Tove Nielsen

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 (

Sediments and sedimentation at the NE Faeroe continental margin; contourites and large-scale sliding

Abstract High-resolution seismic profiles of the NE Faeroe continental margin show a wedge-shaped sedimentary sequence of up to 2 km thickness on top of lower Tertiary basaltic basement. The sedimentary section can be divided into four major seismic sequences separated by unconformities. The uppermost upper slope deposit is interpreted as a contourite which formed later than the Mid-Miocene by the southerly flow of bottom water along the margin of the SW Norwegian Sea, which is subsequently deflected to the east by the NE Faeroe continental margin and Fugloy Ridge. A scoured channel about 85–90 m deep at about 1000 m water depth associated with the contourite partially follows and modifies a preexisting slump scar. Large-scale slumping and sliding of the middle and lower continental slope below 1500 m water depth have affected sediments of presumably Miocene, Pliocene and Quaternary age. A TOBI deep-tow side-scan sonar mosaic composed of ten lines recorded in 1995 semi-parallel to the slope, shows that mass flow deposits are partially covered by recent contourite sediments on the middle slope. The slump complex at the middle/lower continental slope is younger and has a steep, 250–300 m, irregular, deep main scarp and very large, sometimes rotated blocks near the main scarp. At the base of the slide, numerous 10–15 km long, narrow tracks with individual blocks at the end are found. Several types of debris flows have been mapped, some with a longitudinal flow fabric.

Cenozoic sediment distribution and tectonic movements in the Faroe region

Abstract A complex history of Cenozoic vertical movements in the Faroe region has been revealed from interpretation of geophysical and geological data, mainly offshore reflection seismic data, side-scan images, shallow cores, and onshore mapping. The history comprises several phases of tectonic disturbances observed at different scales. On the eastern margin of the Faroe Platform a late Eocene–early Oligocene phase of doming of the Faroe Platform has caused a postdepositional tilting of Eocene strata along the southern margin of the platform; a mid-Miocene phase of compressional tectonics is evidenced on seismic transects as gentle anticlines and associated reverse faults; and possible Pliocene uplift of the Faroe Islands is indicated by a progradational wedge of sediments deposited on the eastern Faroe Platform. At the continental margin/slope north of the Faroe Platform, reflection seismic data imaging the postbasalt sedimentary strata indicate three distinct tectonic events phases in the Eocene–Oligocene, Miocene and Pliocene, respectively. In contrast to the Faroe Platform the Faroe–Shetland Channel was characterised by more or less continuous subsidence dominated throughout the Cenozoic. During the Eocene, sediments deposited in the Faroe–Shetland Channel was mostly derived from a source area on the British shelf.

Norwegian Sea overflow through the Faroe-Shetland gateway as documented by its bedforms

We have used seabed information from three deep-tow side-scan sonar surveys in order to trace the high-energy current core of Norwegian Sea Overflow Water (NSOW) from the Faroe–Shetland Channel (FSC) through the Faroe Bank Channel (FBC) towards the southern flank of the Iceland–Faroe Ridge. A hierarchy of bedform types was used for estimating maximum mean near-bottom current speed, which has been compared with results from current meter measurements. We conclude that in the FSC maximum mean NSOW near-bottom flow speed at some sites may occasionally approach or even exceed 1.0 m/s. Both seabed data and oceanographic information indicate that the current core is concentrated along the Faroes slope at the 500–600-m depth stratum. With a change of the large-scale channel topography towards the southern extremity of the Faroe Plateau, where the FSC turns west towards the FBC, the slope current core gradually descends towards basin depth. Along the Wyville–Thomson Ridge (1100–1200 m water depth) maximum mean NSOW near-bottom current speed decreases slightly towards the west as the high-speed current core runs upslope the ridge. At the entrance to the narrow channel between Faroe Bank and the Faroe Plateau funnelling results in a renewed current acceleration at basin depth. Seabed evidence shows that further west in the FBC the NSOW high-speed current core is detached from the channel floor over larger areas. From the FBC outlet most of the overflow waters continue as a high-energy contour current concentrated along the southern flank of the Iceland–Faroe Ridge between 600 and 1000 m water depth.

