharyi offshore region as designated by the government is shown in Figure 8.1a. It is subdivided into two broadly north–south-trending structural elements: the Tanintharyi Shelf on which the small Tanintharyi and North and South Mali basins are also present, and the distal, southern, part of the Martaban Basin. Fig. 8.1. ( a ) The Tanintharyi region showing MOGE block numbers, the Martaban Basin and the Yetagun gas field development area. ( b ) Eastern part of the Tanintharyi region showing wells, blocks and bathymetric contours. In the Yetagun area some wells have been omitted for the sake of clarity. The Tanintharyi Shelf (also known as the Mergui Shelf or Mergui Terrace) forms the eastern part of the region and mainly overlies a basement of Palaeozoic–Mesozoic indurated sediments intruded by granites of the Phuket-Slate Belt terrane (see Chapter 2). Its western boundary is marked by the dextral Shan Fault (also known as the Mergui Fault in Thailand), and the shelf is located in an overall back-arc setting on continental crust. This shelf area is designated as shallow-water licence blocks M12 and M13 in the north to M18 in the south. The sediments on this shelf were sourced during much of the Early and Middle Miocene from the Thai-Myanmar Peninsula in the east, and the succession thickens westwards. During the Late Miocene–Pleistocene, sediment input from the north became more dominant representing the distal portion of the Thanlwin/Ayeyarwady depositional system. The Yetagun Field and associated discoveries in and around the SE corner of Block M12 (Fig. 8.1a, b) are close to the present-day shelf edge, which coincides approximately with the more gradational western boundary of the Tanintharyi Shelf. Several early wells in the northern half of the shelf area had hydrocarbon shows, but the structures were generally small and partially breached. In the eastern part of …

FORMATION AND EVOLUTION OF THE RUSSIAN ARCTIC CRYOLITHOZONE (ON THE SHELF AND THE CONTINENT) IN THE PLEISTOCENE-HOLOCENE

The paper addresses age nonuniformity of the permafrost of the Russian Arctic shelf. It has been widely accepted that recent permafrost exists on the present-day shelf, which was formed under subaerial conditions during shelf draining in the Late Pleistocene, was flooded during the subsequent transgression, and now exists as a relic zone. However, there is also modern permafrost forming under submarine conditions. The paper considers the mechanism of its formation. The author suggests a mechanism that involves bottom soil freezing and ice formation due to constant natural transformations in seabed sediments. The proposed mechanism is supported by analyzes of certain sections of the bottom sediments of shelf and of the Pleistocene marine plains (ancient shelves) composed of dislocated sequences with massive ice beds. Analysis of the massive ground ice genesis identified different geological history as well as different transgressive and regressive regime of the Russian Arctic western and eastern sectors. The glacial cover has limited distribution in the Russian North and was absent on the Russian Arctic and Subarctic plains to the East of the Kanin Peninsula.

Permafrost (Cryolithozone) of the Russian Arctic Shelf

The problem of age nonuniformity of the permafrost of the Russian Arctic shelf is considered in this paper. It is assumed that recent permafrost exists on the present-day shelf in addition to the cryolithozone, which was formed under subaerial conditions during shelf draining in the Late Pleistocene, was flooded during the subsequent transgression, and exists as a relic zone. It is considered whether permafrost can be formed immediately under submarine conditions. The mechanism of bottom soil freezing and ice formation, based on constant natural transformations in seabed sediments, is proposed. The proposed mechanism is confirmed by analyzing certain sections of the bottom sediments of shelf and of the Pleistocene marine plains (ancient shelves) composed of dislocated sequences with massive ice beds. On the basis of a ground massive ice genetic analysis different geological development as well as different transgressive and regressive regime of the Russian Arctic western and eastern sectors was revealed. The glacial cover limited distribution in the Russian North and its absence on the Russian Arctic and Subarctic plains to the East of the Kanin Peninsula were noted.

