Surficial geology, Gunnell Creek, British Columbia

The Gunnell Creek (NTS 94I/13) map sheet was inundated by the Laurentide Ice Sheet during the late Wisconsinan glaciation (ca. 25 000 - 10 000 years ago). A thick blanket of clay and fine-silt-rich till was deposited by westward flowing ice across the region. Observations of large shear structures in till (some demarcated by intra-till winnowed sand and gravel lenses and larger bodies), and prevalent clast pavements (including three distinct clast pavements in a single 10 m vertical section) suggest till in this area underwent extensive deformation, thrusting and stacking. This likely accounts for the considerable thicknesses of till (>3-10 m) seen here and elsewhere in the region. During deglaciation, the eastward retreat of ice appears to have undergone increasing degrees of topographic constraint. A regional full-glacial flow path curving west-northwest around the Etsho Plateau (to the north) and down the axis of the Sahtaneh River, eventually separated into two lobes. The point of divergence appears to have been a bedrock high situated ~20 km east of the map area. Ice in the northeastern quadrant of the map retreated first, and in an eastward fashion. The second lobe of ice which occupied much of the central and southern half of the map area, became increasingly lobate, and retreated in an east-northeast direction. Thick till deposits (Tb and Th) and prominent ice-marginal meltwater channels recorded in the southern areas of the map are associated with the retreat of this latter ice lobe. A prominent esker in the central part of the map area, paralleling the Sahtaneh River, was identified as part of this surficial geology study. At its proximal (eastern) end, it is up to 7m thick, declining to 2-4 m along the rest of its ~6 km length. The esker is bisected by the South Gunnell Road, and as such has locally been exploited as a source of gravel in support of road construction. Other smaller eskers situated north of here are predominantly composed of sand, offering little in the way of granular aggregate potential. Another esker identified from airphotographs in the southwestern corner of the map area has yet to be verified or its granular aggregate potential assessed by ground reconnaissance. Glaciofluvial terraces situated along Gunnell Creek and Sahtaneh River may also represent potential granular aggregate resources. Only those terraces situated on either side of the road crossing on Gunnell Creek have been inspected in the field, and these were predominantly found to be coarse sand deposits; the granular aggregate potential of other terraces in the map area remains to be assessed. Extensive crevasse-squeeze ridges and the hummocky character of till deposits in the map area are accentuated by bog and fen deposits that infill much of this flat to gently rolling terrain. Unique, long, linear (1-3 m high, 10-40 m wide), generally flat-topped ridges situated in the central part of the map area, paralleling the Sahtaneh River, are considered to be the product of longitudinal shearing in the ice and squeezing of till during full-glacial time. This may mark the separation of more dynamic ice flowing west-northwest around the Etsho Plateau from a more passive regional ice cover, akin to shear moraines and similar deposits associated with ice streams. Given their topographic expression and linear extent, these ridges make ideal corridors for road development.

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