Lobal interactions, rheologic superposition, and implications for a Pleistocene ice stream on the continental shelf of British Columbia

Abstract

Data on structures, stone striae, clast pavements, fabrics, morphology, provenance, and matrix grain size of four pre-late Wisconsinan diamictons are presented from the lower accessible part of an 855 m long sea cliff at Cape Ball, east coast of Graham Island (Queen Charlotte archipelago) British Columbia. The data indicate that an ice stream crossed the continental shelf from the British Columbian mainland to the Queen Charlotte Islands. Local Queen Charlotte piedmont ice coalesced with the western edge of the Cordilleran Ice Sheet, resulting in northward flow deflection along the east coast of Graham Island. Generally consistent structural and stone data indicate that diamictons were deposited as primary subglacial tills and that subglacial dynamics and rheologic conditions were complex. Two of the tills can be traced continuously over the Cape Ball section which provides a north-south transect oblique to ice flow. Each of the two tills reveals five major changes in rheology and genesis ranging from a strong lodgement component to dominantly deformation (mainly by subglacial squeeze flow). True end-member tills are rare and in almost all places the tills are hybrids that formed within a continuum of till-forming processes commonly involving stone rotation and superposition of structures and rheologic states. This could have occurred mainly in response to fluctuating till pore water content. Along the transect, two-dimensional stretches of contrasting till types probably represent a three-dimensional patchwork of contrasting till rheologies and pore waters. If patches of ductily-deforming till were common and/or widespread, they could have accounted for much of the movement of Cordilleran ice over glaciomarine mud on the continental shelf when sea level was much lower than present. Deforming, probably saturated, muddy till (reconstituted glaciomarine) would have possessed negligible shear strength, resulting in a thin, rapidly moving glacier (most likely an ice stream) that reached Cape Ball but failed to remove the record of previous glacial events. The ice stream probably issued out of the mainland's Skeena valley which is aligned with Cape Ball and is approximately the size of valleys in Antarctica that today conduct small outlet glaciers and ice streams. Evidence for this and other ice streams on the northwestern continental shelf of North America imply that destabilization and rapid decay of the western edge of the Cordilleran Ice Sheet may have been accomplished by a combination of ice streams, sea level rise, and extensive calving.