FORMATION AND EVOLUTION OF THE RUSSIAN ARCTIC CRYOLITHOZONE (ON THE SHELF AND THE CONTINENT) IN THE PLEISTOCENE-HOLOCENE

The paper addresses age nonuniformity of the permafrost of the Russian Arctic shelf. It has been widely accepted that recent permafrost exists on the present-day shelf, which was formed under subaerial conditions during shelf draining in the Late Pleistocene, was flooded during the subsequent transgression, and now exists as a relic zone. However, there is also modern permafrost forming under submarine conditions. The paper considers the mechanism of its formation. The author suggests a mechanism that involves bottom soil freezing and ice formation due to constant natural transformations in seabed sediments. The proposed mechanism is supported by analyzes of certain sections of the bottom sediments of shelf and of the Pleistocene marine plains (ancient shelves) composed of dislocated sequences with massive ice beds. Analysis of the massive ground ice genesis identified different geological history as well as different transgressive and regressive regime of the Russian Arctic western and eastern sectors. The glacial cover has limited distribution in the Russian North and was absent on the Russian Arctic and Subarctic plains to the East of the Kanin Peninsula.

The Catalan margin during the Messinian Salinity Crisis: Physiography, morphology and sedimentary record

The Messinian Salinity Crisis (MSC) drastically affected the physiography, morphology and sedimentation all along the Mediterranean area at the end of the Miocene. This paper presents an analysis of the effects of the MSC on the Catalan continental margin, based on a broad database of seismic reflection profiles from the oil industry and academy. We study the sedimentary processes and their controlling factors and the correlation with the nearby and well-known Gulf of Lions. Because of the complexity of the MSC, we define the Margin Erosion Surface/Top Erosion Surface (MES/TES) as the surface marking the end of the Messinian event, which allows differentiating three domains along the Catalan margin: the Creus, Girona and Barcelona domains, separated by the Palamós and Blanes canyons. The Creus domain shows an important structural control on the MES/TES physiography, while the Girona and Barcelona domains are characterized by wide and flat MSC platforms below the present-day shelf. The entire margin developed a series of complex drainage systems as the result of the Messinian sea level drop. The location of the main valleys (Cap de Creus paleo-drainage system, Palamós and Blanes paleo-canyons and the downslope-trending valleys on the Barcelona domain) was directly controlled by the structural framework. In contrast, the Barcelona paleo-drainage system and the tributaries of the Blanes and Palamós systems were excavated by direct subaerial erosion by meteoric rain in the basins limited by basement blocks, and were afterwards totally filled by the Pliocene–Quaternary sedimentation. Regarding the MSC depositional features, the major part of the eroded sediment was deposited as detrital deposits on the foot of the slope. The distribution of detrital bodies and evaporites was also structurally controlled. A Zanclean sea level rise occurring in at least two phases is confirmed by this study, in correlation with interpretations from the Gulf of Lions.

The palaeogeography of Northwest Europe during the last 20,000 years

Please click here to download the map associated with this article. Since the last glacial period, large vertical changes in the height of sea level relative to the land surface have led to considerable horizontal shifts in the position of coastlines around Northwest Europe. Indeed, for much of the last 20,000 years, extensive areas of the present-day shelf seabed were sub-aerially exposed due (primarily) to the glacial eustatic lowering of sea level. Accurate maps depicting these palaeogeographic changes are of great value to a wide spectrum of researchers including (inter alia) archaeologists, marine geomorphologists, climate scientists, biogeographers and palaeobotanists, although a lack of empirical sea level data has often hindered efforts to produce reliable palaeogeographic reconstructions. However, the processes which bring about change in relative sea level can be successfully simulated by computer models that describe the response of the solid Earth to the loading and unloading of glacial ice (‘glacial rebound models’). In addition to simulating relative sea-levels, the output from these models can be combined with modern day bathymetric and topographic data to produce first-order palaeogeographic reconstructions. For this publication and associated map, numerical outputs from a recently published glacial rebound model are used to produce a series of palaeogeographic maps of Northwest Europe since the Last Glacial Termination. These maps, developed using GIS tools and presented here individually at a scale of 1:20,000,000, emphasize that for much of the period from 20,000 years ago to the present, large areas of the Northwest European shelf, now covered by sea, were dry land. However, they also suggest that whilst Britain maintained a ‘land-bridge’ connection with the continent until well into the Holocene interglacial (which began 11,700 years ago), any connection between Britain and Ireland would have been low-lying, probably ephemeral and unlikely to have existed after circa 15,000 years ago.

