High-resolution Multichannel Seismic Data Research Articles

Morphology, seismic characteristics and development of Cap Timiris Canyon, offshore Mauritania: A newly discovered canyon preserved-off a major arid climatic region

We employ a combined interpretation of Hydrosweep swath bathymetry and high resolution multi-channel seismic reflection data to investigate the development of Cap Timiris Canyon, a newly discovered submarine canyon offshore Mauritania. The dominantly V-shaped and deeply entrenched canyon exhibits many fluvial features including dendritic and meander patterns, cut-off loops and terraces, and is presently incising. Distal meander patterns, confined within a narrow fault-controlled corridor, show several stages of evolution, the latest of which is dominated by a down-system meander-loop migration. Terraces exhibit a variety of internal structures suggesting they originated through different processes including sliding/slumping, uplift-induced incision and lateral accretion. We ascribe canyon origin to an ancient river system in the adjacent presently arid Sahara Desert that breached the shelf during a Plio/Pleistocene sea level lowstand and delivered sediment directly into the slope area. Our data suggest that the initial invading unchannelised sheet of sand-rich turbidity flows initiated canyon formation by gradually mobilising along linear seafloor depressions and fault-controlled zones of weakness. We propose that the development of canyon morphology and structure was influenced by the stages of active flow of the coupling river system, and hence could act as a proxy for understanding the paleo-climatic evolution of a ‘green’ Sahara since Plio/Pleistocene times.

Forced regression systems tracts on the Bengal Shelf

A vertical succession of five composite sequences has been identified within the upper 100 m of the outer Bengal Shelf by means of high-resolution multi-channel seismic data. Each sequence consists predominantly of up to 100 km long and some 10 m thick forced regression systems tracts. The internal reflection pattern of the regressive units show mainly prograding oblique clinoforms. Intervening transgressive systems tracts are represented by seismically transparent or chaotic layers. On the outer shelf three of the sequences cause shelf aggradation and retrogradation, and two of them cause mainly shelf progradation. Based on the hierarchy of systems tracts, their calibration by comparison with eustatic sea-level curves and reconstructed paleoshoreline positions the composite sequences are interpreted as eccentricity driven eustatic 4th order (Milankovitch) cycles with a periodicity of about 100 ky. Internal unconformities mark cycles of 5th or higher order. An average subsidence of the outer shelf is estimated to be less than 0.4 mm/year during the last 345 ky. The correlation between the shelf growth pattern and sea-level fluctuations is consistent with the enhanced deposition on the eastern Bengal submarine fan from 465 to 125 ky B.P., as was observed by other authors.

Gas hydrate occurrence on the continental slope of the northern South China Sea

High-resolution multi-channel seismic data and geological samples were collected during two research cruises of the R/V FENDOU 4 in 1999 and 2000. Studies on these data and samples together with results from sites 1143–1145 and 1148 of ODP Leg 184 suggest that the geological structure on the continental slope of the northern South China Sea is favorable for the formation of gas hydrates. Bottom simulating reflectors (BSRs) and geochemical anomalies which indicate the existence of gas hydrates have been recognized in sediments of the Xisha Trough, the Dongsha Rise and the accretionary wedge of the Manila subduction zone. These gas hydrates are generated by two different mechanisms depending on the tectonic regime and the seismic and geochemical characteristics. The first applies to the passive continental margin of the northern South China Sea on the Dongsha Rise and in the Xisha Trough. The gas hydrates are associated with diapiric structures, active faults, slumps and gravity flows as well as high Late Cenozoic sedimentation rates. Their seismic expression includes BSRs, seismic blanking zones and velocity anomalies. The second mechanism is operative on the active continental margin along the Manila subduction zone, especially in the accretionary wedge. Here, gas hydrate occurrence is marked by widespread BSRs and acoustic ‘pull-down’ possibly related to the existence of free gas in the sediments beneath the BSR. The thickness of the seismic blanking zones averages 250 m, suggesting that the stable gas hydrate zone has about the same thickness.

Controls on sequence stratigraphy of a middle Miocene-Holocene, current-swept, passive margin: Offshore Canterbury Basin, New Zealand

The offshore Canterbury Basin exemplifies sequence development on a prograding passive margin influenced strongly by submarine currents. Nineteen middle Miocene–Holocene, regional, sequence-bounding unconformities are interpreted by using high-resolution multichannel seismic data. The sequences can be grouped into larger units based on seismic geometry and facies that reflect different combinations of controls on sequence architecture. Correlation with oxygen isotope records suggests that eustasy controls the timing of sequence boundaries. The number of sequences is similar to that of coeval cycles on a temperature-adjusted, Miocene and early Pliocene δ 18 O record. The late Pliocene–Pleistocene sequence record is of lower frequency than the isotopic record of this period, either because of the limitations of seismic resolution or because of removal of sequence boundaries by erosion associated with high-amplitude eustasy. However, the last two sequence boundaries correlate well with the last two 100 k.y. isotopic cycles. In contrast, sequence architecture is influenced strongly by local processes. Along-strike currents create large, elongate sediment drifts that control sequence thickness; current erosion in drift moats forms diachronous unconformities. Drifts focus deposition on the slope, reducing the rate of basinward advance of the shelf edge, but increasing that of the slope toe, thereby reducing slope inclination. Replacement of along-strike processes by downslope processes increases rates of shelf-edge progradation, and the slope steepens as the reduced accommodation space over the expanded slope is filled. Clinoform geometries along strike from active drifts suggest that currents might influence clinoform formation even in locations lacking seismic evidence of current reworking.

