The continental margin of Uruguay: Crustal architecture and segmentation

The Uruguayan continental margin comprises three sedimentary basins: the Punta del Este, Pelotas and Oriental del Plata basins, the genesis of which is related to the break-up of Gondwana and the opening of the Atlantic Ocean. Herein the continental margin of Uruguay is studied on the basis of 2D multichannel reflection seismic data, as well as gravity and magnetic surveys. As is typical of South Atlantic margins, the Uruguayan continental margin is of the volcanic rifted type. Large wedges of seaward-dipping reflectors (SDRs) are clearly recognizable in seismic sections. SDRs, flat-lying basalt flows, and a high-velocity lower crust (HVLC) form part of the transitional crust. The SDR sequence (subdivided into two wedges) has a maximum width of 85 km and is not continuous parallel to the margin, but is interrupted at the central portion of the Uruguayan margin. The oceanic crust is highly dissected by faults, which affect post-rift sediments. A depocenter over oceanic crust is reported (deepwater Pelotas Basin), and volcanic cones are observed in a few sections. The structure of continental crust-SDRs-flat flows-oceanic crust is reflected in the magnetic anomaly map. The positive free-air gravity anomaly is related to the shelf-break, while the most prominent positive magnetic anomaly is undoubtedly correlated to the landward edge of the SDR sequence. Given the attenuation, interruption and/or sinistral displacement of several features (most notably SDR sequence, magnetic anomalies and depocenters), we recognize a system of NW-SE trending transfer faults, here named Río de la Plata Transfer System (RPTS). Two tectono-structural segments separated by the RPTS can therefore be recognized in the Uruguayan continental margin: Segment I to the south and Segment II to the north.     

西南非海岸盆地中生界构造-沉积特征与成藏条件

西南非海岸盆地位于南大西洋的非洲海岸地区,由上侏罗—下白垩统裂谷盆地和上白垩—全新统被动陆缘盆地叠合形成,是一个热点油气勘探区。在调研国外油气地质研究的基础上,根据地震、测井资料,详细描述了西南非海岸盆地在中生代的构造形态与沉积充填特征,明确了其纵向演化与横向迁移规律,并依据已知油气田的钻井与测试资料,综合分析了盆地油气富集的基本地质条件。研究表明,西南非海岸盆地始形成于侏罗纪晚期,早期以剧烈的断裂与火山活动为特点,裂谷盆地内部填充大量砂岩和页岩;在经历白垩纪巴雷姆期与阿普特期的过渡阶段后,于阿尔布期进入被动陆缘阶段,在过渡与被动陆缘阶段,盆地内部以细粒海相沉积为主,可见少量碳酸盐岩与盐岩。盆地阿普特阶主力烃源岩与其上覆的上白垩统碎屑岩储层共同组成了油气成藏组合。     

南海西南次海盆两侧陆缘新生代构造沉降特征及演化过程

构造沉降史分析有助于认识盆地的形成演化过程,是盆地分析的重要基础。为对比分析南海西南次海盆两侧陆缘新生代构造演化特征,本文选取了横穿南海西南次海盆两侧陆缘的多道地震剖面测线,其中NH973-3测线横跨西南次海盆北侧陆缘中-西沙地块,NH973-1+SO27-04联合剖面跨越西南次海盆南侧陆缘南沙地块,在地震地层解释的基础上,采用回剥法和平衡剖面技术分析了西南次海盆两侧陆缘构造沉降特征及伸展过程。分析结果表明：（1）西南次海盆两侧陆缘的构造沉降曲线特征表现为裂陷初始期曲线斜率平缓,裂陷强烈期和末期曲线斜率较陡,断-拗转换期和拗陷期曲线斜率又回归相对平缓的反“S”形多段式特征;（2）两侧陆缘的构造沉降具有一定的延迟滞后性,造成此现象的原因可能与西南次海盆两侧陆缘岩石圈的分层差异伸展及南海西缘断裂的右旋走滑活动有关,且南海西缘断裂的右旋走滑活动造成两侧陆缘的构造沉降中心向南迁移;（3）两侧陆缘盆地主要形成于晚渐新世,北侧陆缘因受晚渐新世南海西缘断裂右旋走滑活动的改造影响而形成伸展-走滑相关的沉积盆地,南侧陆缘在早中新世因受到挤压碰撞的改造影响而形成伸展-挠曲复合型沉积盆地。这些研究成果可为南海西南次海盆两侧陆缘沉积盆地的油气和天然气水合物的勘探开发提供重要的科学背景支持。     

