Fossilisation of a silica diagenesis reaction front on the mid-Norwegian margin

The diagenetic transformation of biogenic silica from opal-A to opal-CT was recognised on seismic reflection data over an area of 78 × 10³ km² on the mid-Norwegian margin. The opal-A/CT diagenetic boundary appears as a positive, high amplitude reflection that generally cross-cuts the hosting stratigraphy. We demonstrate that it is not a sea bottom simulating reflection (BSR) and also that is not in thermal equilibrium with the present day isotherms. We present arguments that three styles of deformation associated with the opal-A/CT reflection – polygonal faulting, regional anticlines and synclines and differential compaction folding – indicate that the silica diagenesis reaction front is fossilised at a regional scale. Isochore maps demonstrate the degree of conformity between the opal-A/CT reflection and three seismic horizons of Late Miocene to Early Pliocene age that potentially represent the paleo-seabed when ‘fossilisation’ of the reaction front took place. The seismic interpretational criteria for recognition of a fossilised diagenetic front are evaluated and the results of our study are integrated with previous studies from other basins of the NE Atlantic in order to determine if the arrest of silica diagenesis was diachronous along this continental margin.     

Predicting porosity reduction due to silica diagenesis using seismic reflection data

A dramatic reduction in porosity, generally as much as 15–35%, is frequently associated with fine-grained sediment within which the transition of biogenic silica (opal-A) to cryptocrystalline opal-CT (cristobalite and tridymite) occurs. Many of these silica diagenetic boundaries imaged on seismic reflection data, in several basins worldwide, show a variety of undulating morphologies, often with a regular wavelength, where the diagenetic boundaries have variable relief, deformation of the overburden has been identified. Unusual stratal patterns are the result and these have recently been accounted for as a result of differential advancement of the boundary, differential compaction and subsidence of the overburden. The hypothesis developed is that differential advancement of the diagenetic fronts simply causes differential compaction and folding of the strata above the opal-A to opal-CT boundary.We test this hypothesis and then use knowledge of the relief of the diagenetic boundary and the folding above it, to make estimations of the magnitude of the porosity drop. We apply this forward modelling technique to examples of boundaries from the North Sakhalin Basin, Russian Far East and from the Faeroe-Shetland, Vøring and Møre Basins from the Northeast Atlantic margin. This is the first documented case of using seismic data to predict porosity reduction on the basis of seismic geometry alone and should have applicability in some siliceous successions.     

Seismic stratigraphy and structural analysis of the northern East China Sea Shelf Basin interpreted from multi-channel seismic reflection data and cross-section restoration

Analysis of multi-channel seismic data from the northern East China Sea Shelf Basin (ECSSB) reveals three sub-basins (Socotra, Domi, and Jeju basins), separated by structural highs (Hupijiao Rise) and faulted basement blocks. These sub-basins show a typical rift-basin development: faulted basement and syn-rift and post-rift sedimentation separated by unconformities. Four regional unconformities, including the top of acoustic basement, have been identified and mapped from multi-channel seismic data. Faults in the acoustic basement are generally trending NE, parallel to the regional structural trend of the area. The depths of the acoustic basement range from less than 1000 m in the northwestern part of the Domi Basin to more than 4500 m in the Socotra Basin and 5500 m in the Jeju Basin. The total sediment thicknesses range from less than 500 m to about 1500 m in the northwest where the acoustic basement is shallow and reach about more than 5500 m in the south.Interpretation of seismic reflection data and reconstruction of three depth-converted seismic profiles reveal that the northern ECSSB experienced two phases of rifting, followed by regional subsidence. The initial rifting in the Late Cretaceous was driven by the NW-SE crustal stretching of the Eurasian Plate, caused by the subduction of the Pacific Plate beneath the Eurasian Plate. Extension was the greatest during the early phase of basin formation; estimated rates of extension during the initial rifting are 2%, 6.5%, and 3.5% in the Domi, Jeju, and Socotra basins, respectively. A regional uplift terminated the rifting in the Late Eocene-Early Oligocene. Rifting and extension, although mild, resumed in the Early Oligocene; while fluvio-lacustrine deposition continued to prevail. The estimated rates of extension during the second phase of rifting are 0.7%, 0.8%, and 0.5% in the Domi, Jeju, and Socotra basins, respectively. A second phase of uplift in the Early Miocene terminated the rifting, marking the transition to the post-rift phase of regional subsidence. Regional subsidence dominated the study area between the Early Miocene and the Late Miocene. An inversion in the Late Miocene interrupted the post-rift subsidence, resulting in an extensive thrust-fold belt in the eastern part of the area. Uplift and subsequent erosion were followed by regional subsidence.     

