Seismic Stratigraphic Analysis Research Articles

Seismic stratigraphy of the Heuksan mud belt in the southeastern Yellow Sea, Korea

To establish the seismic stratigraphy of the Heuksan mud belt (HMB) and reconstruct its depositional history, approximately 1,600 km of high-resolution seismic data were newly obtained using chirp acoustic sub-bottom profiler, sparker, and air-gun seismic systems. Based on seismic stratigraphic analysis, the HMB can be divided into three major seismic units (I, II, and III, from top to bottom) and four subunits (II-a, II-b, III-a, and III-b) overlying transgressive sands, pre-last glacial maximum (LGM) deposits, and the acoustic basement. Each unit and subunit show different seismic facies and geometry, being clearly separated from each other by bounding surfaces formed since the LGM. The spatial distribution, thicknesses and volumes of the seismic units were determined and plotted to document the sequential formation of the HMB. The correlation between deep drill core data (HMB-101, HMB-102, HMB-103, YSDP-101, and YSDP-102) and the seismic data suggests that subunits III-b and III-a were formed by the continuous accumulation of fine-grained sediment with partial sandy sediment in an estuarine/deltaic environment during the early to middle transgressive stage, accompanied by landward migration of the shoreline. Subunits II-b and II-a were probably formed by re-deposition of large volumes of sediment eroded from unit III during the middle transgressive to early highstand stage. Unit I is interpreted as the most recent mud deposit representing the highstand systems tract when sea-level rise terminated. The careful definition of seismic units and their interpretation proposed in this study, on the basis of the large and partly new seismic dataset covering the entire HMB together with deep drill core data, have been instrumental in reconstructing the depositional environment and formation mechanisms of the HMB.

Análise sismoestratigráfica da seção rifte da Bacia de Santos, Brasil

A Bacia de Santos, apesar de uma das principais bacias atualmente no Brasil exploradas devido à importância do pré-sal, possui pouca informação geológica, como registros de poço, sísmica 3D e sísmica 2D de boa qualidade na seção rifte. O presente trabalho visa à integração da análise sismoestratigráfica e proposição de um modelo de evolução tectonoestratigráfica para a seção rifte da bacia. A análise sismoestratigráfica envolveu a interpretação dos refletores, que é a base da identificação das unidades sismoestratigráficas e a caracterização das sismofácies. Como resultado, foi obtida a definição de 16 unidades sismoestratigráficas e quatro sismofácies (incluindo o sag), e o desenvolvimento das cartas cronoestratigráficas de eventos adaptadas para o contexto sísmico. Com base no ajuste do modelo de padrões de empilhamento para mudanças na atividade tectônica, foram delimitados os tratos de sistemas tectônicos de início de rifte, de alta atividade tectônica, de baixa atividade tectônica e o pós-rifte, representado pelo sag. O desenvolvimento de cada um dos tratos respondeu a variações dos fatores controladores das bacias lacustres, interpretado como o balanço relativo entre as taxas de mudanças do espaço de acomodação gerado pela tectônica e o aporte sedimentar influenciado pelo clima.

Late Mesozoic magmatism and Cenozoic tectonic deformations of the Barents Sea continental margin: Effect on hydrocarbon potential distribution

The paper is focused on the two tectonic-geodynamic factors that made the most appreciable contribution to the transformation of the lithospheric and hydrocarbon potential distribution at the Barents Sea continental margin: Jurassic-Cretaceous basaltic magmatism and the Cenozoic tectonic deformations. The manifestations of Jurassic-Cretaceous basaltic magmatism in the sedimentary cover of the Barents Sea continental margin have been recorded using geological and geophysical techniques. Anomalous seismic units related to basaltic sills hosted in terrigenous sequences are traced in plan view as a tongue from Franz Josef Land Archipelago far to the south along the East Barents Trough System close to its depocentral zone with the transformed thinned Earth’s crust. The Barents Sea igneous province has been contoured. The results of seismic stratigraphy analysis and timing of basaltic rock occurrences indicate with a high probability that the local structures of the hydrocarbon (HC) fields and the Stockman-Lunin Saddle proper were formed and grew almost synchronously with intrusive magmatic activity. The second, no less significant multitectonic stress factor is largely related to the Cenozoic stage of evolution, when the development of oceanic basins was inseparably linked with the Barents Sea margin. The petrophysical properties of rocks from the insular and continental peripheries of the Barents Sea shelf are substantially distinct as evidence for intensification of tectonic processes in the northwestern margin segment. These distinctions are directly reflected in HC potential distribution.