Biological structures as a source of habitat heterogeneity and biodiversity on the deep ocean margins

Biological structures exert a major influence on species diversity at both local and regional scales on deep continental margins. Some organisms use other species as substrates for attachment, shelter, feeding or parasitism, but there may also be mutual benefits from the association. Here, we highlight the structural attributes and biotic effects of the habitats that corals, sea pens, sponges and xenophyophores offer other organisms. The environmental setting of the biological structures influences their species composition. The importance of benthic species as substrates seems to increase with depth as the complexity of the surrounding geological substrate and food supply decline. There are marked differences in the degree of mutualistic relationships between habitat-forming taxa. This is especially evident for scleractinian corals, which have high numbers of facultative associates (commensals) and few obligate associates (mutualists), and gorgonians, with their few commensals and many obligate associates. Size, flexibility and architectural complexity of the habitat-forming organism are positively related to species diversity for both sessile and mobile species. This is mainly evident for commensal species sharing a facultative relationship with their host. Habitat complexity is enhanced by the architecture of biological structures, as well as by biological interactions. Colony morphology has a great influence on feeding efficiency for suspension feeders. Suspension feeding, habitat-forming organisms modify the environment to optimize their food uptake. This environmental advantage is also passed on to associated filter-feeding species. These effects are poorly understood but represent key points for understanding ecosystems and biodiversity on continental margins. In this paper we explore the contributions of organisms and the biotic structures they create (rather than physical modifications) to habitat heterogeneity and diversity on the deep continental margins.     

Results from a detailed aeromagnetic survey across the northeast Newfoundland margin, Part II: Early opening of the North Atlantic between the British Isles and Newfoundland

Poles of rotation for the North Atlantic have been derived from the results of a new aeromagnetic survey northeast of Newfoundland. Reconstruction of the North Atlantic at anomaly 34 time shows a band of large amplitude magnetic anomalies which parallels anomaly 34 on both sides of the Atlantic from Flemish Cap and Goban Spur to the Azores-Gibraltar Fracture Zone. A group of similar anomalies has also been identified in the Bay of Biscay. North of Goban Spur and Flemish Cap, these anomalies follow the ocean-continent boundary. Poles of rotation derived for this anomaly show that it forms an isochron (100–110 m.y.) during the long Cretaceous normal polarity interval. The cause of this anomaly is not definite, but it may represent an increase in the magnetization of the crust during a limited time within the Cretaceous Magnetic Quiet Zone by a process such as replacement of thermoremanent magnetization by chemical remanent magnetization as proposed by Raymond and LaBrecque.The North Atlantic has also been reconstructed at the time of the initial opening in the region between Flemish Cap and the Charlie-Gibbs Fracture Zone, using inferred ocean-continent boundaries on the west and east sides: it has been shown that the entire region could not have saparated at one time, but that spreading between the British Isles and Newfoundland had to progress from south to north. Consequently, when active sea-floor spreading was taking place between Goban Spur and Flemish Cap (about 110 m.y.) the region to the north was still being stretched. The calculated amount of stretching as derived from the reconstructions (about 25%) agrees well with the extension of the lithosphere obtained from modelling the subsidence history of this region, and with the results of deep seismic studies. Active spreading in the north started about 100 m.y. ago.     

Magnetic anomalies in the canary basin and the Mesozoic evolution of the central North Atlantic

The data from a recent magnetic compilation by Verhoefet al. (1991) off west Africa were used in combination with data in the western Atlantic to review the Mesozoic plate kinematic evolution of the central North Atlantic. The magnetic profile data were analyzed to identify the M-series sea floor spreading anomalies on the African plate. Oceanic fracture zones were identified from magnetic anomalies and seismic and gravity measurements. The identified sea floor spreading anomalies on the African plate were combined with those on the North American plate to calculate reconstruction poles for this part of the central Atlantic. The total separation poles derived in this paper describe a smooth curve, suggesting that the motion of the pole through time was continuous. Although the new sea floor spreading history differs only slightly from the one presented by Klitgord and Schouten (1986), it predicts smoother flowlines. On the other hand, the sea floor spreading history as depicted by the flowlines for the eastern central Atlantic deviates substantially from that of Sundvik and Larson (1988). A revised spreading history is also presented for the Cretaceous Magnetic Quiet Zone, where large changes in spreading direction occurred, that can not be resolved when fitting magnetic isochrons only, but which are evident from fracture zone traces and directions of sea floor spreading topography.Deceased 11 November 1991     

