  We report quantum oscillation data on the metallic triangular antiferromagnet
PdCrO$_2$. We find that, to very high accuracy, the observed frequencies of
PdCrO$_2$ can be reproduced by reconstruction of the (nonmagnetic) PdCoO$_2$
Fermi surface into a reduced zone. The reduced zone corresponds to a magnetic
cell containing six chromium sites, giving a $\sqrt{3} \times \sqrt{3}$
in-plane reconstruction, and $\times 2$ interplane reconstruction. The
interplane ordering represents a reduction in lattice symmetry, possibly to
monoclinic, and an associated lattice distortion is expected. In addition, we
report a magnetic transition under an applied in-plane field that is probably
equivalent to the spin-flop transition reported for CuCrO$_2$, and present data
on its field-angle dependence. We also report measurements of the resistivity
of PdCrO$_2$ up to 500 K.
