Martyn S. Stoker

The history of Quaternary sedimentation in the UK sector of the North Sea Basin

Reconnaissance seismic and sampling surveys have shown that up to 500 m of Lower and early Middle Pleistocene deltaic sediments are preserved in the UK sector of the North Sea. The distributions of delta-related facies record the progressive expansion of peripheral delta systems northwards across the southern North Sea during the early Pleistocene. A late Middle Pleistocene unconformity is present throughout the UK sector, and sedimentation above the unconformity was dominated by glacial erosional and depositional processes. A complex system of deeply incised valleys was eroded during each of three regional Middle and Upper Pleistocene glaciations and the valleys have been filled or partially filled by glaciolacustrine or glaciomarine and interglacial marine sediments. The valleys of the earliest (Elsterian) glaciation are up to 400 m deep in the southern North Sea. The Saalian and Weichselian valleys are commonly more than 100 m deep, they are eroded through their contemporary tills nearshore but occur well beyond the limits of these tills offshore. The origins of these valleys are attributed to a combination of fluvial, subglacial and periglacial processes

Sediment drifts and contourites on the continental margin off northwest Britain

Abstract Seismic reflection profiles and short cores from the continental margin off northwest Britain have revealed a variety of sediment-drift styles and contourite deposits preserved in the northeast Rockall Trough and Faeroe-Shetland Channel. The sediment drifts include: (1) distinctly mounded elongate drifts, both single- and multi-crested; (2) broad sheeted drift forms, varying from gently domed to flat-lying; and (3) isolated patch drifts, including moat-related drifts. Fields of sediment waves are locally developed in association with the elongate and gently domed, broad sheeted drifts. The contrasting styles of the sediment drifts most probably reflect the interaction between a variable bottom-current regime and the complex bathymetry of the continental margin. The bulk of the mounded/gently domed drifts occur in the northeast Rockall Trough, whereas the flat-lying, sheet-form deposits occur in the Faeroe-Shetland Channel, a much narrower basin which appears to have been an area more of sediment export than drift accumulation. Patch drifts are present in both basins. In the northeast Rockall Trough, the along-strike variation from single- to multi-crested elongate drifts may be a response to bottom-current changes influenced by developing drift topography. Muddy, silty muddy and sandy contourites have been recovered in sediment cores from the uppermost parts of the drift sequences. On the basis of their glaciomarine origin, these mid- to high-latitude contourites can be referred to, collectively, as glacigenic contourites. Both partial and complete contourite sequences are preserved; the former consist largely of sandy (mid-only) and top-only contourites. Sandy contourites, by their coarse-grained nature and their formation under strongest bottom-current flows, are the most likely to be preserved in the rock record. However, the very large scale of sediment drifts should be borne in mind with regard to the recognition of fossil contourites in ancient successions.

Bottom-current sediment waves, sediment drifts and contourites in the northern Rockall Trough

Four types of large-scale sediment bedform occur in the northeast Rockall Trough: broad sheeted drifts, elongate drifts, sediment waves and thin contourite sheets. A large sheeted drift occupies the northern basin; its crest can be traced for 40 km perpendicular to the bottom-current flow direction. Elongate drifts occur adjacent to steep areas of continental slope, separated from the break of slope by a moat. Drift crests are oriented alongslope, parallel to bottom-current flow. Elongate drifts are asymmetric, with the steeper flank facing upslope; sedimentation rates are higher on the downslope flank. Sediment waves, with a wavelength of 1–2 km and wave height up to 20 m, occur locally on the flanks of sheeted and elongate drifts; larger waves (wavelength of 3–4 km and wave height of 50–100 m) occur on the flank of the Wyville–Thomson Ridge. Wave crests are oblique to bathymetric contours, to drift crests and to bottom-current flow. The association of sediment waves and drifts, their orientation oblique to regional slopes, and the presence of contourite sediments in cores, indicate that these are bottom-current rather than turbidity-current waves. A contourite sheet, typically 10–25 cm in thickness and composed of well sorted sand, covers the present-day seafloor along the lower slope. Bedform evolution through time has involved a complex interaction between bottom currents of variable intensity, sea level change and sediment input. The present sea level highstand is characterised by minimal sediment input and redistribution of sediments by strong bottom currents, giving active bedform growth and contourite development. Variability of sediment input is the main factor affecting sedimentation during sea level lowstands. Where input was high, as when ice sheets reached the shelf edge, downslope sediment transport dominated, overwhelming any bottom currents that were active. When input was low, such as during the last lowstand, some evidence for active currents can be observed, although these were less energetic than those during highstands.