Acquisition of high-resolution multichannel seismic data in the offshore part of the Nuussuaq Basin, central West Greenland

A high-resolution multichannel seismic survey (project NuussuaqSeis 2000) was carried out from 18 July to 2 August 2000 in the offshore part of the Nuussuaq Basin, central West Greenland using the Danish research vessel R/V Dana with seismic equipment from the Geological Institute, Aarhus University, Denmark. Funding for the project was provided by the Danish Energy Research Programme, the Bureau of Minerals and Petroleum, Nuuk, Greenland, the Geological Institute of Aarhus University and the Geological Survey of Denmark and Greenland (GEUS). After completion of the NuussuaqSeis 2000 project, R/V Dana was used for a three-day coring project in Disko Bugt (see Kuijpers et al. 2001, this volume) before the ship returned to Denmark.

Large-scale glaciotectonic thrust structures in the eastern Danish North Sea

Abstract The distribution of shallow, detached thrust fault complexes in the eastern Danish North Sea has been mapped based on 6400 km of high-resolution multichannel seismic data. Individual thrust segments are 200–1000 m long and 100–250 m thick with up to 200 m of horizontal displacement. Thrusting mainly affected upper Middle Miocene to lower Pleistocene strata and predated an extensive system of now buried subglacial valleys. The thrust structures are located along a 200 km long NW-SE trend. The general thrust direction is towards the SW, parallel to inferred directions of ice movement during the Elsterian and Saalian glaciations. This direction is, however, also parallel to the dip of the underlying detachment surface. The detachment surface corresponds to the mid-Miocene unconformity in the SE and the base of the Quaternary to the northwest. Detachment is generally located at depths of 130–270 m below sea level. The structures are interpreted as glaciotectonic thrust structures of Elsterian and/or Saalian age. The driving mechanism is interpreted to be gravity spreading in front of glaciers advancing from the north, NE and east. Thrusting was facilitated by the presence of SW-dipping detachment surfaces.

Deformation of the Caribbean region: One plate or two?

New deep-penetrating high-resolution multichannel seismic reflection data collected in the eastern Caribbean during R/V Ewing cruise EW9501 imaged both the crustal structure and overlying stratigraphic successions. On the basis of this new multichannel seismic data, we define the geologic development of the Beata Ridge and Venezuelan basin. The Caribbean crust was formed by seafloor spreading in Late Jurassic–Early Cretaceous time. Prior to the Senonian, widespread and rapid eruption of basaltic flows began in concert with extensional deformation of the Caribbean crust. Thick volcanic wedges characterized by divergent reflectors are observed along the boundary that separates rough from smooth oceanic crust, are coincident with an abrupt shallowing of the Moho, and appear to be bounded by a large, northwest-dipping fault system. The locus of major extensional deformation migrated through time from the Venezuelan basin to the western flank of the Beata Ridge. Extensional unloading of the Beata Ridge footwall caused uplift and rotation of the ridge. Sediment thicknesses and stratal geometry observed across the Venezuelan basin and Beata Ridge suggest that the majority of the deformation in this region occurred during and soon after the emplacement of the volcanics. Minor fault reactivation in the Neogene along the eastern flank of the Beata Ridge is associated with an accommodation zone (i.e., tear fault) that records a change in the deformation style from bending and subduction of the Caribbean plate along the Muertos Trough south of Puerto Rico to compressional deformation and obduction of the Caribbean plate south of Hispaniola. We propose that this difference in deformational style is, in part, a consequence of the thicker crust on the Beata Ridge, which is more resistant to subduction.

