Laurentide glaciation in west-central Alberta: a single (Late Wisconsinan) event

Abstract

The Grande Prairie region in west-central Alberta shows evidence of Late Wisconsinan Laurentide glaciation in the form of a widespread till, containing abundant erratic clasts derived from the Canadian Shield. Two sections, located on the Smoky River at Watino and on the Simonette River 80 km to the southwest, expose sediment lying stratigraphically below the till. A similar sequence is exposed in both localities: 5–10 m of coarse quartzite gravel overlain by a thick sequence of sand and silt. The gravel contains no Laurentide erratics and is thus preglacial. The contact between the basal gravel and the overlying sand and silt is sharp and conformable. 14 C dates from the base of the Simonette section and throughout the Watino section suggest a Middle Wisconsinan age. Thus, the sediment exposed is interpreted as a conformable Middle Wisconsinan sequence. The sedimentary sequences were deposited in a moderate- to low-energy braided stream system flowing towards the north. The general fining-up trend in the sections was produced as a result of channel abandonment, rather than by a change in regional base level. The sand and gravel units contain no material derived from the Canadian Shield. Consequently, the surface till is Late Wisconsinan, and represents the only Laurentide glaciation in this area of Alberta.