Figure 20. Radiolaria in a chert clast of the Guyet
Formation conglomerate. Clast is 6 mm wide along the
horizontal. (GSC 191018)

the Guyet Formation conglomerate is not associated with
this erosional event. The Guyet Formation conglomerate
has been deposited into Black Stuart Group black shales
and the provenance of the clasts is mainly beyond the
bounds of the present outcrops of the Guyet Formation.

The cherts and cherty sediment clasts of the Guyet
Formation resemble Antler Formation sediments and
were probably derived from a similar terrane. Because
no such terrane is known to have existed east of the map
area the source rocks were west of the depositional site.
The volcanic clasts resemble the Waverly Formation and
may have been derived locally or from Waverly equiva-
lents to the west. Sources for the quartz, siltstone, quartz-
ite, and meager feldspar are not known.

Age and correlation. The Guyet Formation cannot be
dated directly. Its position beneath the Greenberry For-
mation, and most likely below upper Middle Devonian
Black Stuart Group, brackets the conglomerate to the
Late Devonian and/or Early Mississippian. Part of the
conglomerate may be younger, as seen from the relation-
ships on the island at the north end of Swan Lake. There,
clasts of a chert in pebble conglomerate are also found
within the directly overlying Tournaisian Greenberry For-
mation limestone. The Guyet Formation conglomerate

32

is a time and lithological correlative of the Earn Group
of Yukon (equivalents of which are informally called
‘*black clastic’’) and its correlatives in northern British
Columbia (Gabrielse et al., 1977; Taylor et al., 1979).

Paleoenvironmental history of the Black Stuart
Group

The Black Stuart Group was deposited onto a gently
undulating Ordovician unconformable surface (see sec-
tion on Pre-Columbian palinspastic reconstruction for
details). Where exposed the unconformity is nearly paral-
lel to bedding, cutting through underlying stratigraphy
at shallow angles.

The oldest unit of the Black Stuart Group is grapto-
litic shale. The unit is discontinuous and is known with
certainty from only one area. The configuration of the
unconformity may have had some influence on its distri-
bution and facies, but because the shape of the uncon-
formity is not known in detail, this possibility can not
be pursued. Lithostratigraphic details from the graptoli-
tic shale unit are: 1) the fine grained nature, 2) the even
finely laminated bedding, 3) the absence of ripples, cross-
laminations, graded bedding or dewatering and density
inversion structures (flames, loads, convolution), 4) the
absence of active burrowing organisms, 5) the moderate
organic content (from dark grey colour) and 6) the pres-
ence of graptolite remains. Those features are sufficient
to indicate that the shales were deposited in a quiet, stag-
nant basin receiving a limited input of clastics. Benedict
and Walker (1978, p. 583), who summarized paleobathy-
metric indicators into table form, thought that flat thin
laminae, abundant fine grains and graptolitic remains
suggest water as shallow as 40 m but possibly much
deeper. The source area and the direction of transport
of these shales is unknown.

The chert-carbonate unit overlies the graptolitic shale
of the black pelite unit and is restricted to the southern
part of the map area. It changes character from the mas-
sive dolostone of Kimball Ridge southwestward to dolo-
stone breccia. Sedimentary features of the dolostone
breccia include: 1) clasts of chert, quartz, siltstone and
dolostone near Limestone Creek, 2) pebbles and cobbles
of dolostone in quartz sand found interbedded with
dolostone breccia on Anderson Ridge, and 3) a limestone
to limy dolomite matrix to the dolostone breccia frag-
ments. The confinement of the dolostone to the unit sug-
gests dolomitization at the time of deposition. The clas-
tic rock of the southwestern facies indicates mechanical
deposition (not recorded to the northeast) but could be
either transported or in situ debris. The sand and chert
fragments suggest that at least some of the material was
from elsewhere. The dolostone of Kimball Ridge may
have been partly broken up and transported to the south-
west. The chert-carbonate unit is reasoned to have been
deposited in shallow water because 1) penecontempo-
raneous dolomitization is a very shallow-water phenom-
enon and 2) conodonts from the unit are all shallow-water
forms (see Seddon and Sweet, 1971). It may be in part