Chapter IV
STRUCTURAL AND HISTORICAL GEOLOGY

Structural Geology

The scarcity of well-exposed rock and the paucity of information observ-
able in outcrops made interpretation of the structural geology difficult. The Tertiary
groups include few bedded sedimentary rocks so that a structural interpretation
must be based on outcrop patterns and a close examination of all outcrops,
usually impossible to accomplish during this type of mapping. The Mesozoic rocks
offer more information, but even with these groups, the broad valleys and dis-
tance between outcrops preclude a satisfactory interpretation. The information
available is briefly recorded.

Folds

All the volcanic and sedimentary rock groups of this area are folded to
some degree, and practically all folds have a northwest trend. Similarly the Topley
Intrusions have a northwest elongation but not the younger granitic rocks. At no
place can the folds be described as tight or isoclinal except possibly within the
Cache Creek Group strata, for which there is no information in the area. Admit-
tedly the evidence is most incomplete, but nowhere were overturned folds recog-
nized or suspected; the regional metamorphism, even in the Takla Group rocks,
is low-grade (pronounced metamorphic effects, where seen, could be related to
contact metamorphism around granitic bodies), and in general the deformation
of rocks is not intense. Nonetheless, the high proportion of volcanic rocks, com-
monly structureless, may be responsible for masking what may well be a complex
fold pattern.

The Takla Group rocks appear to be the most strongly deformed with dips
up to 70 degrees and rarely steeper. The volcanic rocks of the Takla Group are
commonly structureless so that for large areas there is no hint as to the structure.

The Hazelton Group rocks are composed of relatively competent volcanic
and conglomeratic rocks interlayed with less competent shales. In general, this
group occurs in broad folds with dips up to 45 degrees, but locally, where shales
predominate, the rocks are more tightly folded with dips as high as 80 degrees.
These shales are commonly sheared and badly contorted. The folds that do occur
in these rocks, such as those of Fawnie Nose, Tatelkuz and Kuyakuz Mountains,
and Batnuni Lake valley, cannot be traced far as they are cut off by faults or are
covered by drift.

The Ootsa Lake Group has been warped into broad, open folds with dips
rarely as great as 45 degrees. However the structure of this group is difficult to

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