122

STRUCTURE

The three divisions of the Cache Creek group in the southeastern
corner of the map-area are separated by well-exposed faults. The eastern-
most, or limestone, division, has a thickness within the map-area of about
1,500 feet, and dips gently to the southwest. Investigation of the geology
immediately east of the map-area has shown that the limestone overlies
a unit, apparently about 9,000 feet thick, composed largely of argillaceous
rocks and chert in the upper part, and volcanic rocks, chert, and argillites
in the lower part, which in turn overlies another limestone member at least
800 feet thick. The greenstone division in the map-area appears to be
about 6,000 feet thick, unless there has been repetition of strata by faulting.
In general, the rocks of this division strike about north 15 degrees west
and dip steeply southwest, but several large- and small-scale irregular
folds give individual outcrops a wide variety of attitudes. The structures
of the greenstone division are about parallel with those of the argillite
division to the west. This feature, together with the observation that the
upper members of the greenstone division become progressively richer in
sedimentary material, and that most of the volcanic material of the
argillite division is in its lower members, suggests that there may not
have been much stratigraphic displacement along the fault separating the
two divisions, which may, therefore, be parts of an essentially continuous
section. However, the boundary fault is not quite parallel with the bedding
planes of either division; strata on both sides appear to have been trun-
cated; and there has been some, and possibly much, omission or duplication
of beds by the fault movement.

The argillite division is more intensely deformed than the limestone
or greenstone divisions, and drag-folds and local folds up to 500 feet
across are common. In most places the shears and slaty cleavage traverse
the contorted argillites and ribbon cherts without producing noticeable
offset of the beds. A section of this division measured south of Wasi Lake
has an apparent thickness of about 4,000 feet.

The lower east slopes of Conglomerate Mountain, west of Wasi Lake,
are intersected by innumerable faults and shear zones, with a wide range of
attitudes. A study of the direction of slickensides and order of movement
of these breaks, the pattern of the associated systems of joints and quartz
veins, and the position and form of drag-folds, suggests that the structures
could have been produced by compression acting along an axis that now
strikes northeast and plunges to the southwest at an angle of about 50
degrees.

On the basis of the lithology and general structure of the Cache Creek
group rocks exposed within and east of the map-area, it is suggested,
tentatively, that the greenstone division is the oldest unit; that it is overlain
by the argillite and limestone divisions respectively; and that it is under-
lain by another belt of limestone, which is not exposed in the map-area, but
which is widespread in the unmapped region to the east. The total thick-
ness of the group is postulated to be not less than 10,000 feet. It should be
noted, however, that in the Stuart Lake belt of Cache Creek rocks of Fort
St. James map-area to the south, to which the rocks in the southwest