  The NuSTAR hard X-ray telescope observed the transient Be/X-ray binary GS
0834-430 during its 2012 outburst - the first active state of this system
observed in the past 19 years. We performed timing and spectral analysis, and
measured the X-ray spectrum between 3-79keV with high statistical significance.
We find the phase-averaged spectrum to be consistent with that observed in many
other magnetized accreting pulsars. We fail to detect cyclotron resonance
scattering features that would allow us to constrain the pulsar's magnetic
field in either phase-averaged or phase-resolved spectra. Timing analysis shows
a clearly detected pulse period of ~12.29s in all energy bands. The pulse
profiles show a strong, energy-dependent hard phase lag of up to 0.3 cycles in
phase, or about 4s. Such dramatic energy-dependent lags in the pulse profile
have never before been reported in high-mass X-ray binary (HMXB) pulsars.
Previously reported lags have been significantly smaller in phase and
restricted to low-energies (E<10keV). We investigate the possible mechanisms
that might produce this energy-dependent pulse phase shift. We find the most
likely explanation for this effect to be a complex beam geometry.
