Figure 11. Screenwork replacement of light coloured
chert in the chert-carbonate unit of the Black Stuart
Group on Kimball Ridge. (GSC 191007)

breccia. It is discontinuous, occurs at the base of the for-
mation and has several facies variations throughout the
area. These variations will be described from east to west
for: Kimball Ridge; the north limb of the Black Stuart
Syncline; and Anderson Ridge and the south limb of the
Black Stuart Syncline.

On Kimball Ridge the chert-carbonate unit is prima-
rily dolostone; grey to chalky weathering moderately light
grey to white and fine to medium crystalline. Locally chert
has replaced the dolostone. The chert is very light grey
to white and penetrates the dolostone as irregular layers
or screens (Fig. 11).

On the north limb of the Black Stuart Syncline, the
chert-carbonate unit consists of mainly dolostone breccia
and lesser amounts of the same dolostone as on Kimball
Ridge. Chert replacement of the dolostone is different,
occurring more as regular layers and irregular patch-
works. The dolostone breccia consists of light grey dolo-
stone breccia fragments, from 2 to 20 mm across, fine
to medium crystalline, that compose up to 40% of the
rock. The matrix is sulphurous limy-dolomite to dolo-
mitic limestone; grey to moderately dark grey and finely
crystalline. The dolostone breccia undergoes replacement
by chert in the same manner as the fine grained dolostone.
North of Cariboo River at Limestone Creek the dolostone
breccia contains clasts of angular dolostone, chert, cherty
pelite and minor white sandstone and quartz sand
(Fig. 12). The quartz sand varies in concentration from
near 0 to 40%; locally forming calcareous sandstone.
Sand-size quartz grains are rounded and average 1 mm
across. There is a marked bimodality of quartz grain size
with medium-coarse sand being distinct from the silt-size.
Campbell et al. (1973, p. 56-57) described coral-bearing
limestone from the same outcrop horizon but this was
not found. Thickness of the dolostone horizon varies
from 1 to 60 m, possibly being thicker on Kimball Ridge
where the top is unseen.

Figure 12. Dolostone breccia of the chert-carbonate unit
of the Black Stuart Group near the mouth of Limestone
Creek. (GSC 191008)

On Anderson Ridge, through the keel of the Black
Stuart Syncline, the chert-carbonate unit is mostly chert
breccia (Fig. 13) and minor conglomerate. The chert brec-
cia varies from near white to being streaked in shades of
grey. It consists of 30% angular chert fragments, 5 to
30 mm across, commonly elongate, defining a prominent
lineation parallel to the regional northwest-trending fold
axis. The matrix is chert which may be darker or lighter
than the clasts. The chert breccia appears to be a replace- |
ment of the dolostone breccia and can be up to 100 m
thick, but is commonly less than 40 m thick. Conglom-
erate overlies the chert breccia on Anderson Ridge, 5 km
east-southeast of Black Stuart Mountain. It is 8 m thick
and consists of 1 to 10 cm clasts of dolostone, white and
moderately finely crystalline and grey chert. Clasts are
subrounded to angular and vary greatly in concentration.
The matrix is composed of quartz sand; greenish brown
weathering dark grey.

Bioclastic limestone directly overlies the Lower
Cambrian Mural Formation on Anderson Ridge and is
included in the chert-carbonate unit. It is grey and light
grey weathering grey with very poorly preserved bioclastic
debris, some of which resembles Tentaculitids and shell
fragments. Locally preserved are 5 to 15 cm beds differ-
entiated by various concentrations of debris. In other
places augen shapes lie in the metamorphic foliation
and may be sedimentary or tectonic breccia fragments
(Fig. 14). The limestone may be the one from which
Lenz (1977) collected Silurian brachiopods.

North of Antler Creek and southwest of Waverly
Mountain, the chert-carbonate unit may be represented

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