  We report the finding of two large-scale structures of galaxies in a 40'x35'
field embedded in a 25deg^2 area where two 100 Mpc-scale structures of quasars
and MgII absorbers at z~0.8 and z~1.2 have been previously detected. Using deep
optical (V- and I-band) imaging, we are able to select high-redshift
(0.5<z<1.3) early-type galaxies as those redder than the cluster red sequence
at z=0.5 and having I<23.5. Through comparison with a 35'x35' control field, we
find a 30% excess of these red galaxies, corresponding to 563 galaxies across
the field. The colour distribution of the excess galaxies shows a coherent peak
at 2.7<V-I<3.1 and the magnitude distribution is well described by a Schechter
function, showing unambiguously that the galaxy excess is primarily due to a
population of early-type galaxies at z=0.83\pm0.08. In follow-up NIR imaging of
the four best cluster candidates, we identify three clusters at z=0.8\pm0.1,
each having well defined red sequences of ~20 galaxies at I-K~3.4, forming a
structure 20 Mpc across. The fourth candidate, corresponding to a previously
identified cluster at z=1.2\pm0.1 is confirmed with 28 K<20.5 galaxies having
VIJK colours consistent with passively-evolving galaxies at z~1.2 located
within 1' of the cluster centre. The results of this study support the presence
of two sheet-like large-scale galaxy structures extending across the 20-30 Mpc
scales explored by the optical data, one ``sheet'' at z~0.8 with three
associated clusters, and a second ``sheet'' at z~1.2 with one embedded cluster.
Our results confirm that, at least inside the probed region, the two known 100
Mpc-scale structures at z~1 of quasars and MgII absorbers also mark out volumes
with an enhanced density of galaxies.
