Quaternary geology and stratigraphy of the Assiniboine fan delta area, southwestern Manitoba

Abstract

The Assiniboine fan delta covers an area of 6400 km2, and consists of a lower region separated from a higher region to the west by an escarpment known as the Campbell beach escarpment. The topographically higher part of the fan delta is the target of this thesis. The higher region consists of 5 major depositional settings: a braided river plain leading to the fan delta, a Gilbert-type delta, underflow fan deposits, subaerial debris flow deposits, and pro-delta deposits. The braided river plain deposits are composed of tabular cross bedded and trough cross bedded gravelly sand. The Gilbert delta, which is separated from the braided river plain to the west by an escarpment, is dominated by gravel foreset beds... The Assiniboine River eroded a broad and shallow channel across the northern fan delta, and a broad and deeper channel across its southern side. Coarse channel fill sediments in a river terrace north of Glenboro were dated at 10.600 +-150 BP (GSC 383). During late Wisconsinan time, the Red River Lobe advanced southwestward across the Assiniboine valley to the Darlingford and Alexander moraines. The Alexander moraine, which is the northwestern extension of the Darlingford moraine, became an ice barrier to water flowing from the west and formed Lake Hind. Subsequently Lake Brandon formed to the east of the ice barrier when the Red River Lobe retreated northeastward. At first, Lake Hind drained to Lake Agassiz through the Pembina Spillway along a southern route, bypassing the Assiniboine fan delta. When the ice barrier failed along the Assiniboine bedrock valley, water from Lake Hind and the Assiniboine River flooded into Lake Agassiz near Brandon, starting deposition of the Assiniboine fan delta. The earliest episode of flooding was catastrophic because sheet-like poorly sorted gravels and boulders were deposited over an area of 350 km2 at the base of the proximal fan delta sequence... The Assiniboine River shifted to the southern fan delta before 10,600 BP, and before the deposition of massive sandy gravels in the proximal fan delta. As the level of Lake Agassiz rose during the Emerson phase (9900-9500 BP), fluvial and fluvio-lacustrine sediments were deposited in the Assiniboine River valley. These sediments were later incised as the lake fell during the Nipigon phase (9500-8500 BP), leaving many paired and unpaired terraces at an elevation between 330 and 350 m.