Cretaceous tectonostratigraphy of the Faroe-Shetland region

This report presents a set of observations from the rock record and seismic data that detail aspects of the Cretaceous succession in the Faroe–Shetland region, and which have provided the basis for a regional tectonostratigraphic framework. Timeslice reconstructions showing basin history and lithofacies, combined with seismic stratigraphic information that includes the record of contemporary tectonic events, provide chronological indicators of change in the Faroe– Shetland region. These changes are considered in relation to both local and regional tectonic events; the latter includes key events related to the developing North Atlantic spreading centre, in an attempt to better understand the driving mechanisms and controls on the development of the proto-NE Atlantic margin. The first part of the report (Chapter 1) introduces the project and the context of the study. In particular, the common assumption that, along the NW European margin, Late Jurassic rifting extended into the Early Cretaceous as a single event; a premise that commonly overlooks one of the most fundamental events in the evolution of the NE Atlantic margin – the rotation of the principal extension vector from E–W to NW–SE. Another common misconception is that the Late Cretaceous interval represents a phase of thermal sag and general tectonic quiescence in the Faroe–Shetland region. Such assumptions have generally been the result of speculative application of the North Sea Cretaceous setting to the Atlantic margin; this study aims to rigorously test these assumptions by providing a clear set of regional observational data that provide a direct record of events for the Cretaceous succession in the Faroe–Shetland region. Due to the availability of data, our focus is on the eastern half of the Faroe–Shetland region. This is followed in Chapter 2 by a summary of the regional geological framework, both of the proto-NE Atlantic margin and the Faroe–Shetland region. The latter incorporates published information combined with observations from this study, with a particular focus on the regional seismic-stratigraphic expression of the Cretaceous succession. By choosing a set of seismic profiles across the eastern part of the Faroe–Shetland region, a seismic-stratigraphic scheme has been established, which probably represents the first regional integration of seismic data. This scheme is calibrated with the well database and, thus, provides important chronological information for several regional unconformities. Chapter 3 presents a summary of the Cretaceous tectonostratigraphy of the Faroe–Shetland region, based on a set of timeslice reconstructions that address: 1) basin history; 2) lithofacies; and, 3) lithostratigraphy. These timeslice maps are intended to provide a series of base layers for anyone wishing to undertake future palaeogeographic and/or palinspastic reconstructions. The timeslice reconstructions are underpinned by the set of stratigraphical-range charts and the regional correlation chart presented in Appendices 1 and 2, respectively. The stratigraphicalrange charts detail the chronological range, lithology and lithostratigraphy of the Cretaceous record for each commercial well and BGS borehole, whereas the correlation chart summarises this record on the scale of the individual basin, sub-basin or high. The timeslice reconstructions reveal a highly variable history of differential uplift and subsidence throughout the Cretaceous period, which has important consequences for the tectonic development of this region. Key conclusions include: 1) confirmation that the Early Cretaceous rifting phase is distinct and separate from Late Jurassic rifting; 2) Cretaceous basin development was highly variable and occurred against a backdrop of polyphase extension, uplift and compression, as well as a rising eustatic sea level; and, 3) the Upper Cretaceous succession does not show a simple pattern of ‘post-rift’ subsidence – indeed it is difficult to define a classic post-rift phase to this framework. In Chapter 4, we discuss the implications of our study with regard to the tectonic development of the proto-NE Atlantic margin, by correlation of key local tectonic phases with established regional tectonic events in Europe and the North Atlantic region. The pattern of co-eval extension and compression that is observed is consistent with regional strike-slip associated with transtension and transpression; thus, given the location of the Faroe–Shetland region relative to the developing North Atlantic spreading centre (e.g. Figure 2.1), as well as the Alpine collisional zone, it is not surprising that this area developed as a zone of oblique strike-slip motion. Chapter 5 presents a set of conclusions that summarise the key points of the study. There is no doubt that the highly variable pattern of sedimentation and protracted tectonic history were both influenced by the development of the proto-NE Atlantic margin.