Palaeomagnetism of (Palaeozoic) Peel Sandstones and Langness Conglomerate Formation, Isle of Man: implications for the age and regional diagenesis of Manx red beds

Abstract

Abstract The Peel Sandstones are a sequence of fault-bounded fluvially deposited red beds of uncertain age exposed on the western coast of the Isle of Man. Palaeomagnetic study identifies partial preservation of an A component pre-dating deformation at four of eight sampled sites; the presence of both polarities in this remanence defines a positive class C reversal test. This component adjusted for tilt is D/I = 356.9/−47.1° (48 samples, α 95 = 5.6°) and is interpreted to be a post-depositional detrial remanence. The palaeolatitude (29° S) corresponds to the position of Britain during Late Silurian-Early Devonian times and implies that these sediments are correlative with Lower Old Red Sandstone molasse from elsewhere in the British Caledonides; a post-mid-Carboniferous age is excluded. Palaeomagnetic and geological evidence are used to constrain an age of c. 410–400 Ma for the Peel Sandstones, whilst a declination contrast of 40–50° is interpreted to reflect either: (a) tectonic emplacement of a unit including the Peel Sandstone outcrop; or (b) wider post-Early Devonian and pre-Late Carboniferous counterclockwise rotation between the Isle of Man and the mainland during final shaping of the Caledonides. A post-folding B component of uniform reversed polarity ( D/I = 195.6/−30.8°, 45 samples, α 95 = 4.8°) is present at all sites and was acquired during diagenesis at c. 250 Ma. Red sandstones within the Langness Conglomerate Formation, near the base of the Lower Carboniferous (Chadian-Arundian) succession exposed in the southeast of the Isle of Man, have a uniform reversed magnetization ( D/I = 209.7/−24.2°, 20 samples, α 95 = 6.8°). Like the B magnetization in the Peel Sandstones, this was acquired by diagenesis in Early Permian times ( c. 250 Ma) and during the Permo-Carboniferous Reversed Superchron. It is temporally related to early extensional tectonism in the Irish Sea region.