Processing and Interpretation of Seismic Reflection data from Ogaden Basin, Ethiopia: ABSTRACT

The Ogaden basin is believed to be an extensional basin created during the early rifting of Gondwana and the development of the western Indian Ocean. Preliminary studies conducted by various oil companies and national geoscientists suggest that the area is prospective for hydrocarbon accumulation. Prior to these studies, however, very little was known of the surface geology in this area and virtually nothing was known of the subsurface. The objectives of the study were to implement state-of-the-art seismic and gravity data processing and interpretation techniques in order to identify structural and/or stratigraphic trap sequences and indicate their significance in the hydrocarbon exploration of the basin. In this study, high resolution multi-channel reflection seismic data, acquired in 1993 from the Ogaden Basin, are used. The data were acquired for Hunt Oil Company using Vibroseis as an energy source. Various data enhancement techniques were applied to these data in order to produce an interpretable final seismic section for the identification of possible hydrocarbon habitats. These included discrimination and filtering of coherent noise from the signal, enhancing, focussing of the identified reflectors and static corrections. Statics solutions from shallow wells and automatic refraction statics were compared, with the best results obtained when automaticmore » refraction solutions were applied. Our study has revealed various reflection horizons that can be potential stratigraphic traps at pinch outs and may be porous strata which are terminated by lateral transitions or interconnected lines of either elastic sediments or carbonates. We discuss these results in the light of gravity modelling designed to estimate maximum sediment thickness along the line.« less

Faulted structure of the bottom simulating reflector on the Blake Ridge, western North Atlantic

Abstract : High-resolution multichannel seismic data collected from the Blake Ridge in the western North Atlantic by the Naval Research Laboratory's Deep Towed Acoustics/Geophysics System (DTAGS) show that the bottom simulating reflector (BSR) in this area is the reflection from the interface between an approx. 440-m-thick section of hydrate bearing sediment overlying and approx. 5- m-thick layer of methane gas-rich sediment. The high resolution attainable by the deep-tow seismic system reveals normal fault offsets of approx. 20 m in the BSR. These growth faults may provide a path for vertical migration of methane initially concentrated beneath the hydrate-bearing sediment, enabling hydrate to form throughout sediment above the BSR. Because the BSR represents a methane gas methane hydrate phase boundary rather than a lithologic or diagenetic horizon, the observed off set of the BSR itself reflects discontinuities in the pressure- temperature field across the fault zones where they intersect the BSR. Directional ambient noise, Bottom scattering, Deep-towed array geophysical system, Ocean-bottom seismometer, Towed array.

Seismic expression of shallow structures in active tectonic settings in New Zealand

Late Cenozoic deformation along the Australian/Pacific plate boundary is seen in onshore New Zealand as zones characterised by extension- or transcurrent- or contraction-related structures. High-resolution multichannel seismic reflection data were acquired in several of these tectonic zones and successfully reveal the shallow structures within them. Thirty kilometres of dynamite reflection data in the Rangitaiki Plains, eastern Bay of Plenty, define a series of NE-trending normal faults within this extensional back-arc volcanic region. The data cross surface ruptures activated during the 1987 Edgecumbe earthquake. In the southern North Island, a 20 km Mini-Sosie? seismic profile details the Quaternary sedimentation history and reveals the structure of the active strike-slip and thrust fault systems that form the western and eastern edges of the Wairarapa basin, respectively. This basin is considered to sit astride the boundary between a zone of distributed strike-slip faults and an active accretionary prism. In the Nelson area, northwestern South Island, previously unrecognised low-angle thrust faults of Neogene or Quaternary age are seen from Mini-Sosie data to occur at very shallow depths. Crustal shortening here was previously thought to arise from movement on high-angle reverse faults, and the identification of these low-angle faults has prompted a reassessment of that model. A grid of 18 km of Mini-Sosie seismic data from the central eastern South Island delineates Neogene or Quaternary thrust faults in Cenozoic sediments. The thrusts are interpreted as reactivated Early Eocene normal faults, and the thrust fault geometry is dominated by these older structures.

Results of a deep‐tow multichannel survey on the Bermuda Rise

High‐resolution multichannel seismic data taken with a deep‐towed source and receiving array are used to obtain estimates of the in situ compressional velocity‐depth functions within the upper ∼300 m of deep‐ocean sediments. These data, taken on the Bermuda Rise in an area with a nominal water depth of 5 km, resolve structure consistent with DSDP data from the region. The velocity‐depth functions derived from these data indicate little P‐velocity gradient in the upper 100–170 m of sediments and resolve P‐velocity inversions within the depth range sampled.

Multibeam bathymetric survey and high resolution seismic investigations on the Barbados Ridge complex (Eastern Caribbean): A key to the knowledge and interpretation of an accretionary wedge

An important problem of active margin processes is the way accretionary wedges form in response to oceanic crust subduction. New data have been obtained along the Barbados Ridge, in front of the Lesser Antilles Tertiary Island Arc; they include high-resolution multichannel seismic data and accurate SEA BEAM bathymetric maps. Both have been used to propose a 3-dimensional interpretation of tectonic and sedimentary processes related to subduction, such as: oceanic off-scraping, ductile folding, relative uplift and subsidence, fore-arc basin evolution and mud diapirism. They all improved our knowledge of the evolution of accretionary prisms and, more specifically, of the Barbados Ridge development in Plio-Quaternary time. Previous tectonic events are deduced from recently published geological data from Barbados and the Lesser Antilles Islands.