Palaeomagnetic studies of the Torridonian sediments, NW Scotland

Abstract

Synopsis This study covers thermal demagnetisation results from 172 localities distributed through the Upper Proterozoic Stoer Group and Torridon Group sediments of NW Scotland. Chemical leaching experiments and results from slump horizons confirm earlier studies showing that the bulk of the total NRM in these rocks resides in specularite grains and is related to deposition and/or initial lithification processes; in the Stoer Group and Aultbea Formation a considerable fraction of the remanence is held by magnetite. Thermal demagnetization partially, but not completely, removes that part of the small secondary remanence residing in the high blocking temperature fraction overlapping with the initial remanence. Demagnetization trends suggest that this secondary remanence is a composite one including both a Caledonian ( c . 400 Ma) and a Tertiary/Recent component. The palaeopole estimates in stratigraphic order are: Stoer Group (main outcrop) 253°E, 46°N (A 95 = 10°), Stoer Group (Enard Bay) 263°E, 37°N (A 95 = 11°), Diabaig Formation 233°E, 17°S (A 95 = 21½°), Cape Wrath sediments 237°E, 37°S (A 95 = 16°), Lower Applecross Formation 230°E, 40°S (A 95 = 16½°), Upper Applecross Formation 212°E, 27°S (A 95 = 14½°), - Applecross Formation (Loch Maree), 231°E, 23°S (A 95 = 17°), Aultbea Formation (Coigach Area) 231°E, 12°S (A 95 = 8½°) and Aultbea Formation (main outcrop) 207°E, 7°S (A 95 = 17°). The sampled sections complement and extend earlier studies to define in outline the magnetostratigraphy of these formations, and show that the contact between the Applecross and Aultbea Formations is strongly diachronous. The time-sequence of poles from the Lower and Upper Torridonian coincide with the Laurentian a.p.w. path at c . 1100 and c . 1040 Ma respectively and imply that deposition and magnetization of these sediments predated isotopic closure by c . 130 Ma (Stoer Group, Rb–Sr isochron 968 Ma) and c . 260 Ma (Torridon Group, Rb–Sr isochron 777 Ma). This supports the views that the Torridonian and Moinian rocks are broadly coeval.