Unexpected fast rate of morphological evolution of geologically-active continental margins during Quaternary: Examples from selected areas in the Italian seas

In the last few decades, seafloor imagery systems have drastically changed our vision of a mostly regular and depositional marine landscape, evidencing how erosive and mass-wasting processes are widespread in the marine environments, with particular reference to geologically-active areas. Most of the previous studies have focused on the characterization of these features, whereas a very few ones have tried to estimate what is the extent and order of magnitude of erosion rates in these areas. In this paper, we show several examples from some of the most geologically-active margins off Southern Italy aimed to a) quantify the spatial extent of such processes, b) better understand the role of submarine erosion in the morphogenesis of the coastal sector, and c) try to roughly estimate the order of magnitude of erosion rates in these areas. The results are impressive, with mass-wasting features widespread from coast down to −2600, affecting from the 52% up to 97% of the whole continental slope. Because of the narrow or totally lacking shelves in these areas, mass-wasting processes often occur close to the coast and match embayment of the coast, so indicating a key role in the morphogenesis of coastal sector, with significant implication on the related geohazard. Finally, based on a morphological approach integrated by available stratigraphic constraints we have roughly estimated average erosion rates in these areas, ranging from (at least) some mm/year to a few cm/year, i.e., some hundreds of meters up to kilometers eroded in each eustatic cycle. Despite the large uncertainties of these estimates as well as their spatial and temporal variability in response to regional and local factors, the obtained values are very high and they should be considered for future model of margin evolution, source-to-sink computation and marine/coastal geohazard assessment.     

南海西南次海盆被动陆缘构造单元划分及伸展演化过程

南海西南次海盆被动陆缘洋陆转换带位于陆缘强烈伸展区,蕴含着岩石圈临界伸展破裂和洋盆扩张过程的丰富信息。本文利用多道地震剖面和重力异常数据,对西南次海盆被动陆缘构造单元进行划分,研究陆缘南、北部洋陆转换带结构构造特征,探讨陆缘伸展演化过程。多道地震剖面资料显示,北部洋陆转换带发育有裂陷期断陷和向海倾斜的掀斜断块;南部发育有低角度正断层控制的裂陷期断陷、海底火山以及局部隆起;从陆到洋方向,重力异常值变化明显。根据上述结果南海西南次海盆被动陆缘划分为近端带、洋陆转换带和洋盆三个构造单元,分别对应了其伸展演化过程的三个阶段：前裂谷阶段、陆缘裂陷阶段和海底扩张阶段。     

Structure and evolution of a passive margin in a compressive environment: Example of the south-western Alps-Ligurian basin junction during the Cenozoic

We focus on the northern Ligurian margin, at the geological junction of the subalpine domain and the Ligurian oceanic basin, in order (1) to identify the location of the southern limit of the Alpine compressive domain during the Cenozoic, and (2) to study the influence of a compressive environment on the tectonic and sedimentary evolution of a passive margin.Based on published onshore and offshore data, we first propose a chronology of the main extensional and compressional regional tectonic events.High-resolution seismic data image the margin structure down to ∼3 km below seafloor. These data support that past rifting processes control the present-day margin structure, and that 2800-4000 m of synrift sediment was deposited on this segment of the margin in two steps. First, sub-parallel reflectors indicate sediment deposition within a subsident basin showing a low amount of extension. Then, a fan-shaped sequence indicates block tilting and a higher amount of extension. We do not show any influence of the Miocene Alpine compression on the present-day margin structure at our scale of investigation, despite the southern subalpine relief formed in the close hinterland at that time. The southern front of the Miocene Alps was thus located upslope from the continental margin.Finally, a comparison with the Gulf of Lions margin suggests that the tectonic influence of the Alpine compression on the rifting processes is restrited to an increase of the subsidence related to flexure ahead of the Alpine front, explaining abnormally high synrift thicknesses in the study area. The Alpine environment, however, has probably controlled the sedimentary evolution of the margin since the rifting. Indeed, sediment supply and distribution would be mainly controlled by the permanent building of relief in the hinterland and by the steep basin morphology, rather than by sea-level fluctuations, even during the Messinian sea-level low-stand.     