Australian offshore natural hydrocarbon seepage studies,a review and re-evaluation

Surprisingly few natural hydrocarbon seeps have been identified in Australia's offshore basins despite studies spanning thirty years. Early studies of natural hydrocarbon seepage around the Australian margin were generally based on the geochemical analysis of stranded bitumens, water column geochemical ‘sniffer’ sampling, synthetic aperture radar or airborne laser fluorosensor. Later studies involved the integration of these remote sensing and geochemical techniques with multi-channel and shallow seismic. A review of these earlier studies indicates that many seepage interpretations need to be re-evaluated and that previous data sets, when placed in a global context, often represent normal background hydrocarbon levels. Low Recent burial and subsidence rates are not favourable for high rates of seepage. There are also difficulties in proving seepage on high energy, shallow carbonate shelves, where seabed features may be rapidly re-worked and modern marine signatures are overprinted on authigenic seep carbonates. Thus, the relatively few sites of proven natural hydrocarbon seepage in Australia's offshore sedimentary basins can be reconciled relative to their geological occurrences and the dominantly passive margin setting. Active thermogenic methane seepage on the Yampi Shelf, the only proven documented occurrence in Australia, is driven by deposition of a thick Late Tertiary carbonate succession and Late Miocene tectonic reactivation. Therefore, to increase the success of detecting and correctly interpreting natural hydrocarbon seepage, data need to be analysed and integrated within the context of the local geological setting, and with an understanding of what is observed globally.     

Evolution of the Grenada and Tobago basins and implications for arc migration

The tectonic mechanisms controlling how volcanic arcs migrate through space and geologic time within dynamic subduction environments is a fundamental tectonic process that remains poorly understood. This paper presents an integrated stratigraphic and tectonic evolution of Late Cretaceous to Recent volcanic arcs and associated basins in the southeastern Caribbean Sea using seismic reflection data, wide-angle seismic refraction data, well data, and onland geologic data. We propose a new tectonic model for the opening of the Grenada and Tobago basins and the 50-250-km eastward jump of arc volcanism from the Late Cretaceous Aves Ridge to the Miocene to Recent Lesser Antilles arc in the southeast Caribbean based on the mapping of three seismic megasequences. The striking similarity of the half-graben structure of the Grenada and Tobago basins that flank the Lesser Antilles arc, their similar smooth basement character, their similar deep-marine seismic facies, and their similar Paleogene sediment thickness mapped on a regional grid of seismic data suggest that the two basins formed as a single, saucer-shaped, oceanic crust Paleogene forearc basin adjacent to the now dormant Aves Ridge. This single forearc basin continued to extend and widen through flexural subsidence during the early to middle Eocene probably because of slow rollback of the subducting Atlantic oceanic slab. Rollback may have been accelerated by oblique collision of the southern Aves Ridge and southern Lesser Antilles arc with the South American continent. Uplift and growth of the southern Lesser Antilles arc divided the Grenada and Tobago basins by early to middle Miocene time. Inversion of normal faults and uplift effects along both edges of the Lesser Antilles arc are most pronounced in its southern zone of arc collision with the South American continent. The late Miocene to Recent depositional histories of the Grenada and Tobago basins are distinct because of isolation of the Grenada basin by growth and uplift of the Neogene Lesser Antilles volcanic ridge.     