Cenozoic exhumation of the southern British Isles

Rocks that crop out across southern Britain were exhumed from depths of as much as 2.5 km during Cenozoic time. This has been widely attributed to Paleocene regional uplift resulting from igneous underplating related to the Iceland mantle plume. Our compilation of paleothermal and compaction data reveals spatial and temporal patterns of exhumation showing little correspondence with the postulated influence of underplating, instead being dominated by kilometer-scale variations across Cenozoic compressional structures, which in several basins are demonstrably of Neogene age. We propose that crustal compression, due to plate boundary forces transmitted into the plate interior, was the major cause of Cenozoic uplift in southern Britain, witnessing a high strength crust in western Europe.

Neogene evolution of the Atlantic continental margin of NW Europe (Lofoten Islands to SW Ireland): anything but passive

A regional stratigraphic framework for the Neogene succession along and across the NW European margin is presented, based on a regional seismic and sample database. The stratigraphy provides constraints on the timing and nature of the mid- to late Cenozoic differential tectonic movements that have drivenmajor changes in sediment supply, oceanographic circulation and climate (culminating in continental glaciation). The overall context for Neogene deposition on the margin was established in the mid-Cenozoic, when rapid, km-scale differential subsidence (sagging) created the present-day deep-water basins. The Neogene is subdivided into lower (Miocene–lower Pliocene) and upper (lower Pliocene–Holocene) intervals. The lower Neogene contains evidence of early to mid-Miocene compressive tectonism, including inversion anticlines and multiple unconformities that record uplift and erosion of basin margins, as well as changes in deep-water currents. These movements culminated in a major expansion of contourite drifts in the mid-Miocene, argued to reflect enhanced deep-water exchange across the Wyville-Thomson Ridge Complex, via the Faroe Conduit. The distribution and amplitude of the intra-Miocene movements are consistent with deformation and basin margin flexure in response to enhanced intra-plate compressive stresses during a local plate reorganization (transfer of the Jan Mayen Ridge from Greenland to Europe). The upper Neogene records a seaward tilting (

The Minch palaeo-ice stream, NW sector of the British–Irish Ice Sheet

Geophysical data from the UK continental shelf off NW Scotland reveal strongly parallel subglacial bedforms on the sea bed. Field mapping on the adjacent landmass has identified closely spaced bedrock megagrooves and highly elongate drumlinoid forms. All these mega-scale glacial lineations probably formed beneath the same fast-flowing zone of the last British–Irish Ice Sheet, The Minch palaeo-ice stream. This ice stream drained the NW sector of the British–Irish Ice Sheet during the Late Devensian Glaciation (marine isotope stage 2) and terminated near the edge of the continental shelf. The size of the adjacent trough-mouth Sula Sgeir Fan and the presence of buried mega-scale glacial lineations within the Quaternary stratigraphic record imply that fast ice-sheet flow in The Minch has been a feature of several mid- to late Pleistocene glaciations.

