A. W. B. Jacob

Cretaceous and Tertiary basin development west of Ireland

The Porcupine, Rockall and Hatton–Rockall basins lie on the continental seaboard west of Ireland. Up to 9 km of Cretaceous and Tertiary strata are preserved in the Porcupine Basin, in contrast to 2–3 km in the Rockall and Hatton–Rockall basins. Differences in seismic stratigraphic sequences through the region are attributed to an interplay between the developing North Atlantic Ocean, post-rift thermal subsidence and sea-level changes. Localized Ryazanian fault-controlled alluvial fan clastic rocks in the Porcupine Basin are succeeded by Valanginian to Barremian marine deposits. Sea-level lowstands in Albian and Paleocene–Eocene times, interpreted as rift and ridge-push effects respectively, resulted in delta and submarine fan progradation. Rapid thermal subsidence in Early Oligocene times led to marine deposits and to the onset of geostrophic currents. Miocene slumping triggered shale-dominant turbidite development, followed by tranquil deep marine sediments as subsidence outstripped sedimentation. The Cretaceous sequence in the Rockall Trough is thought to contain extensive igneous bodies overlain by an Upper Cretaceous muddy chalk sequence which is cut by abundant Tertiary sills and dykes. A number of shale-prone Upper Tertiary sequences occur. The Upper Cretaceous to Lower Tertiary succession is thin and frequently absent in the Hatton–Rockall Basin.

A model for the development of a carbonate mound population in the Rockall Trough based on deep-towed sidescan sonar data

A large carbonate mound population is identified on deep-towed TOBI (towed ocean bottom instrument) sidescan data along the eastern margin of the Rockall Trough, west of Ireland. Individual mounds are circular to elliptical in plan view, varying from 50 to 850 m in width and up to about 200 m in height. Strong NE-flowing contour currents at 800 m water depth are inferred from large-scale sedimentary bedforms. Smaller mound arrays are spatially associated with slope-parallel escarpments produced by mass wastage, while around the largest mounds the sharp escarpments may have been smoothed by the vigorous contour currents. Current streamlining effects control the shape of the mounds, which become more elliptical as their size increases, thereby minimising the hydraulic drag force. The frequency distribution of mound size follows a general power law, which is determined by the growth rate of the framework-building coral species and the rate at which they colonised the substrate. Initially, bottom currents support mound growth until the mounds become so large that hydraulic drag forces retard their growth. A model for the evolution of the population predicts that increased hydraulic drag forces on the larger mounds cause a sharp decrease in their number, in agreement with the observations. The model also allows an age structure for the population to be determined and correlations between the growth of the mound population and palaeoclimatic variations in the NE Atlantic to be attempted.

Continental crust under the southern Porcupine Seabight west of Ireland

Abstract Two new seismic refraction/wide-angle reflection profiles demonstrate that the crust beneath the southern Porcupine Seabight, out to water depths in excess of 4000 m, is of continental type. They also reveal the rifted margin of the Porcupine basin on its eastern side. Crustal thickness under the Seabight, inclusive of sediments which are up to 6 km thick, decreases from 23 km in the east to about 10 km at a sharp continent-ocean transition in the west.

Structural setting, geological development and basin modelling in the Rockall Trough

Evidence from wide-angle seismic data in the Irish sector of the Rockall Trough suggests that the basin is underlain by thinned continental crust which has undergone differential stretching. The upper crust has been thinned by a stretching factor of 8–10 while the middle and lower crust (and probably also the lithospheric mantle) was stretched by a factor of 2–3. The latter figure is suggested as being representative of the overall lithospheric stretching. Crustal modelling fails to demonstrate any significant effect of the Iceland plume on the development of the basin. The Rockall Trough contains up to 6 km of sedimentary strata. Well and seismic data from adjacent basins indicate that the succession in the basin is likely to be of Late Palaeozoic to Recent age. The pre-Cretaccous faeies are suggested to be broadly similar to other basins of the Atlantic borderlands. Cretaceous and Tertiary strata show progressive clastic starvation as thermal subsidence outstripped sedimentation in the basin, Sandy facies are likely to be concentrated towards the faulted basin margins and the re-entrant regions where the basin margin changes orientation. Basin modelling, based on normal incidence and wide-angle seismic profiles from a number of areas in the Irish sector of the basin, demonstrates that the observed seismic geometries and subsidence patterns cannot be explained by a single rift episode in the Cretaceous, The best fit of possible models suggests that the basin developed in response to discrete rift episodes in the Triassic, Late Jurassic and Early Cretaceous.

A new look at the Rockall region, offshore Ireland

Abstract A 700 km wide-angle reflection/refraction profile carried out in the central North Atlantic west of Ireland crossed the Erris Trough, Rockall Trough and Rockall Bank, and terminated in the western Hatton-Rockall Basin. The results reveal the presence of a number of sedimentary basins separated by basement highs. The Rockall Trough, with a sedimentary pile up to 5 km thick, is underlain by thinned continental crust 8–10 km thick. Some major fault block structures are identified, especially on the eastern margin of the Rockall Trough and in the adjacent Erris Trough. The Hatton-Rockall Basin is underlain by westward-thinning continental crust 22–10 km thick. Sedimentary strata are up to 5 km thick. The strata in the Rockall Trough and Hatton-Rockall Basin probably range in age from Late Palaeozoic to Cenozoic. However, the basins have different sedimentation histories and differ in structural style. The geometry of the crust and sediments suggests that the Rockall Trough originated by pure shear crustal stretching, associated with rift deposits and Cenozoic thermal sag strata. In contrast, the development of the Erris Trough, located on unthinned continental crust, was facilitated by shallow, brittle extension with little deep crustal attenuation. A two-layered crust occurs throughout the region. The lower crustal velocity in the Hatton-Rockall Basin is higher than that in the Rockall Trough. The velocity structure shows no indication of crustal underplating by upper mantle material in the region.

