  We report the observation of a Griffiths Phase in the geometrically
frustrated antiferromagnet DyBaCo${_4}$O${_{7+\delta}}$. Its onset is
determined using measurements of the thermoremanent magnetization, which is
shown to be superior to conventional in-field measurement protocols for the
identification of the Griffiths Phase. Within this phase, the temporal
relaxation of magnetization exhibits a functional form which is expected for
Heisenberg systems, reflecting the nature of spin interactions in this class of
materials. Interestingly, the effective Co${^{2+}}$/Co${^{3+}}$ ratio tailored
by varying the oxygen non-stoichiometry $\delta$ is only seen to influence the
antiferromagnetic ordering temperature ($T{_N}$), leaving the Griffiths
Temperature ($T{_G}$) invariant.