Review of the chronostratigraphic charts in the Sinú-San Jacinto basin based on new seismic stratigraphic interpretations

Disperse and punctual studies; absence of integration of data ranging from local to regional focus; interpretations based only on lithostratigraphic features; and interpretation of data premised on an allochthonous origin of the Caribbean plate, are some of factors that increase the confusion and uncertainty in understanding the Sinú-San Jacinto Basin. The sedimentary record of Upper Cretaceous to Eocene has been traditionally interpreted as the record of deep-water settings. However, recently these sediments have been related to shallow marine and deltaic settings. Second problematic point is about the deposition environment of the Oligocene to Late Miocene succession. Some studies suggest canyons, turbidites and sediments deposited in deep-water settings. However, recent studies propose deltaic and shallow marine settings. The last stratigraphic problem is related to the controversial fluvial vs. shallow marine interpretations of the Pliocene sediments. Based upon seismic stratigraphic analysis in recent and reprocessed 2D seismic data, integrated with well data, we propose chronostratigraphic charts for the northern, central and southern zones of the Sinú-San Jacinto Basin. Twenty seismic facies based on amplitude, continuity, frequency and geometry of seismic reflectors and twelve seismic sequences were recognized. The seismic stratigraphic analysis in this study suggests that the sediments of Upper Cretaceous to Paleocene/Eocene were associated to continental to shallow marine settings. Lagoons, coastal plain and carbonate platform dominated during this period. The Oligocene to Middle Miocene record was characterized by deep-water deposition, whereas the Late Miocene to recent sedimentation was characterized by falling base level, characterized by deltaic and fluvial deposits. Five syn-rift sequences with wedge-shaped geometry were identified in this study. Three Triassic to Jurassic syn-rift sequences were characterized by seismic facies typical of fluvial to lacustrine and flood plain sedimentation. Two Cretaceous to Paleocene syn-rift sequences were characterized by seismic facies related to lagoons to coastal plain settings. Normal high-angle faults with a northeast-southwest direction related to rifting processes controlled the development of these sequences. The sheet-drape post-rift section was characterized by passive margin settings in the northern part of the Sinú-San Jacinto Basin and by diachronic tectonic inversion of older normal faults during Cenozoic, predominantly in the central and southern zones. The stratigraphic record related to the Mesozoic to Early Cenozoic rifting; the shallow marine sedimentation during Eocene and the tectono-stratigraphic continuity across the northern Colombia and northwestern Venezuela is coherent and well explained by the in situ origin of the Caribbean plate and is not explained by the “allochthonous” model.

