| Summary: | Laterally continuous subglacial sediments are a necessary component for ice streaming in the modern onset regions of the ice streams draining the Siple Coast of West Antarctica on the basis of new seismic data combined with previous results. We present geophysical results from seismic reflection and refraction experiments in the upper reaches of ice streams C and D that highlight continuous sedimentary basins within and upstream of the current onset regions of both ice streams, with streaming ice overlying these sedimentary packages. The subglacial environment changes from no-sediment to discontinuous-sediment to continuous-sediment cover along a longitudinal profile from the ice sheet to tributary C1B. Along this same profile, we observe a speedup of ice flow and then full development of the ice stream tributary. Ice stream D flows above a thick sedimentary package with an uppermost low-seismic-velocity zone indicative of soft till, and the upglacier and lateral extensions of ice stream D are tightly constrained by the extent of continuous sediments. The inland termination of these sediments suggests that future migration of high-velocity, low-shear-stress ice flow in these regions appears unlikely
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