116

STRUCTURE

With the exception of small faulted blocks east of Osilinka River and
west of Uslika Lake, all of these late Paleozoic rocks strike northwest and
dip to the southwest at angles ranging from 30 to 90 degrees and averaging
about 60 degrees. The tops of beds, as shown by grain gradation in almost
all exposures of banded tuffs, consistently face southwest. At the west
edge of the map-area, a few banded tuffs strike north to north 15 degrees
east, with a vertical dip. Local folds and drag-folds are rare, and are
best developed in beds of argillite such as those exposed on Tutizika River
(See Plate VIII A). Some of the tuffs show small, local slump structures.
The minor variations of strike and dip are due principally to the lenticular
nature of the beds.

A series of northwest trending, steeply dipping faults has divided the
rocks of this group, in their southern mountain area, into slices about 2
miles wide; the slice northeast of each fault appears to have been lowered
relative to its neighbour to the southwest. The Uslika formation has
apparently been emplaced along two of these faults, which in places nearly
coincide with the bedding planes (See page 188). All of these rocks and
structures have been offset by a transverse fault along Osilinka River
Valley. The west side of this fault moved northward about 14 miles
relative to the east side. As a result of these fault movements, a block
about 2 square miles in area northeast of Uslika Lake has been tilted to
the east, with dips ranging from 25 to 70 degrees; and another, consider-
ably shattered part of the map-unit, exposed on Thane Creek, has a
westerly strike with dips of 10 to 60 degrees south.

The upper part of the map-unit exposed on Tutizika River is highly
sheared parallel with the bedding, and some shears may represent south-
west dipping faults of considerable displacement (See Plate VIII A). These
faults all indicate that the rocks on the southwest side have dropped and
moved to the south with respect to those on the northeast side, and they
may be subsidiary breaks of a large compound fault zone forming the
contact between this map-unit and the overlying Takla group rocks.

Apparently, few faults occur in these rocks in the Lay Range, and
no appreciable displacement was observed on any that cross the bedding.
About forty carbonatized shear zones were noted in the entire range. Most
of these are less than 5 feet wide, and parallel the bedding. The largest
carbonatized zone is about 100 feet wide and at least 3 miles long, and
cuts obliquely across the beds, striking about north 10 degrees east, near
the west border of the map-area. Beds are, apparently, not offset along
this zone. The hematite-impregnated shatter zone above the large conglo-
merate body between the headwaters of Swannell River and Lay Creek does
not seem to have been the locus of significant net movement. Shear zones
up to 100 feet wide, generally parallel with the bedding and heavily
impregnated with pyrite and pyrrhotite, were observed at the northwest
end of Lay Range and in the canyon on Polaris Creek.

The rocks of this late Paleozoic map-unit are adjoined to the south-
west by volcanic flows and intercalated sedimentary rocks of the Takla
group, which is Upper Triassic and Jurassic in age. The division between
the two map-units is based mainly on structural considerations, for