Small to medium-sized glacitectonic structures in till and in its substratum and their comparison with mass movement structures

Abstract

Ductile and brittle deformation structures have been observed in till of various origins, particularly in subglacial till and flowtill, and in the substratum beneath subglacial till. Although some of the glacitectonic structures have been claimed to be palimpsest structures from debris-rich glacier ice, such a preservation is not proven to be very common. However, deformation structures are found in megablocks and other clasts that occur in melt-out till and in deformation till. Most glacitectonic deformation structures are formed in the substratum or in the already deposited subglacial till by an actively moving glacier, for instance recumbent folds, shears, boudins, thrusts, hook folds, diapirs, tension fractures, till wedges, fissility, glacitectonic breccia, conjugate sets of fractures. Structures similar to those reported from subglacial primary till and its substratum occur also in flowtill and other mass movement deposits, but deformations are less common in the substratum beneath them. The orientation of the deformations in flowtill is unrelated to the direction of glacial movement. The shapes of some shears and folds are different in flowtill, when compared with those in subglacial till and its substratum. Still, the flow and shear structures alone are not sufficient to distinguish certain varieties of till (fir instance subglacial primary till from flowtill) but they are useful as local directional indicators of stress and as genetic criteria in combination with other features, particularly with directional glacial erosion marks and fabric.