Surficial Geology and Quaternary History of the Cleardale Area, Northwestern Alberta (NTS 84D/SW)

This report describes the distribution, character and origin of surficial sediments in the Cleardale area (NTS 84D/SW). It also discusses the Quaternary history of the region. The Cleardale area is south of the Clear Hills, along the British Columbia-Alberta border, and comprises a broad lowland (450-750 metres above sea level) incised by the Peace River.

Three gravel sheets identified in the Cleardale area, each grading to different base levels are attributed to three major cycles of fluvial aggradation and incision. These cycles are attributed to the evolution of regional, braided river systems draining westward from the Rocky Mountains since the Middle Tertiary.

Widely dispersed till documents the advance of the Laurentide Ice Sheet across the Cleardale area during Late Wisconsinan glaciation. This advance covered the Cleardale region, including local uplands, and deposited one major till facies across much of the map area. These sediments provide us with insight into regional stratigraphy and landscape evolution in northwestern Alberta.

During regional deglaciation, the Laurentide Ice Sheet retreated from the Cleardale area about 12,000 years ago, exposing a broad lowland basin flanking the ancestral Peace River. Due to the down-drainage retreat of the Laurentide Ice Sheet, subaerial and glacial meltwater discharge that converged toward the ice margin infilled this basin, forming the western part of a regional proglacial lake termed Glacial Lake Peace. Widespread glaciolacustrine sediments that cover the Cleardale area, up to about 780 metres above sea level, now demarcate this lake.

Glacial Lake Peace progressively drained from the Cleardale area as the retreating Laurentide Ice Sheet exposed lower basins and outlets along the ancestral Peace River valley and surrounding lowlands to the east of the map area.

Following lake drainage, Holocene fluvial processes incised through approximately 120 metres of Quaternary sediments and 30 metres into the underlying bedrock, forming the modern Peace River valley. Accompanying mass movement along the river valley and its tributaries is responsible for the widespread distribution of colluvium, which is a regionally significant surficial material and remains a cause for infrastructure problems in the area.