Seismic stratigraphy of Lake Van, eastern Turkey

More than 1500 km of multi-channel seismic reflection profiles combined with ICDP (International Continental Scientific Drilling Program) drilling data, provide important insights into the stratigraphic evolution of Lake Van, eastern Turkey. Three major basins (Tatvan, Northern and the Deveboynu basins) comprise the main lake basin and are separated by morphological highs (Ahlat ridge and Northern ridge). Moreover, NE–SW faults, parallel to the general tectonic lineament of the area, dominate the entire basin and are in charge of creating graben and half-graben structures. Well-developed prograding deltaic sequences on top of the basement were recognized by seismic stratigraphy analysis. Most likely, they formed during the initial flooding of Lake Van ∼600 ka. The Tatvan basin sediments are dominated by mass-flow deposits of various origins alternating with undisturbed lacustrine sediments including distinct tephra layers. Faulting along the Tatvan basin margins may have triggered margin-wide slope failures. Ahlat ridge started to form between ca 340 ka–290 ka. Since then, Ahlat ridge was sheltered from major mass-flows due to its elevation. Hence, slow lacustrine sedimentation has prevailed throughout lake history on Ahlat ridge, which was the location of the main drill site during the ICDP. Several lake level fluctuations are evident on the eastern slope area but the deep basins were permanently covered by water. A significant lake-level low stand (ca 600 ka BP) was found at ∼610 m below present lake level. The setting of the lake changed at about 30 ka. Tectonic activity appears to have waned significantly as the mass-transport deposition decreased across the Tatvan basin while normal undisturbed lacustrine sedimentation prevailed. A different setting is found in the Northern basin from ca 90 ka to Present, especially due to the strong influx of mostly volcaniclastic turbidites causing sedimentation rates to be about 3.5 times higher (drill Site 1), than at Site 2 (Ahlat ridge).

Seismic stratigraphy of the Lower Cretaceous Valhall Formation (Danish Graben, North Sea)

Detailed seismic stratigraphic analysis of a 3D seismic survey covering the southern part of the Danish Central Graben in the North Sea has revealed a complex stratigraphic basin infill pattern controlled by eustacy and local tectonics in the Lower Cretaceous Valhall Formation. The understanding of the stratigraphic architecture of this mudstone formation is critical for the development of the overlaying chalk reservoirs of the Tuxen and Sola Formations. The applied workflow included an application of flattening in order to approximate the Lower Cretaceous basin morphology, the attenuation of multiples generated by the overlying chalk deposits, a systematic use of both 2D transects and horizon slices in the analysis of the depositional geometries and the application of a Relative Geological Time (RGT) model interpretation technology. The results show an asymmetrical infill of the Lower Cretaceous consisting of westward prograding, mud-dominated clinoforms. The overall progradational pattern is clearly expressed in the most southern part of the basin by coast-parallel features interpreted as shelf margin breaks. The asymmetrical paleotopography, created by the muddy shelf of the Valhall Formation, formed subsequently the substratum for the deposition of the chalks of the Tuxen and Sola Formations, and controlled their facies distribution.

Multi-proxy analysis in defining sedimentary processes in very recent prodelta deposits: the Northern Phlegraean offshore example (Eastern Tyrrhenian Margin)

A multi-proxy analysis of Volturno River prodelta deposits in the outer shelf of Northern Phlegraean margin (Eastern Tyrrhenian Sea) has been carried out to reconstruct the sedimentary processes acting during recent times. The late Holocene sediments were characterized through Subbottom Chirp profiles coupled with sedimentological and petrophysical data. The chronostratigraphic framework was achieved by means of colorimetric parameter a* correlations with nearby dated marine sediment. A time interpretation of about 2,300 years BP is estimated for the sedimentary record collected in the cores. Seismic stratigraphic analysis shows late Holocene outer shelf deposits, characterized by fluid escape features and small-offset faults. However, the undisturbed sedimentation and the preservation of an internal geometry at decimetre scale, as detected by the sedimentological and petrophysical analysis, indicates a slow sliding without sediment reworking for this sedimentological body. So far a possible recent (<2,300 years BP) shear dominated downward displacement of high water content sediments, triggered by the occurrence of seismic activity, is inferred. Based on the depth-age conversion of the detected lithological features, a regular climatically driven change in the sediment supply of the prodelta depositional environment is suggested. The detection of spectrophotometry correlations of Holocene shelf margin sediments, several km apart, goes beyond the previous work and confirms even for continental shelf area the potential value of spectrophotometer data in high-resolution stratigraphic correlations.

