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Palaeomagnetic dating of sub-Torridon Group weathering profiles, NW Scotland: verification of Neoproterozoic palaeosols

¹ Department of Geology & Geophysics, University of Adelaide, Adelaide, SA 5005, Australia & Postgraduate Research Institute for Sedimentology University of Reading, Reading RG6 6AB, UK (e-mail gwilliams{at}geology.adelaide.edu.au)
² CSIRO Division of Exploration & Mining, PO Box 136, North Ryde, NSW 2113, Australia

A palaeoplain that cuts across Lewisian Gneiss in the Cape Wrath area, northwest Scotland, is weathered to depths of 1–6 m beneath fluvial red conglomerates and sandstones of the Applecross Formation of the early Neoproterozoic Torridon Group. Weathered Lewisian rocks are enriched in K₂O and Fe₂O₃, and contain hematite as an alteration product of biotite and magnetite. Palaeomagnetic analysis of the weathering profiles has been carried out to constrain the age of alteration. Thermal step demagnetization of 87 oriented core specimens of biotite-gneiss and amphibolite spanning four profiles at Poll a’ Mhurain and Sheigra reveal a stable, high-temperature component with a mean direction of remanence of D= 149.1°, I=57.2° (₉₅ = 5.0°) (corrected for dip of the overlying Applecross strata) that gives a pole at 9.8°S, 199.2°E (dp = 5.3°, dm=7.3°). This direction differs significantly from that for little-weathered Lewisian at those two localities and also from directions for the late Mesoproterozoic Stoer Group. The remanence direction for the weathered rocks is, however, near the dip-corrected high-temperature remanence direction (D= 122.3°, I=50.5°, ₉₅=10.8°) obtained by us from the thermal step demagnetization of 39 sandstone specimens of the Applecross Formation in the Cape Wrath area. Furthermore, the direction for the weathered rocks agrees closely with other dip-corrected results for the Torridon Group, such as the detrital remanent magnetization (DRM) mean direction (D= 148.3°, I=59.9°, ₉₅=13.8°) for the Applecross Formation on Rum and that for the main outcrop of the Aultbea Formation (D=139.7°, I=52.6°, ₉₅ = 14.5°).

The concordance of palaeomagnetic results for the sediments and weathered rocks implies that the weathering profiles in the Cape Wrath area acquired their remanence penecontemporaneously with deposition of the Applecross and Aultbea formations. The palaeomagnetic results therefore argue strongly that the weathering profiles are palaeosols of Applecross age (c. 980 Ma) that formed at a palaeolatitude of 38° and constitute the oldest known palaeosols in the British Isles—views that the weathering is substantially older or younger than Applecross deposition cannot be sustained. Our findings carry implications for Applecross palaeoclimate and source and strengthen the conclusion that atmospheric oxygen was a substantial fraction of its present abundance by early Neoproterozoic time.

KEYWORDS: Scotland, Proterozoic, Torridonian, palaeomagnetism, weathering

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