Neogene history of Bone Gulf,Sulawesi, Indonesia

Bone Gulf is one of the inter-arm basins of the unusual K-shaped island of Sulawesi. Its age, character and origin are disputed. This study is based on recently acquired 2D seismic lines, seabed multibeam mapping and limited well data, and is linked to stratigraphy on land. The gulf is probably underlain by pre-Neogene volcanogenic, sedimentary, metamorphic and ultramafic rocks, and includes crust of Australian origin. We favour basin initiation in the Miocene rather than Eocene, by extension associated with strike-slip deformation. The main basin trends N–S and is divided into several sub-basins and highs. The highs segment the gulf and their WNW–ESE orientations reflect pre-Neogene basement structures. They are interpreted as strike-slip fault zones active at different times in the Neogene. A southern high was active relatively early, whereas further north there is evidence of young displacements during the Late Neogene. These are visible on the seabed above a high linked to the Kolaka Fault on land. Early basin-bounding faults are oriented NNW–SSE and record extension and strike-slip movements, like the sub-parallel Walanae Fault of South Sulawesi which can be traced offshore into extensional faults bounding the young and narrow Selayar Trough. Sediment in the basins came mainly from the north with contributions from both west and east. Carbonate deposits formed at the margins while deeper marine sediments were deposited in the axial parts of the gulf. An Early Pliocene unconformity can be mapped across the study area marking major uplift of Sulawesi and subsidence of Bone Gulf. This regional event caused major influx of clastic sediments from the north, development of a southward-flowing canyon system, and back-stepping and drowning of carbonates at the basin margins. Hydrocarbons are indicated by seeps, and Bone Gulf has potential sources, reservoirs and seals, but the complex faulting history is a risk.     

Evidence of episodic long-lived eruptions in the Yuma,Ginsburg, Jesús Baraza and Tasyo mud volcanoes,Gulf of Cádiz

High-resolution single channel and multichannel seismic reflection profiles and multibeam bathymetric and backscatter data collected during several cruises over the period 1999 to 2007 have enabled characterising not only the seabed morphology but also the subsurface structural elements of the Yuma, Ginsburg, Jesús Baraza and Tasyo mud volcanoes (MVs) in the Gulf of Cádiz at 1,050–1,250 m water depth. These MVs vary strongly in morphology and size. The data reveal elongated cone-shaped edifices, rimmed depressions, and scarps interpreted as flank failures developed by collapse, faulting, compaction and gravitational processes. MV architecture is characterised by both extrusive and intrusive complexes, comprising stacked edifices (including seabed cones and up to four buried bicones) underlain by chaotic vertical zones and downward-tapering cones suggesting feeder systems. These intrusive structures represent the upper layer of the feeder system linking the fluid mud sources with the constructional edifices. The overall architecture is interpreted to be the result of successive events of mud extrusion and outbuilding alternating with periods of dormancy. Each mud extrusion phase is connected with the development of an edifice, represented by a seabed cone or a buried bicone. In all four MVs, the stacked edifices and the intrusive complexes penetrate Late Miocene–Quaternary units and are rooted in the Gulf of Cádiz wedge emplaced during the late Tortonian. Major phases of mud extrusion and outbuilding took place since the Late Pliocene, even though in the Yuma and Jesús Baraza MVs mud volcanism started in the Late Miocene shortly after the emplacement of the Gulf of Cádiz wedge. This study shows that fluid venting in the eastern sector of the Gulf of Cádiz promoted the outbuilding of large long-lived mud volcanoes active since the Late Miocene, and which have been reactivated repeatedly until recent times.     