Seismic geomorphology and sequence stratigraphy as tools for the prediction of reservoir facies distribution: an example from the Paleocene and earliest Eocene of the South Buchan Graben, Outer Moray Firth Basin, UKCS

Abstract A seismic stratigraphic analysis constrained by well and wireline log data has been undertaken on the Paleocene and earliest Eocene succession of the South Buchan Graben (Quadrants 20 and 21), Outer Moray Firth Basin (OMFB). Two principal sequences have been described relating to two regressive/transgressive second-order cycles of relative sea-level change. The Maureen, Andrew, Glamis Tuff and Balmoral sandstone members are expressed as a stacked set of lowstand basin floor fans separated by mudstone intervals representing four cycles of third-order relative sea-level change. The Sele and Balder formations contain both basinal and shelfal packages as an expression of two cycles of third-order relative sea-level change. The Forties Sandstone Member is deposited within highly mounded, levee-confined channels downlapped by a prograding slope succession with well-defined clinoforms and deltaic topsets attributed to the Dornoch and Beauly formations. The individual parasequences of the prograding wedge are related to higher-order eustatic fluctuations with incision and slope fans, attributed to the Cromarty Sandstone Member, deposited during periods of relative sea-level lowstand. It is demonstrated that through the integration of lithostratigraphic, seismic geomorphological and sequence stratigraphic analyses an understanding of depositional environments and the distribution of facies within them can be obtained. The identification of basinal and slope features with reservoir potential, along with an understanding of their chronostratigraphic relationship to sealing facies, play an important role in regional play fairway mapping and risk analysis in this area and beyond. Future prospectivity within mature basins, such as the OMFB, relies on subtle stratigraphic traps typical of lowstand systems tracts, where the main risk is associated with reservoir quality and containment.

Seismic stratigraphic analysis of the Cenozoic sediments in the NW Faroe Shetland Basin – Implications for inherited structural control of sediment distribution

The post-basalt strata in the Faroese area have been investigated based on interpretation of 2D and 3D reflection seismic data. The post-basalt package is divided into 5 units which have led to the constructions of 6 structural maps and 5 thickness maps. Within the 5 units 12 prograding sediment bodies have been identified. Based on the interpretation it is possible to obtain an overview during time of the location of depocentres and direction of prograding units. Within Eocene time the depocentre was placed in the central part of the basin and the sediment influx was mostly from south and southwest. During Oligocene–Pliocene time the sediment influx was from north and northwest and the depocentre had moved in a westward direction closer to the Faroe Platform area.Emplacement of the Cenozoic sediments in the Faroese sector of the Faroe-Shetland Basin is controlled by thermal subsidence of the basin, and local uplift of sediment source areas. Reactivation of older, Paleozoic and Mesozoic, structural elements seem to control the sediment path way and restrict the depositional areas. Various structural elements being re-activated at different times caused considerable structural complexity. Understanding the older, structural elements and their control on sedimentation is a potential tool for understanding deviations from “normal” thermal subsidence and for predicting the prospectivity in the post-basalt succession in the Faroe-Shetland Basin.

Autochthonous versus allochthonous Upper Triassic evaporites in the Sbiba graben, central Tunisia

Two-dimensional seismic data and well data from the Sbiba graben in central Tunisia (North Africa) reveal its asymmetrical structural geometry affected by E–W, N–S, NW–SE, and NE–SW master strike-slip faults. These faults have been intruded by Upper Triassic evaporites, which can be classified into the following three types: (1) sealed evaporite anticlines and domes along folded structures; (2) extrusional wrinkles of “salt walls” at the borders of the graben related to along-strike displacement of the master fault zone; and (3) evaporite outpouring at the center of the graben interpreted as the result of fault and diapir movement. Detailed seismic-stratigraphic analysis highlights a complex interplay between fault growth and evaporite movement, which strongly controlled the evolution of the graben. During the Mesozoic, tectonic stresses induced multidirectional faulting along inherited faults. The vertical rising of the Upper Triassic evaporites was initiated locally at the intersection of master faults during regional extensional and transtensional events in the sub-evaporite basement. The interaction of normal faulting and evaporite diapirism caused lateral outpouring of surface-piercing evaporites. This phenomenon is well expressed in Upper Cretaceous (upper Cenomanian to lower Turonian) strata. The master faults, which cut the diapir at the border and the crest, have caused the extrusion of evaporites. The downward, gravity-driven flow of these evaporites across the slope surface and subsequent accumulation and preservation in Mesozoic sediments is related to the fault escarpment and the adjacent fault-induced depression. There is an apparent relation between tectonic inheritance and inversions of the graben, where interconnected deep faults and diapirs have been reactivated during extensional and contractional tectonic episodes, causing evolution of the structures with the associated sedimentary lapouts. Several autochthonous Triassic evaporites and associated fault escarpments have induced salt canopy structures.

