Rockall Continental Margin Report. Final geological report (5 volumes)

The Rockall Continental Margin Project was a 3-year research programme, undertaken between April 1992 and March 1995, designed to investigate the geology and resource potential of part of the frontier area west of Scotland. The programme was funded by a consortium comprising the British Geological Survey (BGS) and 8 exploration companies - BP, British Gas, Conoco, EE Caledonia, Elf, Enterprise, Esso and Mobil. The study has focused on the central and northern Rockall Trough, although several long transect lines were run across the Rockall Plateau and into the Iceland Basin to provide a margin-wide assessment of the geological framework. Over the duration of the project, multichannel seismic, gravity, magnetic and bathymetry data, together with boreholes and shallow-sample information were acquired by the consortium. These data form the basis of this 5-volume report. A descriptiono f the geology and prospectivityo f the surveyed area comprises volume 1, whilst the data on which the geological interpretation is based, including biostratigraphy, petrology and geochemistry of the boreholes and shallow samples, are presented in volumes 2 to 5. The widespread distribution of Upper Cretaceous to lower Palaeogene volcanic rocks continues to hinder our understandionfg t he geological frameworkof the Rockall Continental Margin. Whilst it has been proved that Lower Proterozoic gneisses form continental basement on Rockall Bank, and are therefore part of the Islay structural terrane, the pre-Cretaceous supracrustal infill of the Rockall Trough and Hatton-Rockall Basin remains conjectural. Reworked palynomorphs of Carboniferous, Jurassic and Early Cretaceous age have been identified in lower Palaeogene sediments recovered on the western margin of the Rockall Trough, but their provenance is uncertain. The present morphological expressiono f the Rockall Continental Margin largely reflects late Mesozoic-Cenozoic extensional tectonism associated with North Atlantic sea-floor spreading. In mid-Cretaceous time, the Rockall Trough was the focus of extreme crustal attenuation associated with abortive continental breakup along the axis of the basin. The growth of the axid Rosemary Bank and Anton D o h s eamounts may have been initiated duringth is phase of crustal thinning. When the axis of spreading shifted westwards, the margin was affected by extensive volcanism concomitant with the split between Rockall Plateau and Greenland, that formed the North-East Atlantic OceaInn. the study area, this volcanismis manifested by the areally extensive, Paleocene to lower Eocene lavas and sills. The continental margin subsequently underwent regional differential subsidence punctuated by intermittent tectonism. This is reflected in the post-volcanic, sedimentary succession for which, for the first time, a unified seismic stratigraphy linking the Rockall Trough and Hatton-Rockall Basin has been established. Three main post-volcanic seismic-stratigraphical sequences have been defined; upper Paleocene to lower upper Eocene, upper Eocene to middle Miocene, and middle Miocene to Holocene. Stratigraphical control and inter-basin correlation are based on a databaseo f BGS boreholes and shallow samples, DSDP boreholes and well 164/25-2 (courtesy of BP). Late Paleocene to early late Eocene sedimentation occurred amidst continuing tectonic instability across the margin. This has been well demonstrated on the edge of Rockall Bank where a prograding shelf-margin sequence penetratebdy borehole 94/3 preserves a record of fluctuating alluvial to shallow-marine sedimentation, interrupted by phases of uplift, erosion and sporadic volcanism. Late Eocene subsidence in the Rockall Trough and Hatton-Rockall Basin provided the downwarped, basin-margin unconformity onto which upper Eocene to middle Miocene sediments onlap. This is a major sequence boundary and essentially marks the onset of deep-water, current-controlled sedimentation in both basins. In the Rockall Trough, the main buildup of the Feni Ridge sediment drift occurred during this interval. A phase of mid-Miocene tectonism resulted in the initiationo f the Barra Fan, on the eastern margin of the Rockall Trough, and may also have instigated a change in regional palaeoceanography culminating in the developmenotf a widespread unconformity across the Rockall Trough and Hatton-Rockall Basin. On the Hebrides Slope, middle Miocene to Holocene sediments form a thick, prograding, clastic weAd gthei.n ner package of deep-water sediments is preserved in the Rockall Trough and, on the western margin of the trough, an erosional regime has prevailed throughout this interval. This has resulted in a marked asymmetry to the depositional sequence architecture across the Rockall Trough. The Hatton- Rockall Basin was similarly dominated by deep-water processes but, in contrast to the Rockall Trough, a much thicker sedimentary succession has accumulated in this basin. In terms of prospectivity, circumstantial evidence suggests that Carboniferous, Mesozoic or lower Tertiary source rocks may be present in the Rockall Trough, but no definite thermogenic source has been proved. Potential hydrocarbon indicators include gas blanking, fluid-migration structures and locally high methane concentrations in surface sediments. The identification of tilted fault blocks on the western margoifn the Rockall Trough, in bothU K and Irish waters, illustrates one possible hydrocarbon-trapping mechanism that may be applicable to this area. Other potential trapping styles include fault-scarp fans and lowstand slope-apron or basin-floor fans.