General Geology

are easily visible on either a fresh or a weathered surface, suggesting differences
in composition between fragments and matrix. Individual beds vary in thickness
from 4-inch beds of tuff to 5-to-25-foot beds of breccia. These are interlayered with
the flows and with sedimentary rocks.

The sedimentary rocks of the chert-pebble conglomerate unit are the con-
glomerates for which it was named—orthoquartzites, shales, and greywackes. Be-
cause of the high chert and quartz content, most of the rocks fit into the quart-
zite series of Krynine’s classification (1948, pp. 149-152); but with an increase
in chlorite, mica, and volcanic rock fragments, some members of the greywacke
series are present. In general, the conglomerates, orthoquartzites, and shales are
similar to one another in composition but differ in grain size.

The chert-pebble conglomerate beds are distributed around the margin of
the sedimentary basin and were deposited under both marine and non-marine
conditions. They are exposed in the northwest quarter of the area, along Chelas-
lie Lake and in isolated patches southeast from there to the southern boundary
of the area, and in Nechako Range. These conglomerates are interbedded not
only with the volcanic strata of the unit but also with the finer clastic sedimentary
rocks. Individual conglomerate beds vary in thickness from 2 inches to more than
25 feet, but show many irregularities with abrupt thickening or thinning and
lensing out. Laterally the conglomerate beds may pass gradually into finer clastic
rocks, but despite the variable thicknesses, they are fairly persistent. Pebbles in the
conglomerates are well rounded and vary in shape from tabular to almost perfect
spheres. The conglomerates are moderately to well sorted and rarely display graded
bedding; in finer phases, coarse crossbedding is common. Pebbles vary in size
from one sixteenth of an inch to 3 feet in diameter but most are from one half
inch to 2 inches. Most of the conglomerate includes more than 90 per cent quartz
and chert pebbles with about 10 per cent argillite, andesite, greenstone, and
rarely pebbles of limestone, granite, gneiss, and schist. Almost all the pebbles are
similar to rock types of the Permian Cache Creek Group, which outcrops north
and east of the area. The granite pebbles, locally abundant near Francois Lake,
are derived from the Topley Intrusions outcropping nearby. The matrix is mainly
sand-sized particles of the material contained in the pebbles. The cementing mate-
rial of the well-indurated beds is always siliceous, forming an impermeable rock.
Poorly cemented conglomerates mostly have limonite or calcite cement and the
whole rock is very porous and permeable, and commonly friable. The chert
pebbles are mainly of two kinds, black chert and grey chert, and the proportion
of these one to the other varies from 100 per cent black chert to over 90 per
cent grey chert. Black chert pebbles are characteristic of these conglomerates.
With increased distance from the source area of the Cache Creek strata, the per-
centage of andesite pebbles increases substantially to produce a greywacke conglom-
erate. Thus in the southwestern part of the area conglomerates consist of as
much as 50 per cent green andesite pebbles, derived presumably from Takla Group
andesite or the erosion of penecontemporaneous volcanic flows.

228)