3D architecture of Quaternary sediment along the NW Atlantic Moroccan Rharb continental shelf: A stratal pattern under the dual control of tectonics and climatic variations

This study focuses on the evolution of the Atlantic NW Moroccan Rharb continental shelf during the Neogene and Quaternary. This region is part of a foreland basin bounded by the Rif mountain belt and thus provides an interesting geological setting to study the interactions between eustasy and tectonics and the driving mechanisms controlling stratigraphic patterns. The results are supported by an interpretation of new data including high-resolution seismic lines coupled with an interpretation of industrial seismic lines and detailed logs of industrial wells completed by micropaleontologic analysis of cuttings. The stratigraphy reveals a succession of three mega sequences related to the transition from an underfilled to an overfilled stage reflecting the long-term evolution of the foreland system. Moreover, evidence of cyclical sea-level changes are visible in the upper megasequence composed of three depositional sequences assumed to be fourth-order sequences generated in response to the most recent 100-ka glacio–eustatic cycles. This study also shows the peripheral deformation of the Rharb shelf responsible for changes in the geometry of the deposits and thicknesses of the sedimentary fill during the Pliocene and Pleistocene. The most important change was triggered by the uplift of the Lallah Zahra Ridge corresponding to a major Quaternary kinematic boundary and the broad uplift of the southern shelf interpreted as a flexural uplift of the forebulge domain. The deformation-controlled sediment dispersal pattern consists of a progressive growth of the shelf accompanied with a progressive shift of depocenters from the North East to the South West and a general progradation to the North West along the southern border. This progressive filling has led to the confinement of the Rharb paleo-valley across the continental shelf. The complete filling of the palaeo-valley was followed by the development of a more than 70-m thick prodeltaic lobe at the front of the Oued Sebou river mouth during the Holocene.     

Mid-Holocene evolution and paleoenvironments of the shoreface–offshore transition, north-eastern Argentina: New evidence based on benthic microfauna

A sedimentary record spanning 5792–5511 cal yr BP and 3188–2854 cal yr BP was recovered at 36° 45′ 43″ S–56 ° 37′ 13″ W, south-west South Atlantic. The sedimentological features and micropaleontological (benthic foraminifera and ostracoda) content were analyzed in order to reconstruct paleoenvironmental conditions. Considerable environmental fluctuations are indicated by all these proxies. Five different stages were distinguished: Stage 1 (ca. 5800–5000 cal yr BP) consists of muddy sand with abundant microfossils. In this interval, species typical for inner marine shelf environments maintained a high abundance. Stage 2 consists of plastic light greenish grey clays barren of microfossils, and probably represents fluvial input from the de la Plata River to the shelf contemporaneous of a lowering of sea level. Stage 3 is composed of brownish yellow sandy silts, and represents increasing marine conditions in the area as reflected by higher faunal diversity and typical foraminifera of inner shelf environments. Stage 4 is made of homogeneous mud, barren of microfossil, which represents a new pulse of fluvial input to the shelf in consequence of a new fall in sea level. The final part of the core (Stage 5) is a coarsening upward sequence, grading from greeny brown clayey sandy silts to coarse shelly sands and represents the modern sedimentation in the area. This interpretation strengthens the stepped model of late-Holocene sea-level fall between 5511–5792 cal yr BP and 2854–3188 cal yr BP in Buenos Aires coast, and agrees with the relative sea-level history previously proposed by some authors from western South Atlantic coasts.     

