Palaeomagnetism of the (Late Mesoproterozoic) Stoer Group, northwest Scotland: implications for diagenesis, age and relationship to the Grenville Orogeny

Abstract

The Stoer is the lowest of three groups comprising the Torridonian Supergroup, a clastic succession deposited late in Mesoproterozoic times on the (Lewisian) metamorphic foreland to the Caledonian Orogen in northwest Scotland. This study reports a palaeomagnetic, rock magnetic and magnetic fabric investigation through the full stratigraphic thickness of the succession. A primary magnetic fabric of sedimentary origin defines current flow from a westerly veering to northwesterly source. Rock magnetic studies identify the presence of both magnetite and hematite in these sediments. Magnetite is apparently of primary detrital origin whilst the hematite probably results mostly from early diagenesis in an environment of restricted chemical weathering. Palaeomagnetic study of sedimentary slumps shows that magnetic remanence post-dates deposition but was probably fixed by early dewatering and lithification because slumped blocks of Stoer in basal Torridon Group sediments preserve a primary remanence. Tilt adjustment, although inconclusive, also implies that magnetic remanence is older than pre-Torridian Group tectonic deformation. The lower part of the Stoer succession shows a progressive increase of magnetic inclination with shallower components resident in magnetite and steeper components in hematite. The succession above the Stac Fada Member has the steepest magnetic inclination and shows no significant difference between magnetite and hematite component directions. The inferred time sequence of palaeopoles coincides with the Gardar palaeomagnetic track (∼ 1250–1130 Ma) at 1180 Ma, conforming to a Pb–Pb determination of 1199±70 Ma. The Stoer Group was fully lithified and deformed before deposition of the Torridon Group at ∼ 1030 Ma because it contains no vestige of the range of Caledonian and later overprints found extensively in the latter. Sedimentation and lithification of the Stoer Group are therefore linked with a phase of extensional tectonism at 1200–1150 Ma and deformation is attributed to a culminating phase of deformation in the nearby Grenville Belt at ∼ 1100 Ma.