General Geology

interlayered with lesser amounts of fine-grained, water-lain tuff, argillite, and
greywacke. Non-marine rocks are rare.

Within Nechako River map-area a group of rocks typical of the Takla Group,
here mapped as the main unit, are exposed, as well as a red bed sequence that
interfingers with rocks of the main unit and apparently represents the same time
span. The writer has mapped two sequences as distinct units, fully realizing that
the red bed sequence is not typical and should perhaps be mapped as a separate

local group or formation. For the present, however, it is considered as part of the
Takla Group.

Main Takla Unit
Distribution

This unit occurs as two disconnected belts, one forming the core of the
Fawnie Range and the Tetachuck Hills, the other occurring along the western
margin of the Topley Intrusions. These belts form the approximate northeasterly
and southwesterly rims of a northwesterly trending Middle Jurassic trough or
basin. Later folding, faulting, and erosion brought Takla rocks to the surface
at a few places within this basin.

Lithology

No complete section of this part of the Takla Group is exposed in Nechako
River area, only two or three separate sections believed to represent different
parts of the group with little time overlap indicate at least 5,000 feet of inter-
layered volcanic and sedimentary rocks. Andesitic and basaltic pyroclastic rocks
and lavas make up by far the greater part of the group unless repetition or
omission of parts have given an erroneous impression. Well-bedded black
argillite, argillaceous tuff, and argillaceous limestone are the more important
sediments.

The lavas are black to dark green, porphyritic, basic andesites and basalts,
which are best exposed in the Naglico Hills near the southern margin of the
map-area, on Tutiai Mountain, and on the west end of Fawnie Nose Mountain.
Tatuk Hills is made up of similar flows but also includes some purplish andesites.

The flows consist mainly of dark green porphyritic andesite with phenocrysts
of grey plagioclase or dark green to black augite; the phenocrysts are seldom
prominent, and are generally less than a quarter of an inch in any dimension. The
groundmass is a fine-grained aggregate of andesine or labradorite, augite, chlorite,
and magnetite. Saussuritization of feldspars and alteration of augite to hornblende
and chlorite have occurred, but alteration and metamorphism is not a pronounced
feature except near some plutonic masses. Some fine-grained, dark grey to black
basalt, made up of a cryptocrystalline aggregate of plagioclase, magnetite, and

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