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.