  The SPT0311-58 system at z=6.900 is an extremely massive structure within the
reionization epoch, and offers a chance to understand the formation of galaxies
in an extreme peak in the primordial density field. We present 70mas Atacama
Large Millimeter/submillimeter Array observations of the dust continuum and CII
158um emission in the central pair of galaxies and reach physical resolution
~100-350pc, among the most detailed views of any reionization-era system to
date. The observations resolve the source into at least a dozen kiloparsec-size
clumps. The global kinematics and high turbulent velocity dispersion within the
galaxies present a striking contrast to recent claims of dynamically cold
thin-disk kinematics in some dusty galaxies just 800Myr later at z~4. We
speculate that both gravitational interactions and fragmentation from massive
parent disks have likely played a role in the overall dynamics and formation of
clumps in the system. Each clump individually is comparable in mass to other
6<z<8 galaxies identified in rest-UV/optical deep field surveys, but with star
formation rates elevated by ~3-5x. Internally, the clumps themselves bear close
resemblance to greatly scaled-up versions of virialized cloud-scale structures
identified in low-redshift galaxies. Our observations are qualitatively similar
to the chaotic and clumpy assembly within massive halos seen in simulations of
high-redshift galaxies.