The sedimentary, magmatic and tectonic evolution of the southwestern South China Sea revealed by seismic stratigraphic analysis

The southwestern South China Sea represents an area of continental crust frozen immediately before the onset of seafloor spreading. Here we compile a grid of multichannel seismic reflection data to characterize the continent-ocean transition just prior to full break-up. We identify a major continental block separated from the shelf margin by a basin of hyperextended crust. Oligocene-Early Miocene rifting was followed by mild compression and inversion prior to 16 Ma, probably linked to collision between the Dangerous Grounds, a continental block to the east of the study area, and Borneo. The timing of inversion supports models of seafloor spreading continuing until around 16 Ma, rather than becoming inactive at 20 Ma. The off-shelf banks experienced uplift prior to 16 Ma in an area, which had previously been a depocenter. The off-shelf banks continued to extend after this time when the rest of the region is in a phase of thermal subsidence. Post-rift magmatism is seen in the form of scattered seamounts (~5–10 km across) within or on the edge of the deeper basins, and are dated as Late Miocene and Pliocene. They are not clearly linked to any phase of tectonic activity. Further inversion of the off-shelf banks occurred in the Pliocene resulting in a major unconformity despite the lack of brittle faulting of that age. We speculate that this is part of a wider pattern of scattered magmatism throughout the South China Sea at this time. Prograding clinoforms are seen to build out from the shelf edge in the south of the study area during the Pliocene, after 5.3 Ma, and then more towards the north and east during the Pleistocene. At the same time a trough south of the off-shelf banks is filled with >1.35 km of mostly Pleistocene sediment. While we expect the bulk of the sediment to come from the Mekong River, we also suggest additional sediment supply from Borneo and the Malay Peninsula via the Molengraaff River and its predecessors.

Structural style and early stage growth of inversion structures: 3D seismic insights from the Egersund Basin, offshore Norway

High-quality three-dimensional (3D) seismic reflection and borehole data from the Egersund Basin, offshore Norway are used to characterise the structural style and determine the timing of growth of inversion-related anticlines adjacent to a segmented normal fault system. Two thick-skinned normal faults, which offset Permian clastics and evaporites, delineate the north-eastern margin of the basin. These faults strike NNW-SSE, have up to 1900 m of displacement and are separated by an ESE-dipping, c. 10 km wide relay ramp. Both of these faults display exclusively normal separation at all structural levels and tip out upwards into the upper part of the Lower Cretaceous succession. At relatively shallow structural levels in the hangingwalls of these faults, a series of open, low-amplitude, fault-parallel anticlines are developed. These anticlines, which are asymmetric and verge towards the footwalls of the adjacent faults, are interpreted to have formed in response to mild inversion of the Egersund Basin. The amplitude of and apparent shortening associated with the anticlines vary along strike, and these variations mimic the along-strike variations in throw observed on the adjacent fault segments. We suggest that this relationship can be explained by along-strike changes in the propensity of the normal faults to reactivate during shortening; wider damage zones and lower angles of internal friction, coupled with higher pore fluids pressures at the fault centre, mean that reactivation is easier at this location than at the fault tips or in the undeformed country rock. Seismic-stratigraphic analysis of growth strata indicate that the folds initiated in the latest Turonian-to-earliest Coniacian (c. 88.6 Ma) and Santonian (c. 82.6 Ma); the control on this c. 6 Myr diachroneity in the initiation of fold growth is not clear, but it may be related to strain partitioning during the early stages of shortening. Anticline growth ceased in the Maastrichtian and the inversion event is therefore interpreted to have lasted at least c. 20 Myr. This study indicates that 3D seismic reflection data is a key tool to investigate the role that normal fault segmentation can play in controlling the structural style and timing of inversion in sedimentary basins. Furthermore, our results highlight the impact that this structural style variability may have on the development of structural and stratigraphic hydrocarbon traps in weakly-inverted rifts.

