General Geology

and as a syncline on the crest of Fawnie Range. No other strata referable to this
unit are known in Nechako area.

The volcanic flows and pyroclastic rocks are mainly reddish brown, purplish
red, brown, green, and grey. The flows are mainly andesite, rhyolite, and basalt.
A fine-grained rhyolite, mainly a cryptocrystalline mass of quartz, occurs on
Fawnie Nose Mountain and rhyolite breccia on Kuyakuz Mountain. Andesite
is common to all areas of outcrop but andesitic and basaltic tuffs and breccias
are more abundant, particularly on Kuyakuz Mountain. The andesitic and basaltic
rocks are not appreciably different from those of the chert-pebble conglomerate
unit except that red, brown, and purplish types are more abundant.

The sedimentary rocks comprise conglomerate, greywacke, shale, and arkose,
commonly derived from erosion of contemporaneous volcanic rocks, but also
include some debris from older rocks. Conglomerates are uncommon in this
unit but grey to brown conglomerate is prominent in the Fawnie Range section.
Black shales, green tuffaceous shales, and brownish shales occur as numerous
but discontinuous beds in all sections. Green andesitic greywacke with abundant
wood fragments and carbonaceous imprints occurs along Taiuk Creek. Arkose
and arkosic conglomerate occur along Euchiniko Creek.

The arkose that occurs in this unit is an interesting rock type consisting
of euhedral grains of feldspar in a greenish chlorite, biotite, and quartz matrix.
Sorting is poor and on first glance the rock appears to be igneous. However, the
rounding of the quartz grains, crude stratification, and interlayering with fossilif-
erous marine shales point to a marine sedimentary environment. The beds of
arkose are from 2 to 50 feet thick but most are about 3 feet. The lack of sorting
and the fresh, unbroken appearance of the feldspar grains point to rapid trans-
portation for a short distance and rapid burial. An arkosic conglomerate occurs
in one place with irregular pebbles (2 inches in diameter) of granodiorite em-
bedded in an arkosic matrix. The outline of the pebbles is lost on a fresh sur-
face; only on a weathered surface is it distinguishable. The granodiorite pebbles
and feldspar grains in hand specimen are seen to bear a close resemblance to
the rocks and feldspars of the Topley Intrusions. The proximity of the Topley
Intrusions, which were undergoing erosion at the time, suggests the obvious
source of the arkoses. Furthermore, the short distance over which the material
was transported as indicated by the arkoses, as well as the carbonaceous imprints
and coarse clastic fragments present, suggests that the rock formed near the north-
east margin of the sedimentary basin.

The lithology of the rocks in the different sections of this unit varies greatly
but the strata in each are considered to be correlative in time, and deposited in
a single unstable sedimentary basin, hence they should be considered as a
single mapping unit. A lithological characteristic of the unit would be its
heterogeneity rather than homogeneity. It is not known to contain sedimentary
strata identical with or even similar to those of the underlying chert-pebble con-
glomerate unit.

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