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STRUCTURAL GEOLOGY

In 1941 the writer traced the Pinchi fault zone from southeast

of Fort St. James to Kwanika Creek! In 1943 the fault zone was traced
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leeol. Surv., Canada, Paper 42-11.

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northwesterly to Omineca River Valley, and is now known to be at least
130 miles long. ‘This’ zone varies in width from 200 feet, near the mouth
of Vital Creek, to 5,000 feet, cast of Indata Lake, but in most places
does not exceed 1,000 feet. Its eastern margin represents the contact
between stratified Permian rocks on the west and Mesozoic formations
andthe Omineca batholith on the east. Although the fault contact be-

. tween; Permian and Mesozoic strata was nowhere observed, it secms probable
that the Pinchi fault zone marks the site of major thrust faulting from

the west, and that the Permian rocks have moved up with respect. to the
Mesozoic formations. Intense faulting was, however, observed in the
Permian rocks within the fault zone. There, the more important faults
trend northwesterly, dip steeply southwest, and may join 4 major low
angle thrust: fault at depth, In most places it is difficult to recognize

these. thrust faults because of numerous minor faults that strike and dip
in all directions, but along which offsets are relatively small. Along
most of the faults, major and minor, the wall-rocks are brecciated across
widths varying from a few inches to 30 feet, but, in places where the

faults are closely spaced brecciation may be continuous across several
hundred feet. Some of the breccia zones lie between slickensided walls.

Deposits of cinnabar’ occur along, or near, the Pinchi fault zone,
and the wall-rocks are commonly carbonatized or silicified.

Folding

The Permian sedimentary rocks exposed between the Pinchi fault
zone and Takla Lake, 20 miles. to the west, appear to represent part of
a great anticlinorium plunging northwest and widening to the southeast.
The rocks exposed near Quartz Creek and Fall River appear to lio near
the north end of this structure. One component synclinal fold has been
traced from near Fall River, where it is terminated by the Pinchi fault
zone, for 60 miles southeasterly to Trembleur Lake. Other parallel
folds were observed in the mountains at the heads of Dream and Canyon
Creeks, but were not traced far. Each of them is from 1 to 4 or more
miles wide. During the period of folding the limestone beds acted as
relatively competent strata and are not folded to nearly the same ex-
tent as the associated strata. The latter are compressed into many ~
minor anticlines and synclines, and are in part overturned. .7he minor
folds are from 100 to 700 feet wide and their axes are approximately
parallel to those of the major folds. Many drag folds were observed in
the argillaceous quartzites and ribbon cherts. These generally plunge
in the same direction as that part of the major fold on which they lie.

The Jurassic strata along Silver Creek form a series of north-
westerly trending folds, 500:-to 1,200 feet wide.

‘CARBONATI ZATION

Many of the crushed and sheared rocks in the Pinchi fault zone
have been hydrothermally altered. Grey limestones have been partly to
completely changed to buff dolomites, argillaceous quartzites and ribbon
cherts to quartz-carbonate schists, greenstones to chlorite-carbonate