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Atlantic Margin (atlantic + margin)

Distribution by Scientific Domains

Earth and Environmental Science	100%

Selected Abstracts

A quantitative study on the use of converted waves for sub-basalt imaging

GEOPHYSICAL PROSPECTING, Issue 3 2003
Peter Hanssen
ABSTRACT The idea of imaging beneath a high-velocity layer using converted waves has been popular since 1990. Because these wave types have their maximum amplitudes at mid- to far-offsets, the search for pure P-waves at the highly multiple-contaminated near-offsets can be avoided. For the Atlantic Margin, with buried thin-layered basalts, our quantitative study shows that the initial single-layered approach is not viable. Even in an unrealistic ideal geological setting, the amplitude of the symmetrical PSP-mode is far too weak to be recognized on towed streamer data. Furthermore, in the far-offset window, where locally converted waves have their strongest amplitudes, there is a multitude of other reflections, refractions and interbedded multiples, which have similar moveouts and, often, higher amplitudes. Without the removal of these events, a reliable image of the subsurface cannot be produced. We show that even if this problem were solved, it would be far easier to use the P-wave reflection from beneath the basalt at near-offsets. Our study shows that this wave type is by far the strongest response. A borehole-derived model using a thin-layered basalt sequence reveals that the strongest locally converted wave has an asymmetrical path and is 10 times weaker. All our results indicate that the pure P-modes provide the best chance of imaging sub-basalt sedimentary interfaces. [source]

Permo-Triassic development from Ireland to Norway: basin architecture and regional controls

GEOLOGICAL JOURNAL, Issue 6 2009
tolfová
Abstract Extensive occurrences of Permo-Triassic strata are preserved along the Northwest European Atlantic margin. Seismic reflection and well data are used to describe large-scale Permo-Triassic basin geometries along a swath of the continental shelf more than 2000,km long extending from the Irish to the mid-Norwegian sectors. Successions in the Celtic Sea, the flanks of the Irish Rockall Basin, basins west and north of Scotland, and the Trøndelag and Horda platforms west of Norway are described. The large-scale Permo-Triassic depositional geometries commonly represent erosional remnants of larger basins modified by later rifting episodes, uplift, inversion and continental breakup. However, the interpreted geometries reveal spatial and temporal differences in rifting style. The basins developed above a complex mosaic of petrologically heterogeneous crustal terranes with inherited crustal fabrics, which had a significant impact on the depositional basin geometries. Small Permian basins with growth faulting developed in the southern Celtic Sea region. Extensive, uniformly thick Triassic strata are characteristic of the wide rift basins in the southeastern Rockall Basin and northwest of the Solan Bank High. Thick, fault-controlled basins developed in the Horda and Trøndelag platform regions. The main controls on Permo-Triassic basin architecture are (a) crustal thickness and composition, which determined the development of narrow or wide rift basin geometries, (b) inherited Variscan, Caledonian and Precambrian basement structures and (c) pre-rift palaeotopography. Copyright © 2009 John Wiley & Sons, Ltd. [source]

A fossilized Opal A to Opal C/T transformation on the northeast Atlantic margin: support for a significantly elevated Palaeogeothermal gradient during the Neogene?

BASIN RESEARCH, Issue 4 2002
R. J. Davies
ABSTRACT Rock samples , collected from a recent deep-water exploration well drilled in the Faeroe-Shetland Channel, northwest of the UK , confirm that a distinctive high-amplitude seismic reflector that cross-cuts the Upper Palaeogene and Neogene succession and covers an area of 10 000 km2 is an example of a fossilized Opal A to Opal C/T (Cristobalite/Tridymite) transition. Analysis of these rock fragments tied to an extensive two-dimensional and three-dimensional seismic database constrains the time at which the boundary was fossilized and in addition reveals the unusual geometrical characteristics of a relict bottom-simulating reflector. The diagenetic transformation of biogenic silica (Opal A) to Opal C/T is predominantly temperature-controlled and requires sediments that contain biogenic silica. The reflector (termed as Horizon E) probably initially represented a biosiliceous ooze or a siltstone that contained a component of biogenic silica that underwent transformation as the diagenetic front migrated upsection during burial. The parallelism it shows with a shallower early Pliocene reflector and its apparent upsection migration during a compressional episode in the basin indicate that it was active during the middle and late Miocene and ceased activity during the early Pliocene when there was between 200 and 400 m of overburden. The present-day burial depth of the boundary is ca. 700 m and the temperature at the inactive diagenetic front at the well location is 24 °C. Given these temperature and depth constraints, we hypothesize that even if this is an example of a relatively low-temperature Opal A to Opal C/T transformation, a temporarily elevated geothermal gradient of ca. 60 °C km,1 would have been required to initiate and arrest upsection migration of the boundary during the middle and late Miocene. Factors such as climatic deterioration and the onset of cold deep-water circulation are likely to only have had a contributory role in arresting the upward migration of the boundary. [source]

Stratigraphic evolution of the Triassic,Jurassic succession in the Western Southern Alps (Italy): the record of the two-stage rifting on the distal passive margin of Adria

BASIN RESEARCH, Issue 3 2009
Fabrizio Berra
ABSTRACT The Triassic,Lower Jurassic succession of the Southern Alps is characterized by rapid thickness changes, from an average of about 5000 m east of Lago Maggiore to about 500 m in the Western Southern Alps. The stratigraphy reflects the Triassic evolution of the Tethyan Gulf and the Early Jurassic rifting responsible for the Middle Jurassic break-up of Adria from Europe. The succession of the Western Southern Alps starts with Lower Permian volcanics directly covered by Anisian sandstones. The top of the overlying Ladinian dolostones (300 m) records subaerial exposure and karstification. Locally (Gozzano), Upper Sinemurian sediments cover the Permian volcanics, documenting pre-Sinemurian erosion. New biostratigraphic data indicate a latest Pliensbachian,Toarcian age for the Jurassic synrift deposits that unconformably cover Ladinian or Sinemurian sediments. Therefore, in the Western Southern Alps, the major rifting stage that directly evolved into the opening of the Penninic Ocean began in the latest Pliensbachian,Toarcian. New data allowed us to refine the evolution of the two previously recognized Jurassic extensional events in the Southern Alps. The youngest extensional event (Western Southern Alps) occurred as tectonic activity decreased in the Lombardy Basin. During the Sinemurian the Gozzano high represents the western shoulder of a rift basin located to the east (Lombardy). This evolution documents a transition from diffuse early rifting (Late Hettangian,Sinemurian), controlled by older discontinuities, to rifting focused along a rift valley close to the Pliensbachian,Toarcian boundary. This younger rift bridges the gap between the Hettangian,Sinemurian diffuse rifting and the Callovian,Bathonian break-up. The late Pliensbachian,Toarcian rift, which eventually lead to continental break-up, is interpreted as the major extensional episode in the evolution of the passive margin of Adria. The transition from diffuse to focused extension in the Southern Alps is comparable to the evolution of the Central Austroalpine during the Early Jurassic and of the Central and Northern Atlantic margins. [source]