(1926) and Sutherland Brown (1957,1963). They postu-
lated that a major deformational event occurred prior to
the deposition of the Guyet Formation. The deformation
was considered Devono-Mississippian because of the sus-
pected Carboniferous age of the Guyet Formation. It was
named the Cariboo Orogeny by White (1959). From the
revised stratigraphic relationships of this study, the major
unconformity is not at the base of the Guyet Formation,
but at the base of the Black Stuart Group. The uncon-
formity is Ordovician and does not represent a major
deformation within the Cariboo Terrane of the area and
it therefore does not satisfy the definition of the Cariboo
Orogeny.

Stratigraphic control of structure

Pre-existing structures appear to influence the formation
of superimposed structures. Pre-existing structures
include stratigraphic features such as lithology, thickness
change and bedding orientation and conventional tectonic
structures. Within the area structural control is dominated
by the competency of Yanks Peak Formation quartzite,
the discontinuity of Yanks Peak Formation quartzite, the
“‘arches’’ due to thickness changes and the broad-scale
warping that occurred during the lower Ordovician. First,
the structural features seen in the cross-sections of the
map area will be described in relation to the above men-
tioned controls. Then, structural controls will be dis-
cussed with the aid of a clay model.

The most prominent feature of the palinspastic
reconstruction of cross-section E-E’ is the ‘‘arch’’ which
is now located on the ridge northeast of Sunshine Creek.
On the west flank of this arch is a thick wedge of Yanks
Peak Formation quartzite. The area of the arch is a locus
for thrusts which carry the thick stratigraphic package
underlying Kimball and Anderson ridges. Mural Forma-
tion is preserved beneath the unconformity both west and
east of the lens of Yanks Peak Formation quartzite in
the western part of the palinspastic sections (Fig. 33). The
thrust carrying Cunningham Formation over Mural For-
mation cuts through the stratigraphy west of the Yanks
Peak Formation quartzite lens and on the eastern side
of the remnant of Mural Formation. The thrust appears
to have been localized between these two competent units.

Cross-section C-C’ shows the faulted fold pair of the
Roundtop Mountain area. There are two prominent fea-
tures in this area; the rapid westward downcutting of the
unconformity and the local abundance of Yanks Peak
Formation quartzite. The ductile phase thrusts superim-
pose the quartzite and the described unconformity. The
thrust relationship is like that described for the western
thrust of cross-sections E-E’ and D-D’. The thrust cuts
upsection at the western edge of the Yanks Peak Forma-
tion quartzite lens. With a horizontal datum for the base
of the Black Stuart Group, the western limb of the pres-
ent syncline would have existed as an east-dipping mono-
cline prior to the Columbian event.

Clay models were made to have two principal fea-
tures. They had angular unconformities and layers of
different thickness and viscosity. These features are sus-
pected to influence the formation of deformational struc-
tures. The clay models were placed in a vise and slowly
squeezed horizontally. One experiment is shown through
various stages of deformation in Figure 34. The series of
pictures in Figure 34 is only meant to illustrate the idea
that tectonic structures are controlled by pre-existing
features. The experiment is not meant to prove the point,
only to illustrate it. Figure 34A shows the configuration
of layers prior to squeezing. An anticline-syncline pair
with a larger scale anticline is cut off by an unconform-
ity. Of the three viscosities represented the intermediate
grey is the most viscous, the darkest grey the least vis-
cous and the lightest grey the moderately viscous. Follow-
ing the series of pictures in Figure 34 it is obvious that:
1) the thrusts initiate in the vicinity of the hinge zone of
the anticline or ‘‘arch’’, 2) the initial fold pair becomes
accentuated, and 3) the final faulting and folding is con-
centrated on the limbs of the initial central syncline.
Figure 35, a cross-section of the model, shows the con-
trol the most viscous layer had on the position of the
thrust. The thrusts border exactly the area where the most
viscous layer is in contact with the unconformity. A
peculiarity of this feature is that beds separated by the
lower thrust appear not to match.

It is suggested that stratigraphic arches react similarly
to the syncline of the experiment and that the combina-
tion of arches and pre-existing ‘‘warps’’ of the Ordovi-
cian event governed the position and scale of the features
produced during the Columbian Orogeny.

ROCKS OF THE BARKERVILLE TERRANE

The Barkerville Terrane is bounded on the east by the —
Pleasant Valley Thrust across which it adjoins the Cari-
boo Terrane, and on the west by the Slide Mountain and
Quesnel terranes (Fig. 2). It is underlain by an unknown
basement and overlain by the tectonically emplaced Slide
Mountain Terrane. The Barkerville Terrane continues
beyond the map area northwest to near Prince George
and southeast to near Clearwater. It is treated separately
from surrounding rocks because it has a unique strati-
graphic succession and is bounded by faults.

It is recommended that most existing names for units
within this terrane be abandoned. The strata are here
divided into one formal and several informal units
(Table 14). The divisions are mainly informal because of
uncertainties concerning stratigraphic order. The Snow-
shoe Group is the formal unit; includes most rocks of
the Barkerville Terrane and has fourteen informal sub-
divisions: Ramos, Tregillus, Kee Khan, Keithley, Harveys
Ridge, Goose Peak, Agnes, Downey, Eaglesnest, Bralco,
Hardscrabble, unnamed carbonate, Island Mountain and
Tom. The Permian Sugar limestone (Orchard and Struik,
1985) rests with unknown relationship on the upper part
of the Snowshoe Group.

47