Characterization of a deeply buried paleokarst terrain in the Loppa High using core data and multiattribute seismic facies classification

The recognition of paleokarst in subsurface carbonate reservoirs is not straightforward because conventional seismic interpretation alone is generally not sufficient to discriminate karstified areas from their surroundings. In the Loppa High (Norwegian Barents Sea), a protracted episode of subaerial exposure occurring between the late Paleozoic and mid-Triassic—Late Permian to Anisian—resulted in a significant overprinting of the previously deposited carbonate units. Here, we map the extension of the karstified areas using an integrated approach consisting of (1) a core study of critical paleokarst intervals, (2) a three-dimensional (3-D) seismic stratigraphic analysis, and (3) a 3-D multiattribute seismic facies (SF) classification. A core retrieved in the flat-topped Loppa High revealed breccia deposits at least 50 m (164 ft) thick, which probably resulted from cave collapses following the burial of the karst terrain. The SF classification was tested on a 3-D cube to (1) discriminate the respective SF related to the breccia deposits compared with other SF and (2) to estimate their spatial extent. Seismic-facies analysis suggests that breccias occupied the topmost area of the structural high, extending up to 12 km (7 mi) in width, 46 km (29 mi) in length, and tens of meters in thickness. The inference of such a large amount of breccia suggests that a significant part of this terrain was derived from the amalgamation of successive cave-development events—including periods of subaerial exposure and subsequent burial and collapse—resulting in a coalesced collapsed paleocave system. Previous observations from the Loppa High revealed the presence of karst plains associated with sinkholes, caves, and other dissolution phenomena associated with the breccia facies, further suggesting that a large volume of carbonate rocks in this area was affected by subaerial exposure and karstification. Our integrated approach and proposed karstification model could be applied to similar sedimentary basins that accommodate deeply buried carbonate successions affected by protracted episodes of subaerial exposure, where only few wells as well as 3-D seismic data are available.

Structural and stratigraphic evolution of the Connemara discovery, Northern Porcupine Basin: significance for basin development and petroleum prospectivity along the Irish Atlantic Margin

ABSTRACT A detailed seismic stratigraphic interpretation of a previously unpublished, well-calibrated 3D seismic volume and regional 2D seismic lines in the Northern Porcupine Basin, west of Ireland has provided significant insights into the basin development, sedimentary fill and petroleum prospectivity within this area of the Irish Atlantic continental margin. The results of the seismic interpretation presented here provide a detailed description of the evolution of the Connemara discovery and have resulted in a revised model for the tectonic development and stratigraphic evolution of the fault block structure containing the oil accumulation. The new seismic stratigraphic analysis also provides the basis for an alternative interpretation of Early Cretaceous ‘clinoform’ features based on the results of the 3D seismic interpretation which supports a genesis through structural rotation of onlapping horizons rather than depositional downlap favoured by previous interpretations. This new-found structural and stratigraphic understanding not only has significant implications for determining the main controls on the Connemara discovery, but also provides a basis upon which to place other prospective structures in their regional context, including the identification and risking of exploration plays and prospects in the area.