Paleogeographic settings and geodynamics of the northeastern Pacific margin of Asia in the terminal Cretaceous-Early Paleogene

Evolution of the ocean-continent transition zone in the northeastern Pacific margin of Asia in the con� text of general geodynamics continues to receive attention from geologists. This problem is considered in many circumstantial works (1-3). At the same time, these general interpretations largely concern its Meso� zoic history, while the Cenozoic remain insufficiently studied. In addition, these works ignored particular features of paleogeographic settings and variations in the geodynamic regime within short geological peri� ods marked by changes in the character of boundaries between domains with the oceanic crust and continen� tal margins. The purpose of this communication is analysis of the paleogeographic settings and geody� namics in the Late Cretaceous-Early Paleogene in the spacious ocean-continent transition zone between the western part of the presentday Bering Sea shelf and western Kamchatka (Fig. 1). This period is sand� wiched between two geological events: early (pre� Maastrichtian) formation of the main fold-thrust structures in the Koryak Highland and intermediate (Early Paleogene) reorganization related to accretion of the Olyutor-eastern Kamchatka island arc (2). In the northern Koryak region and Bering Sea shelf, this stage preceded the onset of formation of sedimentary basins during the second half of the Paleogene and Neogene in the presentday shelf and adjacent shore areas (Khatyrka, Navarin). In this connection, of significance is the problem of former spatial relationships between coeval (late Cam� panian-Paleocene) sediments of the continental mar� gin (shelf, coastalmarine, paralic, and continental) and deepwater flyschoid sequences that were likely formed on the oceanic crust of the adjacent basin. In the presentday structure, the uppermost Cretaceous- basal Paleogene sediments deposited on the continen� tal margin are known in the western part of the Bering Sea shelf (based on drilling and dredging data (4)) and on land, where they are traceable from the Navarin Peninsula via the southern and central parts of the Koryak Highland, Shelekhov Bay in the Kamchatka Isthmus southward to western Kamchatka. Onshore, coeval deepwater flyschoid sequences (known as the Ukelayat, Lesnaya, and Khozgon flysch sequences) are distributed from the Dezhnev and Anastasiaya bights in the east via the Ukelayat River basin to the upper reaches of the Apuka and Pakhacha rivers and farther southwestward to the Kamchatka Isthmus, where they outcrop from under tectonic nappes near the axial part of the Sredinnyi (Median) Range. In the central part of western Kamchatka, similar sequences constitute the western slopes of the Sredinnyi Range. According to seismic data, the lower part of the conti� nental slope in the western Bering Sea shelf is repre� sented by an accretionary wedge of turbidites (4), which are likely correlative with the onshore flyschoid sequences. It should be noted that deepwater flyschoid sequences are either characterized by tectonic rela� tionships with coeval sediments of the continental margin or occur near them with the contact being cov� ered by young sediments. No facies that could be interpreted as transitional between the two above� mentioned sediment types are recorded in these areas. Moreover, deepwater terrigenous sequences are uni� versally characterized by a quartz-feldspar composi� tion with insignificant admixture of volcanogenic detrital material against the background of the com� plete lack of tephra (5, 6). In the northeast (Ukelayat zone), the Campanian-Maastrichtian and Pale� ocene-Lower Eocene deepwater flyschoid sequences (Taven, Ayaon, Val'en, and Lyapganai formations), which form a continuous succession, contact along the thrust with the upper Campanian (Vachvayam

Transverse segmentation of the Baram-Balabac Basin, NW Borneo: refining the model of Borneo's tectonic evolution

