Abstract

Sediments and bed forms from three braided outwash plains and one steep non-braided mountain stream in the Rocky Mountains of Alberta are compared to sediments and structures exposed in a paleo-outwash sequence.Channel sediments are subdivided into (a) pool facies: ripple- and parallel-laminated silty sand, with current crescents around isolated stones; and (b) riffle facies: parallel-stratified gravel and silt or silty sand.'Transverse ribs' constitute a bed form typical of riffle portions of channels. This bed form occurs as a train of regularly spaced pebble, cobble, or boulder ridges oriented transversely to the flow. Locally, the pebbly ribs are separated by, and rest on, a stone-free silty substratum. It is suggested that transverse ribs constitute a common equilibrium bed form in higher gradient riffle portions of gravelly alluvium.Longitudinal bars dominate interchannel areas of the modern outwash plains. They consist of coarse, poorly sorted gravel, either massive or exhibiting crude horizontal stratification. The paleo-outwash, composed of finer-grained and better sorted gravel, is characterized by sets 5 to 30 cm thick of cross-stratified sand and by sets 75 to 82 cm thick of cross-bedded pebble gravel. These may be related to progradation of dunes and bars, respectively.Standard deviations of channel azimuths in modern outwash plains are appreciably less than that recorded by cross-stratification measurements in the time-transgressive sequence of paleo-outwash. This is partly due to the braided system of channels having swept from side to side across the plain through time.

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