Eocene (Stronsay Group) post-rift stratigraphy of the Faroe-Shetland region

A preliminary stratigraphic framework for the post-rift Eocene Stronsay Group succession in the Faroe–Shetland region has been constructed, which incorporates lithological information from over fifty wells, boreholes and dredge sites, biostratigraphic data provided by Ichron Limited, and seismic stratigraphy. A stratigraphic-range chart was compiled using released UK and Faroese commercial well data, together with BGS and other public domain information. This chart details the chronological range, general lithology and correlation of the post-rift Eocene record for each commercial well, BGS borehole and other data point, e.g. DSDP site. This dataset was used to construct a set of timeslice maps utilising the Ichron Limited T-sequence biozonation scheme, which were used to interpret the spatial and temporal variation of Eocene post-rift sedimentation across the region. Seismic profiles further provided an insight into the large-scale stratigraphic architecture of the Stronsay Group which, in combination with the geological database, provides a context for several regional unconformities and other significant surfaces identified in the study. On the basis of our provisional assessment of the Stronsay Group in the Faroe–Shetland region we have retained the use of the Horda Formation as the main lithostratigraphic unit, as this shelfmargin to basinal unit – defined originally in the North Sea – is consistent with the character of the Eocene succession that we observe preserved offshore NW Britain. Nonetheless, informal subdivision of the Horda Formation is proposed that reflects several discrete phases of sedimentary input into the Faroe-Shetland Basin. Four unconformity-bounded depositional packages have been tentatively identified, and provisionally assigned ages of Ypresian–early Lutetian (phase 1), Lutetian–early Bartonian (phase 2), Bartonian–Priabonian (phase 3), and late Priabonian (phase 4). The phase 1 and 2 depositional packages are separated by the Mid-Eocene (T2d) unconformity which reflects subaerial exposure and channel incision up to 200 m deep during its formation in the Lutetian. The early Bartonian Base-slope-apron (T2c) unconformity separates the phase 2 and 3 depositional packages, and is similarly erosive along the southern and eastern basin margin. Seismic-stratigraphic evidence suggests that synsedimentary deformation may have been active during depositional phases 1 and 2, including growth and uplift of the Munkagrunnur Ridge, and the Judd and Westray anticlines, as well as uplift of the Flett High. The phase 3 depositional package marks a large-scale progradation of the West Shetland margin, which may reflect uplift and rejuvenation of the hinterland. The phase 3 and 4 depositional packages are separated by the Base-marginal-fan (T2b) reflector, which may be a consequence of renewed uplift of the margin and/or further growth of the inversion anticlines during the Priabonian. This stratigraphic framework provides new insights into the early post-rift tectonic and sedimentary history of the Faroe–Shetland region, identifying a sequence of unconformitybounded units. Comparison with the wider NE Atlantic region indicates broad coincidence between the timing of formation of the unconformities in the Faroe–Shetland region and plate reorganisation events in the adjacent Norway Basin; as well as orogenic and related compressional deformation in western Europe. This raises the possibility that plate boundary forcing may be a key mechanism in passive margin development. It is hoped that this framework will form a useful foundation for future studies of the tectonostratigraphic development of the Faroe–Shetland region

An overview of the Upper Paleozoic-Mesozoic stratigraphy of the NE Atlantic region

This study describes the distribution and stratigraphic range of the Upper Paleozoic–Mesozoic succession in the NE Atlantic region, and is correlated between conjugate-margins and along the axis of the NE Atlantic rift system. The stratigraphic framework has yielded important new constraints on the timing and nature of sedimentary basin development in the NE Atlantic, with implications for rifting and the breakup of the Pangaean supercontinent. From a regional perspective, the Permian–Triassic succession records a northward transition from an arid interior to a passively-subsiding, mixed carbonate/siliciclastic shelf margin. A Late Permian–earliest Triassic rift pulse has regional expression in the stratigraphic record. A fragmentary paralic to shallow-marine Lower Jurassic succession reflects Early Jurassic thermal subsidence and mild extensional tectonism; this was interrupted by widespread Mid-Jurassic uplift and erosion, and followed by an intense phase of Late Jurassic rifting in some (but not all) parts of the NE Atlantic region. The Cretaceous succession is dominated by thick basinal-marine deposits, which accumulated within and along a broad zone of extension and subsidence between Rockall and NE Greenland. There is no evidence for a substantive and continuous rift system along the proto-NE Atlantic until the Late Cretaceous

Cenozoic pre- and post-breakup compression in the Faroe-Shetland area, within the context of the NE Atlantic

