PALEOMAGNETIC DATING OF SUB-TORRIDON GROUP WEATHERING PROFILES, NW SCOTLAND - VERIFICATION OF NEOPROTEROZOIC PALEOSOLS

A palaeoplain that cuts across Lewisian Gneiss in the Cape Wrath area, northwest Scotland, is weathered to depths of 1-6m beneath fluvial re d conglomerates and sandstones of the Applecross Formation of the earl y Neoproterozoic Torridon Group. Weathered Lewisian rocks are enriched in K2O and Fe2O3, and contain hematite as an alteration product of bi otite and magnetite. Palaeomagnetic analysis of the weathering profile s has been carried out to constrain the age of alteration. Thermal ste p demagnetization of 87 oriented core specimens of biotite-gneiss and amphibolite spanning four profiles at Poll a' Mhurain and Sheigra reve al a stable, high-temperature component with a mean direction of reman ence of D=149.1 degrees, I=57.2 degrees (alpha(95)=5.0 degrees) (corre cted for dip of the overlying Applecross strata) that gives a pole at 9.8 degrees S, 199.2 degrees E (dp=5.3 degrees, dm=7.3 degrees). This direction differs significantly from that for little-weathered Lewisia n at those two localities and also from directions for the late Mesopr oterozoic Steer Group. The remanence direction for the weathered rocks is, however, near the dip-corrected high-temperature remanence direct ion (D=122.3 degrees, I=50.5 degrees, alpha(95)=10.8 degrees) 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-correc ted results for the Torridon Group, such as the detrital remanent magn etization (DRM) mean direction (D=148.3 degrees, I=59.9 degrees, alpha (95)=13.8 degrees) for the Applecross Formation on Rum and that for th e main outcrop of the Aultbea Formation (D=139.7 degrees, I=52.6 degre es, alpha(95)=14.5 degrees). The concordance of palaeomagnetic results for the sediments and weathered rocks implies that the weathering pro files in the Cape Wrath area acquired their remanence penecontemporane ously with deposition of the Applecross and Aultbea formations. The pa laeomagnetic results therefore argue strongly that the weathering prof iles are palaeosols of Applecross age (c. 980 Ma) that formed at a pal aeolatitude of 38 degrees and constitute the oldest known palaeosols i n the British Isles-views that the weathering is substantially older o r younger than Applecross deposition cannot be sustained. Our findings carry implications for Applecross palaeoclimate and source and streng then the conclusion that atmospheric oxygen was a substantial fraction of its present abundance by early Neoproterozoic time.