Rockall Trough—Cretaceous or Late Palaeozoic?

Abstract

Synopsis The Rockall Trough is the most southerly and widest part of the pre-Tertiary ‘proto North Atlantic’ rift zone running from the Porcupine Bank region west of Ireland to the Vøring Plateau off central Norway. Although the northerly part of this rift is intra-continental, the southern part is quasi-oceanic in origin, but of uncertain age. The many arguments for Cretaceous sea floor spreading are evaluated in turn; the few apparently valid ones remaining are shown to relate not directly to the Rockall Trough, but to a newly-identified spreading phase of approximately mid-Cretaceous age which partially opened up the Hatton-Rockall Basin. The opening of the Rockall Trough predates this. Stratigraphic evidence suggests that the proto North Atlantic is pre-Cretaceous, and probably even pre-Jurassic, in age. The rival arguments for late Palaeozoic (late Carboniferous—early Permian) sea floor spreading are all consistent with an important phase of rifting at that time, but they provide no direct evidence for opening. No detailed arguments have yet been proposed for Jurassic, Triassic, or late Permian sea floor spreading. New modelling of the magnetic anomaly over Rosemary Bank by MV Wood supports a Triassic or earlier age, but the pre-Upper Cretaceous sedimentary infill of 2–3 km in the Trough seems to be rather thin for such an early age. However, by combining the pre-Cretaceous stratigraphic Constraint on the age of opening with the assumption that the major rifting episode was an immediate precursor of spreading, a tentative conclusion is that the Rockall Trough was initiated as a rift, and then opened by a quasi- sea floor spreading mechanism, during the late Carboniferous to early Permian.