Cross-section restoration and one-dimensional basin modeling of the Central Subbasin in the southern Kunsan Basin,Yellow Sea

The petroleum system of the Kunsan Basin in the Northern South Yellow Sea Basin is not well known, compared to other continental rift basins in the Yellow Sea, despite its substantial hydrocarbon potential. Restoration of two depth-converted seismic profiles across the Central Subbasin in the southern Kunsan Basin shows that extension was interrupted by inversions in the Late Oligocene-Middle Miocene that created anticlinal structures. One-dimensional basin modeling of the IIH-1Xa well suggests that hydrocarbon expulsion in the northeastern margin of the depocenter of the Central Subbasin peaked in the Early Oligocene, predating the inversions. Hydrocarbon generation at the dummy well location in the depocenter of the subbasin began in the Late Paleocene. Most source rocks in the depocenter passed the main expulsion phase except for the shallowest source rocks. Hydrocarbons generated from the depocenter are likely to have migrated southward toward the anticlinal structure and faults away from the traps along the northern and northeastern margins of the depocenter because the basin-fill strata are dipping north. Faulting that continued during the rift phase (∼ Middle Miocene) of the subbasin probably acted as conduits for the escape of hydrocarbons. Thus, the anticlinal structure and associated faults to the south of the dummy well may trap hydrocarbons that have been charged from the shallow source rocks in the depocenter since the Middle Miocene.     

Reef exploration in the East Sengkang Basin,Sulawesi, Indonesia

Reconnaissance seismic shot in 1971/72 showed a number of well defined seismic anomalies within the East Sengkang Basin which were interpreted as buried reefs. Subsequent fieldwork revealed that Upper Miocene reefs outcropped along the southern margin of the basin. A drilling programme in 1975 and 1976 proved the presence of shallow, gas-bearing, Upper Miocene reefs in the northern part of the basin. Seismic acquisition and drilling during 1981 confirmed the economic significance of these discoveries, with four separate accumulations containing about 750 × 10⁹ cubic feet of dry gas in place at an average depth of 700 m. Kampung Baru is the largest field and contains over half the total, both reservoir quality and gas deliverability are excellent. Deposition in the East Sengkang Basin probably started during the Early Miocene. A sequence of Lower Miocene mudstones and limestones unconformably overlies acoustic basement which consists of Eocene volcanics. During the tectonically active Middle Miocene, deposition was interrupted by two periods of deformation and erosion. Carbonate deposition became established in the Late Miocene with widespread development of platform limestones throughout the East Sengkang Basin. Thick pinnacle reef complexes developed in the areas where reef growth could keep pace with the relative rise in sea level. Most reef growth ceased at the end of the Miocene and subsequent renewed clastic sedimentation covered the irregular limestone surface. Late Pliocene regression culminated in the Holocene with erosion. The Walanae fault zone, part of a major regional sinistral strike-slip system, separates the East and West Sengkang Basins. Both normal and reverse faulting are inferred from seismic data and post Late Pliocene reverse faulting is seen in outcrop.     

Compressive tectonism along the eastern margin of Malaita Island (Solomon Islands)

New bathymetric and geophysical data were collected in the region east of the island of Malaita during the SOPACMAPS II cruise of the French research vessel L'ATALANTE. This region, part of the Malaita Anticlinorium was interpreted as a piece of oceanic crust from the Ontong Java Plateau obducted over the old Solomon Islands arc during collision between the Pacific and Australian plates. It has been generally accepted that convergent motion between the Australia and Pacific plates since the Late Miocene was absorbed exclusively along the San Cristobal trench, southwest of the Solomon Islands Arc.Bathymetry, imagery, and geophysical data (magnetism, gravity, seismic) acquired during the SOPACMAPS II survey allow us to classify the successive parallel ridges mapped within the region as being recent volcanic, oceanic crust, or deformed sedimentary ridges.Seismic profiling provides evidence of successive compressive events along the Malaita margin caused by the relative motion between the Solomon Islands and the Pacific plate. The main phase of convergence probably occurred during Oligocene-early Miocene time, but some relative motion between the two domains are still being absorbed along the East Malaita boundary. The existence of active faulting in the sedimentary cover throughout the region and the present-day deformation of the outer sedimentary ridge is a good illustration of this phenomenon.     

