Depositional controls on the magnetic characteristics of lodgement tills and other glacial diamict facies

Abstract

Genetic interpretation of massive, unstructured diamict and diamictite facies is a commonly encountered problem faced by sedimentologists. Data are presented concerning the magnetic characteristics, namely, anisotropy of magnetic susceptibility (AMS) and natural remanent magnetism (NRM) of facies deposited by (1) lodgement processes at a glacier base (lodgement till) and (2) subaqueously by pelagic mud deposition and ice-rafting ("rain-out" diamicts). Lodgement tills have an NRM that is distorted around the geomagnetic pole position to form a girdle approximately 90° in length, either transverse or parallel to ice flow direction. This distension appears to be the result of subglacial shear processes because other diamicts, deposited passively by melt-out below stagnant ice and modelled by a laboratory experiment, show a nondeformed NRM clustering around the geomagnetic pole. The AMS data show that lodgement tills have only a weakly orientated magnetic microfabric.Glaciolacustrine "rain-out" diamicts show a precise NRM clustering as in marine and lacustrine muds. This reflects the lack of nongeomagnetic forces acting upon magnetic grains during deposition followed by postdepositional remanence "locking" at depth in the sediment column. These facies show both random AMS fabrics, typical of undisturbed pelagic sediments, and preferred microfabrics resulting from local sediment flow on the lake floor.It is concluded that NRM and AMS offer considerable assistance in genetic studies of massive diamict facies; AMS is particularly useful because the large populations of samples can be rapidly processed. The wider use of this technique by sedimentologists—for investigating other sedimentary facies types—is anticipated.