Application of seismic and sequence stratigraphic concepts to a lava‐fed delta system in the Faroe‐Shetland Basin, UK and Faroes

ABSTRACTDetailed seismic stratigraphic analysis of 2D seismic data over the Faroe‐Shetland Escarpment has identified 13 seismic reflection units that record lava‐fed delta deposition during discrete periods of volcanism. Deposition was dominated by progradation, during which the time shoreline migrated a maximum distance of ∼44 km in an ESE direction. Localised collapse of the delta front followed the end of progradation, as a decrease in volcanic activity left the delta unstable. Comparison with modern lava‐fed delta systems on Hawaii suggests that syn‐volcanic subsidence is a potential mechanism for apparent relative sea level rise and creation of new accommodation space during lava‐fed delta deposition. After the main phase of progradation, retrogradation of the delta occurred during a basinwide syn‐volcanic relative sea level rise where the shoreline migrated a maximum distance of ∼75 km in a NNW direction. This rise in relative sea level was of the order of 175–200 m, and was followed by the progradation of smaller, perched lava‐fed deltas into the newly created accommodation space. Active delta deposition and the emplacement of lava flows feeding the delta front lasted ∼2600 years, although the total duration of the lava‐fed delta system, including pauses between eruptions, may have been much longer.

Evidence for current-controlled sedimentation along the southern Mozambique continental margin since Early Miocene times

Major plastered drift sequences were imaged using high-resolution multichannel seismics during R/V Meteor cruises M63/1 and M75/3 south of the Mozambique Channel along the continental margin of Mozambique off the Limpopo River. Detailed seismic-stratigraphic analyses enabled the reconstruction of the onset and development of the modern, discontinuous, eddy-dominated Mozambique Current. Major drift sequences can first be identified during the Early Miocene. Consistent with earlier findings, a progressive northward shift of the depocenter indicates that, on a geological timescale, a steady but variable Mozambique Current existed from this time onward. It can furthermore be shown that, during the Early/Middle Miocene, a coast-parallel current was established off the Limpopo River as part of a lee eddy system driven by the Mozambique Current. Modern sedimentation is controlled by the interplay between slope morphology and the lee eddy system, resulting in upwelling of Antarctic Intermediate Water. Drift accumulations at larger depths are related to the reworking of sediment by deep-reaching eddies that migrate southward, forming the Mozambique Current and eventually merging with the Agulhas Current.

Tectonic evolution and sequence stratigraphy of the Central Pacific margin of Costa Rica

On the Central Pacific of Costa Rica, more than 1600 km of seismic reflection data, and potential field data, located offshore and onshore, joined to the surface geology data have been analyzed in order to clarify the structural configuration, tectonic evolution and characteristics of the sedimentary infill. Two main depocenters are recognaized in the area: the Nicoya basin comprising about 3.5 km of Cretraceous-Tertiary sediments in the offshore region of the Nicoya Gulf and extending toward the noretheast on the onshore region; and the Parrita basin located offshoreand onshore with a sedimentary fill of more 4.5 km. Both basins are limited on the south by a shallow uplift of the basement which represents an extension of the outer arc over the platform.It is interpreted that in the Cretaceous, the area now occupied by the Nicoya and Patarrita basins, was part of an extense forearc system that spanned along the Pacific margin of Costa Rica encompassing the Tempisque and Terraba basins. The present structural configuration of the Parrita and Nicoya Basins initiates in the Middle Eocene, relatedto shear stresses caused by the rotationin clockwise direction of the southern portion of the Costa Rica territory along a main transcurrent fault zone. Such movement displaced the Tempisque and Terraba basins on the south and north respectively, developing the half-graben configuration and the extense system of strike-slip faults of the transtension and the subsidence along this series of faults, transverse to the coast, which are in some cases, still active.The seismic stratigraphy analysis defines five seismic sequences and a equal number of time structure maps, besides the top of basement. This analysis has been tied up to the surface geology information and discussed under the light of sequence stratigraphy concepts. In general, the sediment in depocenters of this central Pacific region of Costa Rica, can be characterized as a shallowing upward megasequence, including deposits ranging from slope to shallow marine.