Metalliferous Sediment and a Silica-Hematite Deposit within the Blanco Fracture Zone,Northeast Pacific

A Tiburon ROV dive within the East Blanco Depression (EBD) increased the mapped extent of a known hydrothermal field by an order of magnitude. In addition, a unique opal-CT (cristobalite-tridymite)-hematite mound was discovered, and mineralized sediments and rock were collected and analyzed. Silica-hematite mounds have not previously been found on the deep ocean floor. The light-weight rock of the porous mound consists predominantly of opal-CT and hematite filaments, rods, and strands, and averages 77.8% SiO₂ and 11.8% Fe₂O₃. The hematite and opal-CT precipitated from a low-temperature (≥ 115° C), strongly oxidized, silica- and iron-rich, sulfur-poor hydrothermal fluid; a bacterial mat provided the framework for precipitation.

Samples collected from a volcaniclastic rock outcrop consist primarily of quartz with lesser plagioclase, smectite, pyroxene, and sulfides; SiO₂ content averages 72.5%. Formation of these quartz-rich samples is best explained by cooling in an up-flow zone of silica-rich hydrothermal fluids within a low permeability system. Opal-A, opal-CT, and quartz mineralization found in different places within the EBD hydrothermal field likely reflects decreasing silica saturation and increasing temperature of the mineralizing fluid with increasing silica crystallinity.

Six push cores recovered gravel, coarse sand, and mud mineralized variously by Fe or Mn oxides, silica, and sulfides. Total rare-earth element concentrations are low for both the rock and push core samples. Ce and Eu anomalies reflect high and low temperature hydrothermal components and detrital phases.

A remarkable variety of types of mineralization occur within the EBD field, yet a consistent suite of elements is enriched (relative to basalt and unmineralized cores) in all samples analyzed: Ag, Au, S, Mo, Hg, As, Sb, Sr, and U; most samples are also enriched in Cu, Pb, Cd, and Zn. On the basis of these element enrichments, the EBD hydrothermal field might best be described as a base- and precious-metal-bearing, silica-Fe-oxide-barite deposit. Such deposits are commonly spatially and temporally associated with volcanogenic massive sulfide (VMS) ores. A plot of data for pathfinder elements shows a large hot spot at the northwestern margin of the field, which may mark a region where moderate to high temperature sulfide deposits are forming at depth; further exploration of the hydrothermal field to the northwest is warranted.     

Sedimentation settings and evolution history of the Kuril Basin (Sea of Okhotsk) in the Cenozoic

A study of the rocks from the Cenozoic sedimentary cover of the Kuril Basin slopes revealed two sedimentation stages in this area: the Late Oligocene-Early Miocene and Late Miocene-Pleistocene, which are separated by erosion in the Middle Miocene. They are characterized by dominant siliceous and terrigenous sediments, respectively. The former largely accumulated in neritic settings, while the latter were deposited in the bathyal zone under a strong influence of explosive volcanism. The change in the sedimentation regime probably occurred in the Middle-Late Miocene during the formation of the slopes of the present-day Kuril Basin. The rocks constituting crustal blocks with a granite-metamorphic layer served as a source of terrigenous material for the Cenozoic sedimentary cover, which indicates the sialic nature of the underlying basement.     

Neogene igneous intrusions in the northern South China Sea: Evidence from high-resolution three dimensional seismic data

Igneous intrusions emplaced within the prospective intervals of sedimentary basins can exert a significant impact on petroleum systems and hence are of considerable interest particularly when risking exploration plays. A number of discordant high amplitude seismic anomalies (DSAs) with a range of geometries are documented in high resolution 3D seismic data in the northern South China Sea. Their distribution and seismic characteristics are analysed and compared with similar seismic anomalies documented within sedimentary basins in the Atlantic margins and other locations. The DSAs occur mainly within Early Miocene strata and are interpreted as igneous intrusions that were emplaced close to the palaeo-seabed and are dated as early Middle Miocene using seismic-stratigraphic methods. A number of vents are also identified above the intrusions within early Middle Miocene strata. Several geometrical forms are observed, referred to here as saucer-shaped, lensoid-shaped, stacked and composite intrusions. The seismic expression of these is increasing complex towards the palaeo-seabed, with a tendency for the saucer-shaped sills to be the deepest intrusive forms. The igneous intrusions observed in this study only could be identified using 3D seismic data and they are important for the future evaluations of petroleum systems, basin evolution and tectonic analysis in the Pearl River Mouth Basin.     

