Subglacial and Proglacial Deformation of the Late Wisconsinan Puget Lobe

Abstract:

A range of brittle and ductile deformation structures is present within subglacial and proglacial sediments of the late Wisconsinan Puget Lobe. Sediments are well exposed on islands within the Puget Sound (Washington State, USA) and offer insight into the processes and environments during overall Puget Lobe retreat (~17-13 kyr BP). Although the macroscale stratigraphy of this ice retreat is known fairly well, and has a simple layered structure, different brittle and ductile deformation structures within the sediments reveal more complex patterns of sediment deposition and reworking associated with ice margin stillstands and/or oscillations during overall ice retreat. Brittle deformation structures include clastic dikes, subglacial shears and mud bed brecciation/rip ups. Ductile deformation structures include passive density settling and fluidization in particular in proglacial outwash. Together, these structures are found in association with each other (i.e. they are commonly parasitic) and formed by high marginward porewater fluxes driven by high subglacial water availability and high sediment permeability. Drivers for this record of deformation may have been very rapid lateglacial relative sea-level changes and incursions of marine water, rapid ice retreat from its maximal position, and high sedimentation rates.