Coeval brittle and ductile deformation beneath the late Wisconsinan Puget Lobe, Washington State, USA

Jasper Knight

doi:10.1017/aog.2019.33

Abstract

AbstractLate Wisconsinan glacial sediments, exposed on Whidbey Island and Camano Island, Puget Sound (Washington State, USA), were deposited in a proglacial shallow marine/outwash environment during northward retreat of the Puget Lobe of the Cordilleran ice sheet. Sediments mainly comprise massive and cross-bedded sand and gravels, and rhythmically-bedded clay and silt/fine sand couplets, interbedded with diamictons that were deposited by a range of mass flows of different viscosities. Although sediment stratigraphy and ice advance–retreat patterns are well established for the Puget Lobe, brittle and ductile deformation structures within, and separating, these sediment units are less well understood. These structures record the nature of ice–bed interactions taking place in subglacial and proglacial environments. This study examines evidence for these processes and environments. Key deformation structures identified include open to overturned folds, normal and reverse faults, clastic dikes and hydrofractures and passive-loading structures. Evidence for coeval development of ductile and brittle deformation structures shows the close relationship between porewater changes, sediment rheology and sediment system responses to changes in strain caused by ice–bed interactions.

Highlights

Subglacial and proglacial processes and environments during ice advance–retreat cycles are commonly reconstructed from different glacitectonic and soft-sediment deformation structures present within subglacial, overridden or bulldozed sediments of different types (e.g. Piotrowski and others, 2006; Lee and Phillips, 2008; Rijsdijk and others, 2010; Phillips and others, 2013a, b, 2018a; Knight, 2015)
This study examines the different sediment types and sedimentary and glacitectonic structures present beneath and in front of the retreating late Wisconsinan Puget Lobe of the Cordilleran ice sheet, in order to interpret the nature of subglacial and proglacial processes and environments
This location provides a good case study of these processes because the ice advance–retreat cycle of this ice lobe is well constrained both in time and space, and sediment sections are well exposed within the Puget Sound region (Fig. 1), discussed below

Summary

Introduction

Subglacial and proglacial processes and environments during ice advance–retreat cycles are commonly reconstructed from different glacitectonic and soft-sediment deformation structures present within subglacial, overridden or bulldozed sediments of different types (e.g. Piotrowski and others, 2006; Lee and Phillips, 2008; Rijsdijk and others, 2010; Phillips and others, 2013a, b, 2018a; Knight, 2015). In addition there are significant feedbacks between substrate properties and overlying ice flow, mediated through the development of glacitectonic structures that give rise to variations in porewater pressure within subglacial and proglacial sediments, and variations in meltwater availability at the ice–bed interface (Boulton and Hindmarsh, 1987; Piotrowski and Kraus, 1997; Denis and others, 2009; Phillips and others, 2018b) These structures, indicative of different subglacial strain regimes and their expression in the proglacial environment, may be partially preserved within the sediment profile, or overprinted by later strain events. In detail this study (1) describes the regional glacial context and Puget Lobe dynamics; (2) presents new sedimentological and structural data from coastal exposures on Whidbey Island and Camano Island (Puget Sound), including from a range of deformation structures and (3) discusses this evidence in the context of subglacial processes and ice dynamics

Methods

Results

Discussion

Conclusion