The Le Danois Contourite Depositional System: Interactions between the Mediterranean Outflow Water and the upper Cantabrian slope (North Iberian margin)

The Le Danois Contourite Depositional System (CDS), located in an intraslope basin along the Cantabrian margin, is unique with respect to the known sedimentary systems along the upper slope of the Biscay margin. Whereas the steep Biscay slopes are dominated by downslope processes, the Le Danois CDS has been generated by alongslope processes and has a strong potential to contain a record of the Neogene palaeoceanography. This paper will focus on the onset, development and present-day functioning of this system with respect to its unique morphological control and the responsible local oceanographic processes. New bathymetric and seismic reflection data show that the past and present Le Danois CDS is shaped by the Mediterranean Outflow Water, conditioned by seafloor irregularities and two topographic highs; the large Le Danois Bank and the smaller Vizco High. The seismic stratigraphic analysis carried out on the contourite deposits has allowed to identify 3 seismic sequences, separated by 3 major regional discontinuities. Changes in depositional styles, the vertical stacking of seismic units and the nature of the discontinuities suggest a correlation with the development of the Cadiz CDS and well-known palaeoceanographic events along the NE Atlantic margin. The first clues for bottom-current deposits are identified in the Lower Sequence, which is developed after tentatively the Lower Pliocene. The drift deposits of both the Lower and Middle Sequences were confined into two palaeobasins within the intraslope basin. However, from the Middle Pleistocene Revolution (0.9 Ma) onwards, the contouritic deposition is intensified due to the switch to a “full glacial” mode with 100 ka cyclicity. This has allowed the development of the present-day depositional and erosive features, such as respectively elongated mounded and separated drifts, plastered drifts, moats and slide scars.

Slope and fan deposition in deep-water turbidite systems, East Antarctica

Detailed seismic stratigraphic analysis of 2D reflection seismic data from the East Antarctic rifted continental margin provides an ancient analogue for turbidity fan systems at high-latitude margins. Reflector sequences characteristic of turbidity flow channel migration and aggradation are identified, along with other depositional elements integral to the understanding of deep-water deposition, such as channel levees, splays and wavy bedforms. Wavy bedforms are contrasted with slump-related features, which although present do not explain the morphological features interpreted here as sediment waves. Slump-related features suggest, qualitatively, a relationship between a basal detachment layer and a widespread bottom-simulating reflector. The influence of the initial morphology on slope deposition and distributary patterns for turbidite deposits is clear in these data. Healed-slope accommodation on an above-grade slope is evident in seismic profiles. Ponded and/or healed-slope accommodation is associated with pre-existing bathymetric highs, interpreted as rift-related, serpentinised, peridotite ridges.

Normal fault growth and fault-related folding in a salt-influenced rift basin: South Viking Graben, offshore Norway

Three-dimensional seismic data were analysed to reconstruct the structural and stratigraphical development of a salt-influenced rift basin and thus gain an understanding of the relationships between normal fault growth, salt tectonics and the evolution of syn-rift depocentres. The Sleipner Basin, South Viking Graben, northern North Sea, is ca. 30 km long by 8 km wide and is bound to the east by a major extensional fault zone (Sleipner Fault Zone). Two types of fault-related fold are identified within the basin: (1) A fault-parallel monocline, interpreted as an extensional forced-fold, which formed through the upward propagation of the Sleipner Fault Zone through ductile evaporites of the Zechstein Supergroup and (2) three fault-perpendicular, salt-cored anticlines that compartmentalise the basin into four sub-basins and are related to displacement gradients along-strike of the Sleipner Fault Zone. Detailed seismic-stratigraphic analysis of pre- and syn-rift stratal units reveals a complex interplay between fault growth and salt movement which strongly controlled the evolution of syn-rift depocentres. During the early syn-rift, a series of depocentres, separated along-strike by the fault-perpendicular folds, were offset into the axis of the basin ( ca. 3–4.5 km to the west of the Sleipner Fault Zone) by the fault-propagation fold. Later in the rift event, the influence of the fault-perpendicular folds depleted, resulting in a larger, interconnected depocentre that shifted into the immediate hangingwall of the fault as the surface of the fault-propagation fold was breached. The results of this study have implications for normal fault growth and sedimentary depocentre development in salt-influenced rift basins, and contribute to the general understanding of the controls on salt migration.