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CHAPTER III

STRUCTURE
FAULTS

GENERAL DESCRIPTION

Faults are outstanding structural features of the map-area, and as
already stated most of those recognized lie within a belt some 12 miles
wide, the northeast edge of which extends northwesterly from Omineca
River at longitude 126 degrees to Niven River near longitude 127 degrees.
The abrupt change in the attitude of Sustut strata northwest from Niven
River near longitude 127 degrees, as observed on air photographs, suggests
that the faulted belt extends well northwest of the map-area; whereas
its southeastern counterpart is to be found in the adjacent Takla map-area
(Armstrong, 1946). Faults are exposed in a few places, mainly in creeks
and above timber-line, and their extensions are commonly indicated by the
courses of valleys and shallow depressions. A few faults, whose existence
has been inferred by abrupt structural discordances or by apparent gaps
in the stratigraphic sequence, are, so far as known, not exposed. Most
of the faults strike northwesterly, about parallel with the folds in the
enclosing strata; others strike west, west-northwest, north, or northeast.
In most places the inclinations of the fault planes are not known, but
where observed they are steep, and the topographic expression of all
faults is such as to suggest that all are steep. Where examined, the mapped
faults are marked by altered and disturbed zones ranging from about 25
feet to possibly 1,000 feet or more in width. These zones are characterized
by various combinations of schistose, fractured, crumpled, or carbonatized
and otherwise altered rock. Replacement by buff or rusty weathering fer-
ruginous carbonate characterizes many parts of the fault zones, particularly
southeast of Asitka River; but not all carbonatized zones mark significant
faults. Directions and magnitudes of the lateral displacements are not
known. The apparent vertical displacements may be greatest in the north-
east half of the faulted belt, along the Omineca, Carruthers, and Ominicetla
faults where, in places, as much as 10,000 feet of lower Takla strata
appear to be missing. The Omineca fault marks the northeast edge
of the faulted belt, and available evidence suggests that the strata
on the southwest side of this fault have moved upwards relative
to those on the northeast side. A similar displacement has been described
along its southeasterly extension, the Pinchi fault zone, which has been
interpreted (Armstrong, 1946) as the site of major thrust faulting from the
west. The apparent relative movement on other northwesterly trending
faults in McConnell Creek map-area is in the opposite direction, as if the
northeast sides had moved relatively upwards.