Towards an understanding of the Neogene tectonostratigraphic framework of the NE Atlantic margin between Ireland and the Faroe Islands

Abstract The Neogene succession preserved on the continental margin between Ireland and the Faroe Islands can be divided into two megasequences of Miocene and Pliocene–Holocene age. The base of each megasequence is marked by a regional unconformity. These are of latest Oligocene/early Miocene and early Pliocene age respectively and reflect major phases of Neogene margin evolution. The megasequences and their bounding unconformities reflect a gross two-stage depositional history; predominantly a response to intra-plate tectonism that modified sedimentation patterns and palaeoceanographic circulation. The latest Oligocene/early Miocene event marks a major change in the oceanographic regime, and can be linked to the establishment of deep-water exchange between the Arctic and North Atlantic oceans. The submergence of the Greenland–Scotland Ridge, which includes the Iceland–Faroe Rise and the Wyville–Thomson Ridge in the area of study, resulted in locally vigorous deep-water erosion as northern source deep water flowed through the Faroe conduit, which is part of the Southern gateway for deep-water exchange. Coeval deposition of sediment drifts occurred in the Faroe–Shetland Channel and Rockall Trough as the deep-water currents stabilised. The early Pliocene event marked the widespread instigation of shelf-margin progradation to the north–west of Britain and Ireland, and to the east of the Faroe Islands. This event is linked to the uplift and erosion of Britain, Ireland and the Faroe Islands, and is part of a larger North Atlantic-scale uplift. Significant deep-water erosion in the Rockall Trough suggests that the water circulation pattern was also modified by this event. The formation of the key megasequence boundaries occurred during times of significant plate rearrangement, and suggests that these events may be correlatable across the whole NW European margin.

Seismic stratigraphy and sedimentary processes at the Norwegian Sea margin northeast of the Faeroe Islands

Abstract Sedimentary processes on the continental slope north of the Faeroe Islands during the Cenozoic were studied in detail using high-resolution multichannel reflection seismic data. A wedge-shaped succession of Cenozoic sediments with a maximum thickness of approximately 2 km overlies the basaltic basement, which forms the underlying structure of the slope. The sedimentary succession represents four depositional sequences and a contourite deposition. Also some slump deposits are recognised. The oldest sequence, which is possibly of Eocene–Late Oligocene age, was prone to repeated mass movements during the deposition of the sequence. At the end of the Oligocene, the sediments on the slope stabilised, and the depositional pattern was strongly influenced by sea level changes. In the Late Pliocene, a renewed instability of the slope occurred, resulting in mass movements causing erosion at the middle of the slope and creating a major slide escarpment at the base of the slope. Subsequently, the sedimentary processes change from mainly downslope, progradational forms to along-slope, current-induced contourite deposition. This change most likely marks the formation of a Norwegian Sea Deep Water current pattern similar to the present. Variations of the current strength and sediment supply caused by glacial–interglacial cycles probably results in an alternating sedimentation pattern of the contourite deposit. Another impact from the glaciation of the Northern Hemisphere is the presence of iceberg turbate at the upper parts of the northeastern Faeroes slope.

Cenozoic evolution of the Faroe Platform: comparing denudation and deposition

Abstract Throughout Paleocene and Eocene time the Faroe-Shetland Channel and the eastern part of the Faroe Platform was a subsiding marine basin. In Early Paleocene time, basin-floor fans of a British provenance were deposited in the eastern part of the basin. In Late Paleocene time, c. 6 km of basalt entered the basin from the west and north, and the basin was constricted by the large volumes of basalt that entered the basin, creating the Faroe-Shetland Escarpment. In Eocene time subsidence continued in the basinal areas. Again, sediments of a dominantly eastern provenance were deposited. Throughout Eocene time, erosion products from the Faroe Platform were possibly deposited in the Faroe Bank Channel and the Norwegian Sea Basin, but only to a limited degree in the Faroe-Shetland Channel. The oldest sediments of documented western provenance on the eastern margin of the Faroe Platform are of Early Oligocene age. During a compressional phase commencing in Mid-Late Miocene time some basinal areas emerged and erosion took place on the top of emerged anticlines. However, denudation throughout Late Miocene and Early Pliocene time was apparently rather limited compared with a Late Pliocene phase of denudation. During this phase of denudation, a large progradational wedge was deposited on the eastern margin of the Faroe Platform. On the basis of a structural analysis of the Faroe Platform, the amount of basalt removed from it during Cenozoic time is estimated to be c. 46 000 km3 (131 100 × 1012 kg). Using 2900 kg m−3 as the density of basalt and 2300 kg m−3 as sediment density the estimated amount of removed basalt is in fair agreement with the estimate of the volume of sediments derived from the platform (c. 56 000 km3, 114 800 × 1012 kg). The greatest deposition rates on the eastern Faroe Platform and in the Faroe-Shetland Channel apparently occurred after two distinct inversion or compression events in Mid-Eocene and Mid-Late Miocene time. However, uplift of the Faroe Platform could have been forced by denudation rather than endogenous processes.

