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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

Notes on Glacial History

The Trutch map area was glaciated during the last (Late Wisconsinan) glaciation (ca. 25 000-10 000 years ago) by the continental Laurentide Ice Sheet flowing from the northeast and by Cordilleran glaciers flowing from the west. The Cordilleran glaciers included valley glaciers originating from local cirques and the Cordilleran Ice Sheet originating west of the continental divide.

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.

References

Bednarski, J.M. 1999: Preliminary report of the Quaternary geology of the Trutch map area, northeastern British Columbia; in Current Research 1999-A; Geological Survey of Canada, p. 35-43

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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