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Drift Composition
and Surficial Geology of the Trutch Map Area (94G), Northeastern British
Columbia
Geological Survey of Canada Open File D3815 |
The Cordilleran Ice Sheet dispersed distinctive slate and schist erratics
(Hadrynian exposures east of the Rocky Mountain Trench) to the eastern
edge of the Rocky Mountain Foothills. The ice was sufficiently thick to
flow unhindered by the north-south trending mountain divides. At the same
time, the Laurentide Ice Sheet advanced to the mountain front from the
northeast, dispersing crystalline erratics from the Canadian Shield to
elevations of 1566 m above sea level. Some of these erratics originate
from the Great Bear batholith, at least 600 km to the northeast. It is
very likely that during the height of the last glaciation, about 18 ka
BP, the Cordilleran and Laurentide ice sheets were in contact. Cosmogenic
exposure dating of erratics in southern Alberta shows that the Laurentide
and Cordilleran ice sheets were in contact during the last glacial maximum
(Jackson et al. 1997). Nonetheless, no moraine systems marking the contact
were preserved. This is probably because, as each of the ice sheets withdrew,
out of phase fluctuations of the ice margins effectively smeared the contact
zone.
Cosmogenic chlorine-36 exposure dating of striated surfaces shows that
Cordilleran ice retreated from summits along the mountain front as late
as 14 020±760 to 13100±1560 calendar years ago. Shortly after
the Cordilleran ice retreated, the last Laurentide readvance penetrated
some valleys along the mountain front. For example, Laurentide till mantles
the upper Buckinghorse River valley as far as Nevis Creek, 10 km west of
the mountain front.
When the Cordilleran Ice Sheet thinned during deglaciation, its flow
became directed by the underlying topography. The dominant eastward flow
of the glacial maximum was replaced by a northward flow in the main valleys
bordering the main ranges and foothills. With further thinning of the ice,
the northward flow was diverted again to the east as the Cordilleran ice
assumed the character of a valley glacier system. The Laurentide Ice Sheet
blocked eastward drainage during its eastward recession from the area.
This created glacial lakes in many mountain valleys. Ice-dammed lakes previously
confined to the valleys expanded onto the plains as the Laurentide ice
front retreated eastward. Most of these lakes were short-lived because
of changing base levels as new spillways formed. The largest glacial lake
occupied the lowland between the Muskwa and Prophet rivers and the lower
Sikanni Chief River. Complete deglaciation was marked by glacial lake drainage
and rapid fluvial incision.
Quaternary sediments and weakly lithified Cretaceous rocks were deeply
incised by rivers causing extensive mass wasting. Active and relic landslides
suggest that mass wasting has been occurring throughout the postglacial
time. Some large failures appear to have been catastrophic (e.g. south
of Mount Stearns and along Crehan Creek), whereas, a very large landslide-earthflow
occupying ~7 km² along Besa River appears to be ongoing for decades.
In the eastern part of the map area, extensive mass wasting occurs along
deeply incised valleys and thick colluvial deposits mantle most of the
valleys. Radiocarbon dates on buried trees indicate large mass movements
throughout Holocene time.
Jackson, L.E. Jr., Phillips, F.M., Shimamura, K., and Little, E.C. 1997. Cosmogenic 36Cl dating of the Foothills Erratics Train, Alberta Canada; Geology 25: 195-198.
Chlorine-36 surface exposure dates were undertaken by Dr. F.M. Phillips, Department of Earth and Environmental Science, New Mexico Tech, Socorro, New Mexico, U.S.A.
Geochemistry and geophysical properties of surficial sediment samples
are described elsewhere in this Geological Survey Open File.
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