LOBAL INTERACTIONS, RHEOLOGICAL SUPERPOSITION, AND IMPLICATIONS FOR APLEISTOCENE ICE STREAM ON THE CONTINENTAL-SHELF OF BRITISH-COLUMBIA

Data on structures, stone striae, clast pavements, fabrics, morphology , provenance, and matrix grain size of four pre-late Wisconsinan diami ctons are presented from the lower accessible part of an 855 m long se a cliff at Cape Ball, east coast of Graham Island (Queen Charlotte arc hipelago) British Columbia. The data indicate that an ice stream cross ed the continental shelf from the British Columbian mainland to the Qu een Charlotte Islands. Local Queen Charlotte piedmont ice coalesced wi th the western edge of the Cordilleran Ice Sheet, resulting in northwa rd flow deflection along the east coast of Graham Island. Generally co nsistent structural and stone data indicate that diamictons were depos ited as primary subglacial tills and that subglacial dynamics and theo logic conditions were complex. Two of the tills can be traced continuo usly 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 theology and genesis ranging from a strong lodgement component to d ominantly deformation (mainly by subglacial squeeze flow). True end-me mber tills are rare and in almost all places the tills are hybrids tha t formed within a continuum of till-forming processes commonly involvi ng stone rotation and superposition of structures and theologic states . This could have occurred mainly in response to fluctuating till pore water content. Along the transect, two-dimensional stretches of contr asting till types probably represent a three-dimensional patchwork of contrasting till theologies and pore waters. If patches of ductily-def orming till were common and/or widespread, they could have accounted f or much of the movement of Cordilleran ice over glaciomarine mud on th e continental shelf when sea level was much lower than present. Deform ing, probably saturated, muddy till (reconstituted glaciomarine) would have possessed negligible shear strength, resulting in a thin, rapidl y moving glacier (most likely an ice stream) that reached Cape Ball bu t failed to remove the record of previous glacial events. The ice stre am probably issued out of the mainland's Skeena valley which is aligne d with Cape Ball and is approximately the size of valleys in Antarctic a that today conduct small outlet glaciers and ice streams. Evidence f or this and other ice streams on the northwestern continental shelf of North America imply that destabilization and rapid decay of the weste rn edge of the Cordilleran Ice Sheet may have been accomplished by a c ombination of ice streams, sea level rise, and extensive calving.