ABSTRACT

Braided outwash streams deposited sands and gravels during the withdrawal of the Wentworth and Halton (both late Wisconsin) ice sheets from the Credit River Valley of Southern Ontario. The gravels are crossbedded on a scale of several feet, and fine upward into sandy cross-laminated and tabular cross-bedded facies. The Wentworth outwash sediments rest on Ordovician bedrock, and are separated from the Halton outwash sediments (above) by varved clays with dropstones and the Halton Till.

Within the sandy facies are several newly recognized coarsening-upward sequences. These begin with very fine clay, coarsening upward into faintly cross-laminated silts and ripple-drafted sands. Above the sands there is normally an erosive surface, then tabular cross-bedded coarse and pebbly sands. Above a second erosive surface lies large scale cross-bedded sandy gravels.

The sequence is interpreted as a channel fill. The clay represents overbank flooding into an old disused channel within the braided system. As the flood stage rises, more and more suspended sediment is washed overbank, and flow within the channel gives rise to the cross-laminated silts and sands. The tabular cross-bedded coarse and pebbly sands indicate bed load material being washed into the new channel, and the uppermost erosion surface, followed by cross-bedded sandy gravels, suggests levee-breaching, and final incorporation of the old channel into the active part of the braided system.