东海新生代沉积盆地的类型和成盆期

东海新生代沉积厚度最大可达10km。分为三个发展时期。第一阶段从晚白垩世至中始新世,由于中国大陆向东濡散和掀斜断块作用,在大陆边缘由陆缘裂谷盆地转化为浅海沉积盆地。第二阶段从晚始新世至中中新世,由于喜马拉雅陆缘造山带的形成和中国大陆边缘的隆升联合作用结果,在大陆边缘由环绕大陆分布的带状地堑转化为前陆盆地。第三阶段从晚中新世至第四纪,由于太平洋板块向西俯冲,形成弧后断陷及弧前坳陷。从横向上看,不同性质和时代的沉积,由西向东,由老到新,依次排列。从盆地性质上看,由老到新,张性盆地和压性盆地交替形成,叠置在一起。因此不同时代和性质的盆地,具有不同的石油地质条件和油气成藏规律。     

Constraining the origin and evolution of confined turbidite systems: southern Cretan margin,Eastern Mediterranean Sea (34°30–36°N)

Bathymetric, 9.5-kHz long-range sidescan sonar (OKEAN), seismic reflection and sediment-core data are used in the analysis of two tectonic troughs south of Crete, Eastern Mediterranean Sea. Here, up to 1.2 s two-way travel time (TWTT) of strata have accumulated since the Middle Miocene in association with extension in the South Aegean region. The study area comprises >100-km- long by >25-km-wide basins filled by sediments subdivided into two seismic units: (1) an upper Unit 1 deposited in sub-basins which follow the present-day configuration of the southern Cretan margin; (2) a basal Unit 2, more than 500 ms (TWTT) thick, accumulated in deeper half-graben/grabens distinct from the present-day depocentres. Both units overlap a locally stratified Unit 3 comprising the pre-Neogene core complex of Crete and Gavdos. In this work, the interpreted seismic units are correlated with the onshore stratigraphy, demonstrating that denudation processes occurring on Crete and Gavdos in response to major tectonic events have been responsible for high sedimentation rates along the proximal southern Cretan margin. Consequently, topographically confined sedimentary units have been deposited south of Crete in the last 12 Ma, including turbidites and other mass-flow deposits fed by evolving transverse and axial channel systems. Surface processes controlling facies distribution include the direct inflow of sediment from alluvial-fan systems and incising mountain rivers onto the Cretan slope, where significant sediment instability processes occur at present. In this setting, seismic profiles reveal eight different types of stratigraphic contacts on basin-margin highs, and basinal areas show evidence of halokinesis and/or fluid escape. The acquired data also show that significant changes to the margin’s configuration occurred in association with the post-Alpine tectonic and eustatic episodes affecting the Eastern Mediterranean.     

Miocene-to-recent structure and basinal architecture along the Central Range strike-slip fault zone, eastern offshore Trinidad

