Significance of soft-sediment clasts in glacial outwash, Puget Sound, USA

Soft-sediment clasts composed of silt and clay are contained within glacial outwash sands in the Puget Sound, Washington State, USA. The outwash was deposited during ice retreat of the Cordilleran ice sheet around 17 cal kyr BP. The soft-sediment clasts have a distinctive and consistent morphology and disposition within the sand beds. The sedimentology, sedimentary structures and presence of soft-sediment clasts suggest sand was deposited as proglacial outwash with silts and clays deposited in meltwater pools. Following drying-out of the pools and subaerial cracking, lumps of silt and clay were excavated by meltwater and transported distally as soft-sediment clasts within high-density flows. The most likely final depositional setting is as a Salisbury-type ‘delta’ in which subaqueous outwash grades distally into deeper water. This interpretation shows the power of soft-sediment clasts to inform on past processes and palaeogeography for which there is often little evidence in the geologic record.