COEVAL DUCTILE DEFORMATION AND BRITTLE FRACTURE BENEATH THE LATE WISCONSINAN PUGET LOBE, WASHINGTON STATE, USA

Glacial sediments (< 35 m thick) exposed at Camano Island, Puget Sound, were deposited during retreat of the Puget Lobe of the Cordilleran ice sheet c. 15 kyr BP in a shallow marine setting. Sediments mainly comprise massive and cross-bedded sand and gravels, and rhythmically-bedded clay and silt/fine sand couplets. These sediments are overlain by poorly sorted glacial diamictons that mantle the exposure. Folds developed in the lowermost sands and gravels (< 1.5 m high, wavelength of 4 m) vary in form and shape but strike to and are asymmetric towards the south-east. Parasitic clastic dikes which are infilled with sorted medium to coarse sand feed upwards from the fold apices. The dikes have a zigzag morphology and consistent width throughout and are interpreted as brittle fracture fills associated with strain release following extensional deformation of the ‘deforming bed'. Other infilled upward- and downward-pointing structures developed in the sand and gravel unit are wedge-shaped and are interpreted as due to water escape. These structures all terminate at the flat-lying and erosional boundary between the sand/gravel and overlying diamicton. This boundary is loaded and deformed by sediment density inversion. The presence of injection and water-escape structures suggest multiple strain cycles in which sediments are variously put under subglacial compressional and extensional regimes during development and collapse of the ‘deforming bed'. These strain regimes are manifested variously as folds (compression) and faults (strain relaxation). Such coeval ductile and brittle deformation processes show the close relationship between pore water, sediment rheology, and response to strain. They also provide evidence for lateral variability in strain response reminiscent of stick-slip basal ice flow.