The sandstone unit is found south of Cariboo River
on Black Stuart Mountain and 3.5 km east-southeast and
2.7 km southeast of the mountain. The thickness of the
unit is less than 30 m but nowhere is the top seen. The
lower contact is sharp and conformable onto black pelite
and siltite of the black pelite unit.

The predominant quartzite and siltite is brownish-
olive-grey weathering grey with thin dark grey lamellae,
fine grained and micaceous (Fig. 16). Thin section exam-
ination of an impure quartzite from Black Stuart Moun-
tain revealed the rock as composed of mostly subrounded
quartz and a matrix of the secondary minerals calcite,
dolomite, sericite and clay with minor quantities of
plagioclase, potassium feldspar, detrital and secondary
opaques, zircon and tourmaline. Bedding with grey pelite
is from 1 to 20 cm thick and commonly graded. Local
sedimentary features include load and flame structures,
fine scale lensing and ripples (1 cm amplitude, 3 cm wave-
length). Quartzite beds are very evenly bedded with con-
stant thickness maintained for at least 30 m. At Black
Stuart Mountain a bed of quartzite is spotted with pink
lenticles of medium grained quartz grains. The sandstone
unit exposed 2.7 km southeast of Black Stuart Mountain
includes white moderately fine grained clean quartzite and
black to white chert (Fig. 17). White quartzite and black
impure chert form 2 cm interbeds. The chert has irregu-
lar patches of white. Flames of chert protrude upward
into the white quartzite, suggesting loading of the quartz-
ite onto a soft less dense horizon. The black impure chert
is probably a silicified micrite, which would account for
the density differential required for production of the
load and flame structures associated with overlying
quartzite.

Age and correlation. Mansy and Campbell (1970) and
Campbell et al. (1973, p. 57) reported the age of the chert-
carbonate unit of the Black Stuart Group as Lower Devo-
nian from identification of corals and conodonts. Mansy
(1970, p. 70) reported some organic looking small rods
from beneath the sandy unit 2.7 km southeast of Black

Ry,
4

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Figure 16. The bedding style of fine grained sandstone
of the sandstone unit of the Black Stuart Group. (GSC
191012)

Figure 17. Chert of the sandstone unit of the Black
Stuart Group near Black Stuart Mountain. (GSC 191013)

Stuart Mountain. T.E. Bolton (in Mansy, 1970) specu-
lated that they may be a type of coral found generally
in the Upper Ordovician. Lenz (1977) described brachio-
pods from limestone breccia blocks taken from the basal
unit at its easternmost exposure on Anderson Ridge
(52°53’N, 121°03’W). Lenz proposed these brachiopods
as Llandoverian or Wendlockian.

The black pelite unit has yielded Ordovician and
Devonian fossils; ages that are older and younger than
the chert-carbonate unit. The black pelite unit must there-
fore contain at least two separate sequences interrupted
by deposition of the chert-carbonate unit. The two
sequences were not divisible during mapping.

The oldest black pelite unit has yielded deformed
graptolites from dark grey slate on the ridge 2.5 km south-
southeast of Black Stuart Mountain (52°52’27.5’’N, »
121°06’59"’W; GSC Locality C-53427). B.S. Norford
identified them as Climacograptus? sp., Dicellograptus
sp. and Orthograptus sp. indicating late Middle or Late
Ordovician (Caradoc or Ashgill). The exact position of
this collection in the stratigraphic sequence is unknown.
The base of the dark grey slate is thrust onto the chert-
carbonate unit and onto Cambrian Mural Formation.
The fossil locality can be linked to components of the
black pelite unit underlying the sandstone unit of Black
Stuart Mountain.

The chert carbonate unit has yielded a conodont frag-
ment from the same locality at Limestone Creek
(52°58’59.4’’N, 121°11’10.7’’W; GSC Locality C-82852)
as reported by Campbell et al. (1973). It was identified
by T.T. Uyeno as Icriodus woschmidti Ziegler, and
assigned to the latest Silurian to Early Devonian (Prido-
lian to Lochkovian). This is in complete agreement with
ages determined for the fauna collected by Campbell
(Campbell et al., 1973).

The younger black pelite unit has yielded two Devo-
nian fossil localities. One of them occurs beneath the
sandstone unit 2.8 km southeast of Black Stuart Moun-

Pa