Previous GPS-based geodetic studies and onland paleoseismologic studies in Trinidad have shown that the 50-km-long, linear, onland segment of the Central Range fault zone (CRFZ) accommodates at least 60% of the total rate of right-lateral displacement (∼20 mm/yr) between the Caribbean and South American plates. 2D and 3D seismic reflection data from a 60-km-long and 30-km-wide swath of the eastern shelf of Trinidad (block 2AB) were used to map the eastern offshore extension of this potentially seismogenic and hazardous fault system and to document its deeper structure and tectonic controls on middle Miocene to recent clastic stratigraphy. Two unconformity surfaces and seafloor were mapped using 3D seismic data to generate isochron maps and to illustrate the close control of the CRFZ and associated secondary faults on small, clastic basins formed along its anastomosing strands and the east-west-striking North Darien Ridge fault zone (NDRFZ) that exhibits a down-to-the-north normal throw. Mapped surfaces include: 1) the middle Miocene angular unconformity, a prominent, regional unconformity surface separating underlying thrust-deformed rocks from a much less deformed overlying section; this regional unconformity is well studied from onland outcrops in Trinidad and in other offshore areas around Trinidad; 2) a Late Neogene angular unconformity developed locally within block 2AB that is not recognized in Trinidad; and 3) the seafloor of the eastern Trinidad shelf which exhibits linear scarps for both the CRFZ and the east-west-striking North Darien Ridge fault zone. Clastic sedimentary fill patterns identified on these isochron maps indicate a combined effect of strike-slip and reverse faulting (i.e., tectonic transpression) produced by active right-lateral shear on the CRFZ, which is consistent with the obliquity of the strike of the fault to the interplate slip vector known from GPS studies in onland Trinidad. The NDRFZ and a sub-parallel and linear family of east-west-striking faults with normal and possibly transtensional motions also contributed to the creation of accommodation space within localized, post-middle Miocene clastic depocenters south of the CRFZ.     

Structural evolution of the northern East China Sea Shelf Basin interpreted from cross-section restoration

The northern East China Sea Shelf Basin consists of three depressions (the Domi, Jeju, and Socotra Depressions), separated by basement highs or rises. Reconstruction of depth-converted seismic reflection profiles from these depressions reveals that the northern East China Sea Shelf Basin experienced two phases of rifting, followed by regional subsidence. Initial rifting in the Late Cretaceous was driven by the NW?CSE crustal stretching of the Eurasian plate, caused by the subduction of the Pacific plate beneath the plate margin. Major extension (~15 km) took place during the early phase of basin formation. The initial rifting was terminated by regional uplift in the Late Eocene-Early Oligocene, which was probably due to reorganization of plate boundaries. Rifting resumed in the Early Oligocene; the magnitude of extension was mild (<1 km) during this period. A second phase of uplift in the Early Miocene terminated the rifting, marking the transition to the postrift phase of regional subsidence. Up to 2,600 m of sediments and basement rock were removed by erosion during and after the second phase of uplift. An inversion in the Late Miocene interrupted the postrift subsidence, resulting in an extensive thrust-fold belt in the eastern part of the area. Subsequent erosion removed about 900 m of sediments. The regional subsidence has dominated the area since the Late Miocene.     

Basin modeling of the Late Miocene Zeit source rock in the Sudanese portion of Red Sea Basin: Implication for hydrocarbon generation and expulsion history

The Late Miocene Zeit Formation is exposed in the Red Sea Basin of Sudan and represents an important oil-source rock. In this study, five (5) exploratory wells along Red Sea Basin of Sudan are used to model the petroleum generation and expulsion history of the Zeit Formation. Burial/thermal models illustrate that the Red Sea is an extensional rift basin and initially developed during the Late Eocene to Oligocene. Heat flow models show that the present-day heat flow values in the area are between 60 and 109 mW/m². The variation in values of the heat flow can be linked to the raise in the geothermal gradient from margins of the basin towards offshore basin. The offshore basin is an axial area with thick burial depth, which is the principal heat flow source.The paleo-heat flow values of the basin are approximately from 95 to 260 mW/m², increased from Oligocene to Early Pliocene and then decreased exponentially prior to Late Pliocene. This high paleo-heat flow had a considerable effect on the source rock maturation and cooking of the organic matter. The maturity history models indicate that the Zeit Formation source rock passed the late oil-window and converted the oil generated to gas during the Late Miocene.The basin models also indicate that the petroleum was expelled from the Zeit source rock during the Late Miocene (>7 Ma) and it continues to present-day, with transformation ratio of more than 50%. Therefore, the Zeit Formation acts as an effective source rock where significant amounts of petroleum are expected to be generated in the Red Sea Basin.     