The West Baram Line separates NW Borneo’s two petroleum systems. Oligocene sandstone and Lower Miocene carbonate reservoirs of the gas-prone Luconia system lie SW of that line. Northeast of the West Baram Line, the oil-rich Baram-Balabac Basin produces from Middle Miocene to Early Pliocene sandstones deposited in a foreland basin on the western side of the mountainous interior of Borneo. On the present-day shelf exploration efforts spanning nearly four decades have focused on the Champion and Baram deltas and associated extensional growth fault structures. Many of these structures have experienced youthful inversion owing to ongoing tectonic shortening. Recent discoveries prove this petroleum system extends into deep water beyond the modern shelf edge where an active fold-thrust belt has formed above autochthonous rifted continental crust of the Dangerous Grounds province in the South China Sea. New regional mapping (c. 100 000 km2) integrating seismic, borehole and gravity data shows that the Baram-Balabac Basin is segmented into four structural domains whose boundary zones trend NW-SE similar to the strike of the West Baram Line. Domain boundaries appear to control the position of the basin’s palaeoshelf edges, turbidite depositional systems, major unconformities and the position of the basin’s largest fields. These observations suggest that the domain boundaries are the expression of deep structures, probably within the underlying rifted continental crust. Two of the domain boundaries can be projected onshore to align with fault systems separating contrasting geological elements indicating they represent features of tectonic interest. The onshore geology of NW Borneo represents the early history of the Baram-Balabac Basin. The onshore geology is reviewed and new outcrop, biostratigraphic and palaeomagnetic data from Sabah are discussed in light of current models for the region’s tectonic evolution. A hybrid model is proposed in which the Baram-Balabac Basin post-dates the Sarawak Orogeny. The Sarawak Orogeny, which appears to be more regionally extensive than previously believed, is attributed to Eocene to Early Oligocene collision of the Dangerous Grounds and Reed Bank with Sabah and Palawan. In the proposed model Oligo-Miocene subduction of oceanic crust under NW Borneo is minimal. The Sabah Orogeny and younger inversion events are related to underthrusting of the Dangerous Grounds driven by both the opening of the South China Sea and by NW-directed subduction beneath SE Sabah in the Semporna-Dent Peninsula. In this context the structural complexity of the Baram-Balabac foreland basin reflects the adjustment of deep crustal blocks to far-field tectonic stress.

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.

Variation in marine total organic carbon through the type Kimmeridge Clay Formation (Late Jurassic), Dorset, UK

Bulk geochemical data from the NERC Rapid Global Geological Events project are used to appraise the pattern of variation in mean organic content through the cymodoce to rotunda zones of the type Kimmeridge Clay Formation in southern Dorset. There is a symmetrical stratigraphic pattern in mean and modal total organic carbon (TOC) values, which increase from 1–2% at the top and base to a peak of 8–9% in the middle of the formation. To remove distortion caused by the strong positive skewness of the TOC values, the mean TOC per zone was recalculated for the first mode of the TOC distribution only, and combined pragmatically with pyrolysis estimates of the reactive (marine) TOC fraction to derive marine TOC values; these also vary symmetrically from c . 0.5% to peak values of c . 5.0%. The mean TOC is negatively correlated with sedimentation rate, indicating that dilution is a significant controlling variable. Mean palaeoproductivity was assessed using modern marine sediment relationships between carbon burial efficiency, sedimentation rate (derived from RGGE cyclostratigraphic analyses), the likely dissolved oxygen range, and water depth. Best estimates of palaeoproductivity range from 40 to 150 g C m −2 a −1 (low to moderate by comparison with present-day shelves) and are positively but nonlinearly correlated with mean δ 13 C TOC values.

Reply to comment of Emile Okal on “Source of the great tsunami of 1 April 1946: a landslide in the upper Aleutian forearc”

