  We investigate the onset of irreversibility in a dense granular medium
subjected to cyclic shear in a split-bottom geometry. To probe the micro and
mesoscale we image bead trajectories in 3D throughout a series of shear strain
oscillations. Though beads lose and regain contact with neighbors during a
cycle, the topology of the contact network exhibits reversible properties for
small oscillation amplitudes. With increasing reversal amplitude a transition
to an irreversible diffusive regime occurs.