Slope instability and sediment redistribution in the Rockall Trough: constraints from GLORIA

Abstract A recent GLORIA (Geological LOng Range Inclined Asdic) sidescan survey covered 200 000 km2 of the sea bed in the Irish Rockall Trough. It revealed a range of sedimentary features on the trough floor and its steep (>6°) margins. The western margin is characterized by large-scale (of the order of hundreds of kilometres in length) downslope mass movement. Smaller-scale slides and slumps (tens of kilometres across) occur on the eastern margin, but they are subordinate to canyon, channel and fan systems. The western and central parts of the trough floor contain the Feni Sediment Ridge, a 600 km long contourite sediment build-up covered by large sediment waves trending sub-parallel to the dominant modern current pattern. Strong, northward-flowing bottom currents are thought to have eroded the base of the slope in the east and redeposited the sediments on the western margin and the trough floor. Mass wasting and terrigenous sediment input through canyons is regarded as the primary source of sediment in the region. The increase in the degree and frequency of canyon incision along the NE margin of the trough reflects increased terrigenous input from the Irish mainland and a possible glacial influence on the basin margin. The GLORIA images reflect a broad interplay of alongslope and downslope sediment transport processes in the Rockall Trough with sediments sourced from the NE margin and redistributed by currents along the western margin. Although alongslope and downslope processes are the major controlling factors, basin subsidence, Quaternary glaciations and glacio-eustatic sea-level fluctuations have also influenced the pattern of sedimentation in the Rockall Trough.

Basin development and petroleum prospectivity of the Rockall and Hatton region

Abstract The Rockall region, offshore west of Ireland, contains a number of large, virtually unexplored, frontier sedimentary basins: Rockall Trough, Hatton Basin and Hatton Continental Margin. It is suggested that the sedimentary section, up to 6 km thick, is of Late Palaeozoic to Tertiary age. Normal incidence reflection seismic data from the region are typically of poor quality owing to the effects of shallow Tertiary sills. Recent wide-angle reflection seismic profiles have helped to resolve the basin geometry and structural setting. Backstripping (reverse modelling) and forward modelling of the wide-angle model are consistent with development in a complex multi-phase rift setting. Rift events are suggested in the Jurassic and Cretaceous, and possibly locally during the Permo-Triassic. The variation in structural style across the region is interpreted as resulting from the interplay of intraplate stresses, magmatic underplating, thermal doming and thermal subsidence adjacent to the continent/ocean margin, and seafloor spreading dynamics in the region to the west. The basins are predicted to contain a range of petroleum plays. Structural, stratigraphic and combination traps are indicated from a range of data. Reservoirs are predicted at several levels, with source rocks in the Jurassic and lowermost Cretaceous.

P-wave anisotropy in the lower lithosphere

Abstract Long-range controlled source seismic experiments yield detailed information about the velocity depth structure of the lower lithosphere. Between Ireland and Northern Britain three such profiles sample almost the same portion of the subcrustal lithosphere. Three zones with steep velocity gradients have been detected between 30 km and 90 km depth. Both the pattern and velocity of the arrivals are incompatible with isotropy. Preferential alignment of olivine crystals, with an azimuth ca. N25°E for the fast axis, could explain the observations, with the more highly aligned zones occurring in bands, or layers, separated vertically by zones in which the degree of alignment is slight or absent. We suggest that a shear heating mechanism may have played a part in producing these patterns. This deformation is most likely to be of Mesozoic or early Cenozoic age. It is argued that the upper mantle is not necessarily a strain marker for the last major orogenic episode, as recent findings have suggested [1], since it may undergo deformation which decouples from the brittle upper crust and hence is not “transmitted” to the Earths surface.

Slope failure features on the margins of the Rockall Trough

Abstract A TOBI sidescan sonar survey in the Irish sector of the Rockall Trough reveals the presence of a range of slope failure features of various sizes and extent along both the eastern and western margins. A number of different types are identified. These include incipient cuspate slides, slab failures and evolved slides, and debris flows. It is suggested that the incipient cuspate slides, slab failures and evolved slides represent slope failure of muddy sediments whereas the failures that gave rise to debris flows lie on steeper slopes and may be of less muddy composition. Many of the slope failure features are relatively recent (probably <15 ka), although some evidence points towards either a prolonged period of movement or a number of phases of slope movement locally along the margins. A comprehensive understanding of the nature, distribution, age and controls on the formation of the slope failure features will be necessary in planning the likely location of sea-bed structures in the event of petroleum development in the region.

Evidence for chaotic behaviour in seismic wave scattering

Scattering of seismic waves by lithospheric heterogeneities produces seismic coda which typically look random. Consequently stochastic modelling methods are usually applied to these coda [1–6]. Here, we introduce a technique, based on previous work [7], for evaluating the predictability of time-series data as an indicator of underlying determinism. When analyzed using this method both earthquake and explosive source seismic coda display strong short term predictability which is not consistent with a random generation mechanism.