ABSTRACT

The youngest sedimentary facies exposed in erosional hummocky terrain in south-central Alberta, Canada, is composed of alternating beds of massive silty clay diamicton, and sorted sand and/or silt. Diamicton beds are similar in thickness (5-20 cm), both laterally and vertically, within the sequence, and sedimentary structures and consistent clast orientations support a subglacial origin for both the diamicton and the sorted sediment. Alternation between diamicton and sorted strata document rhythmic sedimentary sequences: deposition of melt-out till followed by current deposition. Laterally extensive sorted beds indicate that the currents responsible for sand and silt deposition must have flowed as a sheet. This flow was shallow, no more than 20 cm, indicated by scours below boulders that were suspended from the base of glacial ice. Sheet flow requires that the water was stored beneath the ice prior to drainage. Consequently, some of the diamicton may have been deposited by rain-out through the shallow stored water and, hence, each diamicton bed may represent a combination of direct melt-out onto the bed and rain-out through the water column. Inferred cycles of water accumulation and drainage may have been annual.