Outcrop and seismic expression of stratigraphic patterns driven by accommodation and sediment supply turnarounds: Implications on the meaning and variability of unconformities in syn-orogenic basins

Tectonically-complex settings present accommodation and sediment supply changes with patterns and rates for which the current sequence stratigraphy paradigms are not designed. In the Tertiary Piedmont Basin (TPB) and Peri-Adriatic Basin (PAB), outcrop and seismic examples demonstrate that the observed stratal and stacking patterns cannot be entirely explained using conventional sequence-stratigraphic models. Therefore, it is of paramount importance to use a model-independent more comprehensive approach encompassing advanced sequence-stratigraphic concepts combined with process changes, while being able to consider the morphostructural complexity that characterizes these margins and their changes induced by basin reshaping.Abrupt relative sea level falls generated by uplift or basin inversion may exceed several hundreds of meters, resulting in wedge-margin progressive unconformities characterized by subaerial and subaqueous erosional truncation. A progressive increase in sediment supply occurs, expressed by increasing volume and size of mass-transport complexes overlain by forced-regressive deltas, as the maximum sediment supply is delayed until after the main uplift. Different accommodation/sediment supply ratios may also occur at the same time along different margins of the same basin, generating a diachronism in the T-R or R-T cycles, adding further complexity to the variability produced by autogenesis.On clastic shelf margins characterized by an increasing rate of relative sea level rise, such as in case of increasing rollback velocities and related flexural tilting, or following an orogenic collapse, sediment supply may not keep pace with increasing accommodation so that initially retrogradation and basinward condensation occur, marked by omission surfaces. However, when the rate of subsidence increases, the succession is punctuated by multiple subaqueous erosional unconformities marking phases of basinward tilting leading to the oversteepening of basin margins and abrupt deepening. The downwarping usually produces large-scale subaqueous erosional surfaces passing laterally into paraconformities, so hinged-margin drowning unconformities affecting clastic shelves occur, associated with regional stratigraphic gaps.The re-establishment of the slope equilibrium profile implies high volume of sediments eroded from drowned deltas and shelves, feeding turbidites deposited at the toe of above-grade slopes. These turbidites can be therefore considered as high accommodation-high sediment supply systems. This suggests that turbidites are delivered basinward not only due to bypass at sequence boundaries or during the highstand progradation of supply-driven deltas, but also due to abrupt accommodation creation on hinged-shelf margin wedges.The great variability of tectonically-driven unconformities generated under either decreasing or increasing accommodation suggests that the features described in the TPB and the PAB are probably not uncommon, controlled by linked dynamic turnarounds of accommodation, sediment supply and stratigraphy taking place throughout the development of basin reorganizations.     

Eocene post-rift tectonostratigraphy of the Rockall Plateau, Atlantic margin of NW Britain: Linking early spreading tectonics and passive margin response

A regional study of the Eocene succession in the UK sector of the Rockall Plateau has yielded new insights into the early opening history of the NE Atlantic continental margin. Data acquired from British Geological Survey borehole 94/3, on the Rockall High, provides a high-resolution record of post-rift, Early to Mid-Eocene, subaqueous fan-delta development and sporadic volcanic activity, represented by pillow lavas, tuffs and subaerial lavas. This sequence correlates with the East Rockall Wedge, which is one of several prograding sediment wedges identified across the Rockall Plateau whose development was largely terminated in the mid-Lutetian. Linking the biostratigraphical data with the magnetic anomaly pattern in the adjacent ocean basin indicates that this switch-off in fan-delta sedimentation and volcanism was coincident with the change from a segmented/transform margin to a continuously spreading margin during chron C21. However, late-stage easterly prograding sediment wedges developed on the Hatton High during late Mid- to Late Eocene times; these can only have been sourced from the Hatton High, which was developing as an anticline during this interval. This deformation occurred in response to Mid- to Late Eocene compression along the ocean margin, possibly associated with the reorganisation to oblique spreading in the Iceland Basin, which culminated at the end of the Eocene with the formation of the North Hatton Anticline, and the deformation (tilting) of these wedges. A series of intra-Eocene unconformities, of which the mid-Lutetian unconformity is the best example, has been traced from the Rockall Plateau to the Faroe-Shetland region and onto the Greenland conjugate margin bordering the early ocean basin. Whilst there appears to be some correlation with 3rd order changes in eustatic sea level, it is clear from this study that tectonomagmatic processes related to changes in spreading directions between Greenland and Eurasia, and/or mantle thermal perturbations cannot be discounted.     

Source and magmatic evolution of ocean island basalts from the Pohnpei Island,Northwest Pacific Ocean: Insights from olivine geochemistry

The compositional variability of ocean island basalts (OIBs) is thought to reflect partial melting of a lithologically-heterogeneous mantle source dominated by ...