Ocean basins near the Scotia–Antarctic plate boundary: Influence of tectonics and paleoceanography on the Cenozoic deposits

The distribution of seismic units in deposits of the basins near the Antarctic–Scotia plate boundary is described based on the analysis of multichannel seismic reflection profiles. Five main seismic units are identified. The units are bounded by high-amplitude continuous reflectors, named a to d from top to bottom. The two older units are of different age and seismic facies in each basin and were generally deposited during active rifting and seafloor spreading. The three youngest units (3 to 1) exhibit, in contrast, rather similar seismic facies and can be correlated at a regional scale. The deposits are types of contourite drift that resulted from the interplay between the northeastward flow of Weddell Sea Bottom Water (WSBW) and the complex bathymetry in the northern Weddell Sea, and from the influence of the Antarctic Circumpolar Current and the WSBW in the Scotia Sea. A major paleoceanographic event was recorded by Reflector c, during the Middle Miocene, which represents the connection between the Scotia Sea and the Weddell Sea after the opening of Jane Basin. Unit 3 (tentatively dated ∼Middle to Late Miocene) shows the initial incursions of the WSBW into the Scotia Sea, which influenced a northward progradational pattern, in contrast to the underlying deposits. The age attributed to Reflector b is coincident with the end of spreading at the West Scotia Ridge (∼6.4 Ma). Unit 2 (dated ∼Late Miocene to Early Pliocene) includes abundant high-energy, sheeted deposits in the northern Weddell Sea, which may reflect a higher production of WSBW as a result of the advance of the West Antarctic ice-sheet onto the continental shelf. Reflector a represents the last major regional paleoceanographic change. The timing of this event (∼3.5–3.8 Ma) coincides with the end of spreading at the Phoenix–Antarctic Ridge, but may be also correlated with global events such as initiation of the permanent Northern Hemisphere ice-sheet and a major sea level drop. Unit 1 (dated ∼Late Pliocene to Recent) is characterized by abundant chaotic, high-energy sheeted deposits, in addition to a variety of contourites, which suggest intensified deep-water production. Units 1 and 2 show, in addition, a cyclic pattern, more abundant wavy deposits and the development of internal unconformities, all of which attest to alternating periods of increased bottom current energy.     

Evolution of Late Oligocene - Early Miocene attached and isolated carbonate platforms in a volcanic ridge context (Maldives type), Yadana field,offshore Myanmar

This study investigates the stratigraphic evolution of the Late Oligocene - Early Miocene carbonate platforms of the Yadana area (offshore Myanmar). Well data, regional 2D and local 3D seismic surveys allow the identification of three shallow-water carbonate platforms (Yadana, 3DF and 3DE) showing various morphologic and stratigraphic patterns influenced by the presence of a paleohigh. The identification of seven seismic sequences in the Yadana area constrains the stratigraphic evolution in three stages: (1) development of aggrading attached and isolated platforms during the Chattian; (2) a period of platform emersion during the Oligocene - Miocene transition; (3) drowning of the smaller buildup (3DE) associated with km-scale backstepping on the large platforms (3DF and Yadana) during the Aquitanian. The Aquitanian marks the onset of renewed volcanic activity associated with the development of fringing carbonate reefs during the Burdigalian. The rapid (∼6 My) development of these wide (∼5–70 km) and thick (∼300–850 m) carbonate platforms has been mainly controlled by the subsidence. However, the results highlight a strong overprint of eustatic fluctuations on the rates of change in accommodation, and hence on the stratigraphic architecture of the carbonate platforms. Based on an alternative model for the Cenozoic geodynamic evolution of the Yadana area, our results suggest that the platforms developed on a volcanic ridge of hotspot origin located in the Indian Ocean and not on a volcanic arc. Subduction jump processes are interpreted to have played a key role in the demise of all platforms by drastically changing the paleoenvironmental conditions during the Early Miocene, and led to the present-day location of the Yadana Ridge in a back-arc setting. The carbonate platforms from the Yadana area are thus a representative example of the interplay between global mechanisms and local paleoenvironmental parameters on carbonate platform initiation, growth and demise.