38

exposed for about 2 miles downstream from the mouth of Asitka River,
display consistent and fairly uniform southwesterly dips. Throughout
the above described region dips are commonly less than 55 degrees, but in
a few localities are steeper.

The structure of these formations is notably affected by the previously
mentioned northwesterly trending belt of faults, about 12 miles wide, that
includes most of the known major faults of the map-area. Thus, in general,
dips steeper than 55 degrees are most common within these strata adjacent
to the southwest edge of the faulted belt, that is, near the Saiya fault and
the Red Creek and associated faults. For example, an isolated area of
Sustut rocks, about 7 miles long, lying on the southwest side of and
parallel with Red Creek, displays four consecutive fold axes within a space
of about 14 miles. These folds are overturned towards the northeast,
and the axial planes trend about northwest and dip about 60 degrees south-
west. The formations on the southwest flanks of the synclines are about
vertical or, as indicated by crossbedding, grain gradation, and fracture
cleavage, are overturned towards the northeast as much as 30 degrees,
whereas strata on the northeast flanks commonly dip about 45 degrees
southwest.

The progressively tighter folds and steeper dips exhibited by these
strata as they approach the faulted belt, and the overturned axial planes
in that vicinity, suggest that the Sustut formations were folded by a thrust
from the southwest.

The strata near Thutade and Thorne Lakes and Niven River, which
lie northeast of the faulted belt, are much more gently inclined than those
within and southwest of the belt. Thus, northwest of Thutade Lake they
are horizontal or inclined as much as 10 degrees. Between Thutade and
Thorne Lakes they commonly trend about north and dip westerly at angles
up to 25 degrees. although dips of about 10 degrees are probably more
general. West of Niven River, and nearer the faulted belt, dips of as much
as 20 degrees are common and the distances between fold axes are probably
several miles.

The boundary between the gently inclined strata northeast of the
faulted belt and their much more steeply inclined counterparts to the
southwest crosses the west boundary of the map-area near Niven River
and, as shown by air photographs, extends northwesterly from this point
for many miles.

Local plunges of as much as 10 degrees were noted, some towards the
northwest and others towards the southeast. In general, the folds involving
the Sustut group probably plunge gently towards the northwest, because
he formations of this group appear to become more widespread in that
direction.

External Structural Relations

The Sustut formations rest on strata of the upper, Jurassic division of
the Takla group, and almost surely on those of the lower division of the
Takla group and the Asitka group. They are not known to occur in areas
underlain by the Omineca intrusions or by rocks of the Cache Creek group.

The youngest Takla rocks are of Jurassic age, and the oldest Sustut,
Upper Cretaceous. Thus, a long interval of erosion must have intervened
between the deposition of the upper division of the Takla group and the