Deformation styles as a key for interpreting glacial depositional environments

Abstract

Lithostratigraphical and lithofacies approaches used to interpret glacial sediments often ignore deformation structures that can provide the key to environment of formation. We propose a classification of deformation styles based on the geometry of structures rather than inferred environment of formation. Five styles are recognised: pure shear (P), simple shear (S), compressional (C), vertical (V) and undeformed (U). These dictate the first letter of the codes; the remaining letters conveying the evidence. This information can be used to reconstruct palaeostress fields and to infer physical properties of sediments when they deformed. Individual structures are not diagnostic of particular environments but the suite of structures, their relative scale, stratigraphical relationships, and orientation relative to palaeoslopes and to palaeoice-flow directions can be used to infer the environment in which they formed. This scheme is applied at five sites in west Wales. The typical succession is interpreted as subglacial sediments overlain by meltout tills, flow tills and sediment flows. Paraglacial redistribution of glacial sediments is widespread. Large-scale compressional deformation is restricted to sites where glaciers readvanced. Large-scale vertical deformation occurs where water was locally ponded near the ice margin. There is no evidence for glaciomarine conditions.

Distribution of strain signatures in Quaternary sediments, Regional Municipality of Waterloo, Ontario, Canada

“…Compressional strain records the horizontal shortening of a sedimentary unit in response to an encroaching mass (McCarroll and Rijsdijk 2003), and includes open to closed folds, overturned folds, nappes, compressional fractures, and dikes (e.g. Boulton et al 1999).…”

“…Simple shear strain records the effects of tangential and/or horizontal stress on or within a sedimentary body involving the displacement and elongation of sedimentary structures in the direction of maximum shear (McCarroll and Rijsdijk 2003). Simple shearing is common in subglacial and ice-marginal environments where ice and/or sedimentary bodies over-ride underlying sediment (Hart and Roberts, 1994;McCarroll and Rijsdijk 2003).…”

“…Simple shearing is common in subglacial and ice-marginal environments where ice and/or sedimentary bodies over-ride underlying sediment (Hart and Roberts, 1994;McCarroll and Rijsdijk 2003). Rooted and detached shear folds, attenuated beds and thrust faulting are commonly observed simple shear structures.…”

A morphological, sedimentological and geophysical investigation of the Woore Moraine, Shropshire, England

“…The younger extensional faulting relate either to the melt-out of buried ice or to gravitational spreading following ice-retreat and consequent pressure release (e.g. McCarroll and Rijsdijk, 2003). Unfortunately, due to the small scale of the exposure it is impossible to determine whether this pattern of deformation is indicative of pervasive deformation throughout the whole landformassemblage or whether it represents localised surface phenomena associated with minor oscillations of the former ice margin.…”

The limitations of Quaternary lithostratigraphy: an example from southern Ireland

“…If so, seasonal ground saturation (and destabilization) associated with rapid summer thawing is a likely process (cf. McCarroll & Rijsdijk 2003). The capping sand unit at Ballycroneen Strand and Cnoc na nAcrai is considered to be the lateral equivalent of the breccia unit, and deposited by enhanced wind activity in an exposed extra-glacial (periglacial) environment (e.g.…”