The sliding bed facies in esker sands and gravels: a criterion for full-pipe (tunnel) flow?



The Guelph esker (Ontario, Canada) consists of a sinuous, steep-sided and segmented ridge which comprises poorly sorted, matrix-supported sands and gravels. These sands and gravels were probably deposited during the sliding bed stage which has been observed by others in closed-conduit hydraulic experiments. The poor sorting probably resulted from a high concentration of bed-material load in the lower part of a subglacial tunnel, sorting being restricted to that produced by particle collisions. Inclusive graphic standard deviation is characteristically large for the sands and gravels, indicating that virtually all sizes available were in transport. The overall grain size distribution shows a characteristic undulatory shape on arithmetic probability paper, mostly because of selective removal of pebble gravel and granule sizes. This poorly sorted fades is believed to be diagnostic of transport in a subglacial tunnel flowing full of water, and may be used to identify subglacial conditions in other eskers. Deltaic sands and gravels occur downcurrent of the esker and contain a greater diversity of structures; climbing-ripple cross-laminae, parallel laminae and massive structure, deposited in large-scale foresees at the end of a subglacial tunnel. These deltaic sands and gravels grade distally into outwash sands and gravels.