The complex structure of the North-East Greenland margin is the consequence of two rifting events: the initial separation of Greenland and Scandinavia around 56 Ma, and the breaking off of the Jan Mayen microcontinent from Greenland around 33 Ma likely driven by the arrival of the Iceland Plume beneath the east coast of Greenland. The boundary between these two rifting events is the Jan Mayen Fracture Zone. While seismic lines north of this pronounced topographic structure document the presence of a high velocity lower crust, existing data south of the fracture zone show no indications that the lower crust consists of a large, high velocity body. This gap is closed by a 500 km long seismic transect, which starts in Kong Oscar Fjord and extends out beyond the Kolbeinsey Ridge. It provides a detailed view of the crustal structure just south of the Jan Mayen Fracture Zone. The transect includes parts of the Caledonian crust of Greenland, crosses the present-day shelf and the oceanic crust formed since the separation of the Jan Mayen microcontinent. The seismic refraction line shows a constant ~ 9 km thick oceanic crust between the extended continental margin and the mid-ocean ridge. Evidence for an only 3 km thick high-velocity lower crustal body is observed within the continent-ocean transition zone. While such high velocity crust is ubiquitous beneath the shelf to the north of the fracture zone, to the south it only occurs beneath Mesozoic basins that have been attributed to extensional collapse of the Caledonian orogeny. Inline with earlier interpretations of high velocities in the lower crust of extended continental margins, we suggest that Kong Oscar Fjord is underlain by the products of excess magma production that were focused along the Jan Mayen Fracture Zone during the breakup of the Jan Mayen microcontinent from Greenland.

Shelf petroleum system of the Columbus basin, offshore Trinidad, West Indies. II. Field geochemistry and petroleum migration model

Petroleum migration patterns in the present-day shelf area of the Columbus basin reflect a complex interaction of structural evolution, stratigraphic architecture, and fault-seal behavior. Thermogenic charge access is ultimately controlled by the geographic distribution of sand-prone carrier beds and how these vertically stacked stratigraphic units relate spatially to one another along their basinal limits. Migration in this system consists of two components, cross-stratal migration through mud-dominated, deep-water (slope and basin floor) sediments followed by horizon-parallel flow along laterally extensive, sand-prone, shelf carrier beds. Thermogenic charge access to any individual sand occurs only in a linear geographic area where that sand is not shielded from vertical migration by stratigraphically older sands. The geochemical characteristics of the trapped petroleum in this system are primarily a reflection of migration distance, with the earliest-expelled (lowest maturity) products having progressed farthest along the migration pathway. Because of the stratigraphic architecture and structural geometry involved, a pattern of increasing thermogenic content and maturity with depth at any location is created, despite the fact that a significant portion of the migration is horizon-parallel. In parts of the basin, this simple pattern has been overprinted by complex mixing of maturity fractions in single accumulations as a result of late-stage differential source-rock uplift. The observations are inconsistent with previously proposed models for this basin that emphasize petroleum fractionation accompanying vertical migration through the shelf stratigraphic section.

Paleoenvironments, diagenesis, and sequence stratigraphy of the mid-cretaceous section (albian/cenomanian) in the Main Pass Field area, northeastern Gulf of Mexico

The Main Pass Block 253 Field, northeastern Gulf of Mexico, produces oil from mid-Cretaceous carbonate reservoirs at 8,700 to 8,900 feet. The field is located on Main Pass Blocks 253 and 254 and is situated along the present-day continental shelf margin, which closely approximates the shelfedge during Early Cretaceous time. This field is significant because it demonstrates the oil potential of Lower Cretaceous carbonates in the northeastern Gulf of Mexico. Core and thin-section studies of the Washita reservoir rocks, Late Albian to Early Cenomanian in age, show that these carbonates accumulated as part of a reef-shoal complex in a carbonate rimmed-shelf system. A general progradational trend in the section is evident: shelf-margin reef facies are overlain by back-reef and patch-reef facies represented by middle-to-inner-shelf carbonate deposits. Overall, 10 distinct cycles grading upward from mud-supported bafflestone, wackestone, and packstone to grain-supported rudstone and grainstone are observed in core. These coarsening-upward cycles reflect an increase in energy upsection. The boundaries of the cycles are defined by physical surfaces of erosion or non-deposition, or by abrupt changes in facies. The major constituents in the reef facies are rudists, including caprinids and radiolitids. These organisms are in life position in the lower part of the sequence. A major unconformity is observed in the core. Washita carbonates are overlain by Campanian chalk. The chalk was dated using nanofossil associations. This unconformity is the regional mid-Cretaceous unconformity of the northeastern Gulf of Mexico as recognized on seismic profiles. This unconformity probably contributed to the intense diagenetic alteration of the Washita carbonates observed in core. Diagenetic processes observed in the core include dolomitization, neomorphism, pressure solution, and karstification.