82

EXTERNAL STRUCTURAL RELATIONS

The general structural conformity between the Ingenika group and
the underlying Tenakihi group has been noted. This relationship holds
for all exposed contacts between the two groups in the map-area except
on the south flank of Tenakihi Range north of Osilinka River near the
east border of the map-area, and west of Swannell River, east of Mount
Lay, where in each place the Ingenika group strata have been faulted
against Tenakihi group rocks of different structural orientation. The belt
of limestones southwest of Chase Mountain may possibly be down-faulted
against the Tenakihi group, but the actual contact is nowhere exposed,
and outcrops of each respective group, separated by less than 100 feet,
show approximately the same bedding plane attitude. The possibility of
such a fault is suggested mainly by the lack of conglomerates and quartz-
ites, which form the lower Ingenika group northeast of Chase Mountain,
at an equivalent stratigraphic position (with regard to the Tenakihi group)
on the southwest side of the anticlinorium, indicating, if faulting does not
occur, a pronounced lenticularity of the Ingenika group beds; and by the
fact that the contact at this place is marked by an alined series of
topographically low areas, in contrast with the known conformable con-
tacts that have no topographic expression.

The upper contact of the Ingenika group is exposed at only one place
within the map-area; at its extreme western border, between the two main
branches of the south fork of Wrede Creek (See Structure-section C-D).
At this locality the Ingenika group and the overlying late Paleozoic rocks
are strongly sheared, somewhat pyritized, and the bedding relations are
obscure. The actual contact is a shear zone about 75 feet wide, striking
north 45 to 50 degrees west, and dipping vertically. No evidence indicating
the direction of movement along this break was found, but subsidiary
parallel shears on the Ingenika group side in chlorite-sericite slate have
bent and dragged the beds, suggesting that in these shears the northeast
side has moved up and apparently to the west relative to the southwest
side.

In an attempt to obtain further information on the relations between
the Ingenika group and the overlying rocks, the contact was followed north-
westerly to where a good exposure was found. On the crest of the Wrede
Range, 7 miles west of the west border of the map-area, and about 13
miles along the contact from the aforementioned exposure, black, calcareous
slates, with interbedded blue-grey phyllites and light grey limestones of
the Ingenika group, are in definite fault contact against fresh, bedded,
andesitic tuffs, included in the lower division of the Takla group, of pre-
Lower Jurassic age (Lord, 1948, p. 27). The bedding in the Ingenika group
here strikes north 45 degrees west, dips 85 to 90 degrees northeast. and is
minutely contorted into folds whose axial planes strike north 30 to 50
degrees east and dip 25 to 45 degrees southeast, and whose axes dip about
60 degrees southeast. The fault plane strikes north 35 degrees west, and
dips 80 degrees southwest, and it appears that the northeast side has moved
up and to the northwest with respect to the southwest side. This exposure
would appear to indicate that, at least on the crest of the Wrede Range,
the younger rocks have been faulted against the Ingenika group strata, and