Large submarine slides on the NE Faeroe continental margin

Abstract High-resolution seismic profiles of the NE Faeroe margin show large-scale slumping and sliding of the middle and lower continental slope, affecting sediments of presumed Miocene, Pliocene and Quaternary age. Mass-flow deposits on the upper slope are partially covered by more recent deposits, presumed to be contourites. A TOBI deep-tow side-scan sonar mosaic, in combination with deep-towed penetrating echosounder results, shows that the slump complex on the upper slope consists of a buried slide scar, bottleneck slides, debris-flow lobes and a number of shallow slides. Sliding at the middle and lower continental slope seems more recent and shows a steep, irregular main slump scar and very large, intact, angular blocks. The base of the slide at the lower continental slope shows numerous narrow and diverging tracks, 10–15 km length, that end at individual blocks. Several types of debris flows have been mapped, some with longitudinal flow fabrics.

Landslide and Tsunami 21 November 2000 in Paatuut, West Greenland

A large landslide occurred November 21, 2000 at Paatuut, facing the Vaigat Strait onthe west coast of Greenland. 90 million m3 (260 million tons) of mainly basalticmaterial slid very rapidly (average velocity 140 km/h) down from 1,000–1,400 maltitude. Approximately 30 million m3 (87 million tons) entered the sea, creatinga tsunami with an run-up height of 50 m close to the landslide and 28 m at Qullissat,an abandoned mining town opposite Paatuut across the 20 km wide Vaigat strait. Theevent was recorded seismically, allowing the duration of the slide to be estimated tocirca 80 s and also allowing an estimate of the surface-wave magnitude of the slideof 2.3. Terrain models based on stereographic photographs before and after the slidemade it possible to determine the amount of material removed, and the manner ofre-deposition. Simple calculations of the tsunami travel times are in good correspondencewith the reports from the closest populated village, Saqqaq, 40 km from Paatuut, whererefracted energy from the tsunami destroyed a number of boats. Landslides are notuncommon in the area, due to the geology with dense basaltic rocks overlying poorlyconsolidated sedimentary rocks, but the size of the Paatuut slide is unusual. Based onthe observations it is likely at least 500 years since an event with a tsunami of similarproportions occurred. The triggering of the Paatuut slide is interpreted to be caused byweather conditions in the days prior to the slide, where re-freezing melt water inpre-existing cracks could have caused failure of the steep mountain side.

Snapshots of the Greenland ice sheet configuration in the Pliocene to early Pleistocene

The geometry of the ice sheets during the Pliocene to early Pleistocene is not well constrained. Here we apply an ice-flow model in the study of the Greenland ice sheet (GIS) during three extreme intervals of this period constrained by geological observations and climate reconstructions. We study the extent of the GIS during the Mid-Pliocene Warmth (3.3–3.0Ma), its advance across the continental shelf during the late Pliocene to early Pleistocene glaciations (3.0–2.4Ma) as implied by offshore geological studies, and the transition from glacial to interglacial conditions around 2.4Ma as deduced from the deposits of the Kap København Formation, North Greenland. Our experiments show that no coherent ice sheet is likely to have existed in Greenland during the Mid-Pliocene Warmth and that only local ice caps may have been present in the coastal mountains of East Greenland. Our results illustrate the variability of the GIS during the Pliocene to early Pleistocene and underline the importance of including independent estimates of the GIS in studies of climate during this period. We conclude that the GIS did not exist throughout the Pliocene to early Pleistocene, and that it melted during interglacials even during the late Pliocene climate deterioration.