The paleoclimatic significance of deformation structures in Neoproterozoic successions

Abstract

This paper reviews the different types of soft sediment deformation structures that can form in glacial and non-glacial settings and explores the potential use of these structures in resolving long standing debates in paleoenvironmental reconstructions of Neoproterozoic glycogenic successions. Soft sediment deformation structures are created when compressional, gravitational or shear stress is applied to unlithified sediments during or shortly after deposition. In subglacial or ice marginal glacial settings, shear and compressional stress imparted by ice moving on top of a deformable substrate or advancing ice buldozing unlithified ice marginal sediments can result in a wide range of folding, faulting and shear structures. In glaciofluvial or stagnant ice marginal setting, gravitational collapse and remobilization of sediments associated with the melting of buried ice can result in normal faulting and broad folding. In glaciolacustrine or glaciomarine settings, compressional, shear and gravitational types of deformation structures can occur as a result of grounding ice or icebergs, rapid sedimentation and reworking downslope associated with high sedimentation rates. In non glacial settings, similar deformation structures can form as a result of slope instability and reworking of sediments downslope, rapid sedimentation, seismic shaking, wave induced shearing or loading. In this context, two case studies are presented to demonstrate the type of paleoenvironmental information that an analysis of deformation structures can provide. In the first case study, analysis of deformation in the Port Askaig Formation (Scotland) reveals a distinctive stratigraphic distribution of deformation structures. The types of deformation observed together with their recurrence over several 100s of metres and their basinal context are used to infer a seismic origin for the deformation, which in turn suggests a significant tectonic control on sedimentation atop a record of ice margin fluctuations in a glaciomarine setting. In the second example, analysis of deformation in the Smalfjord Formation (northern Norway) provides strong evidence for deformation by active ice overriding glaciofluvial deposits. The types of deformation in this example, together with its complexity, scale and associated facies, provide the strongest case for ice marginal deformation. In sum, analysis of deformation structures together with analysis of structural geology, stratigraphy, facies and facies associations can provide additional constraints on paleoenvironmental conditions at the time of deposition, which can help us refine or test paleoenvironmental models proposed for this critical time period in Earth history