Nechako River Map-Area

Orthoquartzites and low-rank greywackes are interbedded with the con-
glomerates and differ from them only in grain size. They are of coarse sand
size and have a ‘salt and pepper’ appearance. Well-stratified, crossbedded or
ripple-marked beds are common. Individual beds vary in thickness from 2
inches to 12 feet, most being between 2 and 3 feet. The appearance of the
orthoquartzites is deceptive; the black chert grains suggest at first a greywacke,
but a microscopic examination shows a very high chert and quartz content,
commonly 90 per cent. Whereas the conglomerates are both marine and non-
marine, the orthoquartzites and greywackes everywhere suggest marine or near-
shore features and the common presence of belemnite moulds confirms their marine
origin. These rocks are more abundant near the centre of the sedimentary basin
and outcrop in the Fawnie Range, north of Chelaslie Lake, and north of Euchiniko
Lake.

Shales and argillites form an important part of the sedimentary section of
the chert-pebble conglomerate unit. These rocks are invariably dark grey or black,
fine grained, banded or massive, well sorted, and, because they are the least com-
petent rocks in this unit, are commonly sheared, contorted, and drawn out. In
composition many of the shales and argillites are merely finer equivalents of the
orthoquartzites and conglomerates and some may be referred to as quartzose
or siliceous shales and argillites. However, such purity is rare, and carbonaceous,
chloritic, and micaceous shales are more abundant, though all have a high
quartz or chert content. The shales become tuffaceous when interbedded with
pyroclastic strata. Calcareous shales and limestone are not known in the unit.
The shales are interbedded with conglomerates, orthoquartzites, greywackes, tuffs,
breccias, and flows and occur in marine and non-marine sections as indicated
by the presence of both non-marine flora and marine fauna. Although widespread,
the shales form thicker sections near the centre of the basin of sedimentation.

Middle Jurassic Unit

This unit is characteristic of the Hazelton Group both in lithology and age.
It is primarily a sedimentary unit with some interlayered flows, breccias, and
tuffs. The sedimentary rocks are commonly tuffaceous, mostly marine, but show
by the type of fragments that land areas were nearby. Many rock types are
present but none can be said to be typical, although the rocks of the unit are,
on the whole, less siliceous than those of the chert-pebble conglomerate unit.
This characteristic may be used to a limited extent to define the unit, but for
positive identification, palsontological evidence and knowledge of its relation
to the underlying unit are necessary. The rock types are andesite, rhyolite, related
tuffs and breccias, greywacke, conglomerate, shale, argillite, and arkose; no one
type is widespread or particularly predominant in sections examined. The unit
is exposed in three parts of the area, as a syncline trending along Big Bend Creek
and Euchiniko River, as a syncline forming the crest of Kuyakuz Mountain,

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