  We study the global geometry of a general class of spacetimes of relevance to
the supersymmetric $AdS_3/CFT_2$ correspondence in eleven-dimensional
supergravity. Specifically, we study spacetimes admitting a globally-defined
$\mathbb{R}^{1,1}$ frame, a globally-defined frame bundle with structure group
contained in Spin(7), and an $AdS_3$ event horizon or conformal boundary. We
show how the global frame bundle may be canonically realised by
globally-defined null sections of the spin bundle, which we use to truncate
eleven-dimensional supergravity to a gravitational theory of a frame with
structure group Spin(7), SU(4) or Sp(2). By imposing an $AdS_3$ boundary
condition on the truncated supergravity equations, we define the geometry of
all $AdS_3$ horizons or boundaries which can be obtained from solutions of
these truncations. In the most generic case we study, we reproduce the most
general conditions for an $AdS_3$ manifold in M-theory to admit a Killing
spinor. As a consistency check on our definitions of $AdS$ geometries we verify
that they are satisfied by known gauged supergravity $AdS_3$ solutions. We
discuss future applications of our results.