Late Cenozoic sediment drift complex, northeast Rockall Trough, North Atlantic

Seismic reflection profiles and two shallow cores have revealed a sediment drift complex in the northeastern Rockall Trough.The drift complex consists of an elongate drift with associated sediment waves, a broad sheeted drift with a larger field of sediment waves, and smaller, moat-related, isolated drifts. The internal reflection configuration of these features indicates consistent upslope migration throughout the late Cenozoic. The larger field of sediment waves has been described previously with a southerly migration; a reevalution of these waves suggests a migration direction to the east, conforming with the other drift complex features. The most vigorous current activity probably occurred during the Miocene, with a more reduced current flow prevailing during the Pliocene to Holocene interval. Core evidence from the elongate drift and the moat has revealed upper Pleistocene and lower Holocene glaciomarine sediments reworked to produce muddy-silty and sandy contourites. Sedimentation rates for the early Holocene (pre-7.5 ka) are up to 4-times greater on the drift compared to the moat. It is suggested that the sediment drift complex has formed through the interaction of a northward flowing slope current, of North Atlantic Deep Water origin, with an area of complex bathymetry at the northeastern end of the Rockall Trough, where the Wyville-Thomson Ridge intersects the Hebridean Margin.

A record of late Cenozoic stratigraphy, sedimentation and climate change from the Hebrides Slope, NE Atlantic Ocean

A punctuated 103.3 m thick succession of upper Palaeogene to Quaternary sediments has been recovered in a borehole from the upper Hebrides Slope, west of Britain. The borehole proved 11.2m of upper Oligocene, carbonate-rich muds at the base, unconformably overlain by 2.85 m of middle to upper Miocene, glauconitic sands. This is in turn unconformably overlain by 89.25 m of predominantly Plio-Pleistocene sands and muds, with a Holocene sea-bed veneer. The post-Miocene succession is subdivided into two units: the sand-dominated, Pliocene to lower middle Pleistocene, Lower MacLeod sequence between 89.25 and 67.82 m, and the mud-dominated, middle Pleistocene to Holocene, Upper MacLeod sequence above 67.82 m. Regional mapping indicates that these sequences are commonly associated with large-scale shelf-margin progradation and slope-front fan construction. The borehole core provides an excellent record of the transition from pre-glacial to glacial conditions in the mid-latitude NE Atlantic Ocean. Climatic conditions warmer than present prevailed in the late Oligocene, mid- to late Miocene and Pliocene, although the influx of ice-rafted detritus in the late Pliocene marks the onset of climatic deterioration. This deterioration continued, in a fluctuating manner, until the early mid-Pleistocene (0.44 Ma) when fully glacial conditions were established on the Hebridean Margin.

Mid- to late Cenozoic sedimentation on the continental margin off NW Britain

The present morphological expression of the continental margin off NW Britain is a mid- to late Cenozoic phenomenon, initiated by a major phase of rapid subsidence in the Rockall Trough and Hatton-Rockall Basin during the early late Eocene. This led to a deepening of the basins and the onset of bottom-current activity in this region. On seismic-reflection profiles, this basin-subsidence event is manifest in the form of a widespread deep-water unconformity caused by bottom-current erosion. This boundary is particularly enhanced at the basin margins and adjacent to the axial seamounts of Rosemary Bank and Anton Dohrn, where the downwarped and eroded surface of lower upper Eocene and older strata is onlapped by middle to upper Cenozoic sediments. The latter comprise two megasequences of late Eocene to mid-Miocene and mid-Miocene to Holocene age, which consist predominantly of deep-marine contourites both in the Rockall Trough and the Hatton-Rockall Basin, although a clastic wedge has built out along the Hebridean shelf-margin since the mid-Miocene. These megasequences reflect a gross, two-stage, depositional history; a response to intra-plate tectonism which modified sedimentation patterns and palaeoceanographic circulation. The development of the bounding unconformities (early late Eocene and mid-Miocene) was coincident with major phases of regional tectonism.

Submarine end-moraines as indicators of Pleistocene ice-limits off northwest Britain

High-resolution (sparker) seismic profiles reveal evidence for glaciation of the outer part of the northern Hebrides and West Shetland shelves. Submarine end-moraines suggest glacial ice which locally extended out to the shelfbreak. Seismic stratigraphic analysis of the glacial succession suggests that the moraines on the northern Hebrides Shelf are older than those developed on the West Shetland Shelf. Whist the stratigraphic evidence indicates at least two phases of widespread outer shelf glaciation, the timing of these events remains uncertain.