This report is primarily based upon the interpretation of oil industry 2D seismic data, and aims to elucidate aspects of Cenozoic tectonostratigraphic development in the Faroe–Shetland region, especially with regard to post-breakup compression. Evidence of Cenozoic and Late Cretaceous pre-breakup compression and deformation is briefly reviewed. We have utilised established seismo-stratigraphic frameworks and a recently updated scheme for the post-breakup Eocene (Stronsay Group) succession, which are largely based upon the recognition of units bounded by regional unconformities. The seismic expression, extent and thickness of the seismo-stratigraphic units are illustrated by geoseismic profiles, structure contour maps and isochore maps, which are used to analyse the spatial and temporal development of post-breakup compression and deformation within the Faroe-Shetland region. The Faroe-Shetland region records a complex spatial and temporal pattern of departures from the thermal subsidence normally associated with passive margins, including broad uplifts and accelerated basinal subsidence together with fold development up to kilometre scale. The phases of latest Eocene / earliest Oligocene ‘sagging’ (accelerated subsidence) and early Pliocene uplift and exhumation (tilting) appear to be coeval with compression. Indeed, compression appears to have been active throughout post-breakup times, although the loci of deformation have varied both spatially and temporally. Conceivably, some of the large scale sagging, tilting and uplift may be associated with lithospheric folding. Much of the intra-Eocene folding appears to be focused in the southwestern part of the Faroe-Shetland region, around the Munkagrunnur Ridge and Judd area, where phases of shelf progradation are preserved and may be associated with contemporaneous uplift. However, there also appears to be evidence of episodic intra-Eocene and younger uplift in the area around the northern Fugloy Ridge. The overall shaping of the Faroe-Shetland Channel appears to have been initiated at the end of the Eocene, associated with uplift on the Fugloy Ridge and Faroe Platform areas, and with accelerated subsidence in the Faroe-Shetland Basin; this shaping was further developed during the Neogene. A Neogene opening of the ‘Faroe Conduit’ oceanic gateway is favoured on the basis of regional evidence of faunal isolation and restricted environment of deposition together with uncertainty regarding the nature of the ‘Southeast Faroes drift’. A significant phase of Miocene folding is associated with the Intra-Miocene Unconformity (IMU), whereas the Mid Miocene Unconformity (MMU) represents a relatively minor break with a restricted distribution in the NE Faroe-Shetland region. Seabed relief on some folds and late Neogene seismic onlaps may indicate that fold development persisted into Recent times. Lateral offsets and local basin inversion associated with the folding, suggest a strong structural inheritance from the underlying rift architecture. A broad coincidence between the timing of formation of the unconformities and plate reorganisation events in the adjacent Norway Basin and wider region may suggest that these events made important contributions to the forces shaping the margin. The development of Miocene and younger folds may have been influenced by gravitational potential energy / body forces associated with the density structure of the Iceland Insular Margin and the Southern Scandes, or with modulations to ridge-push resulting from transient changes in ridge elevation associated with plume-related temperature (buoyancy) variations in the underlying asthenosphere. Far field stresses associated with, for example, collision between Eurasia and Iberia may also have exerted significant influence on deformation within the Faroe-Shetland region

Dynamic evolution of the Faroe-Shetland region

This report presents a series of twenty palaeoenvironment maps that span the interval between the Late Jurassic and the Quaternary, and which form the basis of a generalised reconstruction of the late Mesozoic–Cenozoic development of the Faroe–Shetland and adjacent region. The database behind this study is firmly grounded within the portfolio of existing FSC stratigraphic reports (Cretaceous to Eocene), though it also includes new work that has extended the stratigraphic time series back to the Kimmeridgian (Late Jurassic), and forward to the Mid-Pleistocene. A synthesis of the main structural elements – basins, highs, faults, folds – is also included as these features provide a reference framework on the maps, as well being indicators of contemporary deformation spanning the pre-, syn- and post-breakup stages of NE Atlantic development in this region. By considering the palaeoenvironment maps (our observations) we identify the following key stages in the late Mesozoic–Cenozoic ‘dynamic evolution’ of the Faroe–Shetland region: • Intermittent and localised rifting in the Late Jurassic (mid-Kimmeridgian–earliest Berriasian) and Early Cretaceous (late Berriasian–Hauterivian). • Early Cretaceous (Aptian–Albian) instigation of rifting in the Faroe-Shetland Basin with maximum extension and basin widening in the Late Cretaceous (Coniacian–Maastrichtian). Localised uplift, compression and folding in various basins, particularly in Cenomanian–Turonian. • The Paleocene onset of major extrusive volcanism initiated close to Danian/Selandian boundary; growth of major basaltic shield of Selandian–Thanetian age overlying continental crust in the vicinity of the Faroe Platform; plate breakup and associated volcanism in the earliest Eocene north and west of the Faroe Islands. • Eocene (post-breakup) episodic uplift and erosion along the southern and eastern flanks of the Faroe-Shetland Basin; this was followed by a period of major compressive structuration across the entire Faroe–Shetland region spanning the end-Eocene/Oligocene–Mid-Miocene interval; this set the template for the shape of the modern-day continental margin, including the formation of the deep-water Faroe Conduit which facilitated the transfer of intermediate- and deep-water masses across the Greenland-Scotland Ridge. By comparing the timing of these key phases of geological development of the Faroe–Shetland region with European and North Atlantic plate tectonics we identify a first-order correlation between the pattern of deformation that we observe and established changes in intraplate and/or plate boundary stresses. This raises the possibility that additional forces, including those postulated to be related specifically to the internal dynamics of a mantle plume, may not be a prerequisite to the evolution of the Faroe–Shetland region