Nechako River Map-Area

mafic minerals, occurs as a minor part of the lava assemblages. Some so-called
porphyritic andesites should be more properly called basalts on the basis of
feldspar composition but as yet sufficient information is not available to estimate
the proportion. At present it is believed that the bulk of the lavas in the Takla
Group are andesites.

The lavas occur in flows 25 to 100 feet thick but large structureless masses
occur that may be very thick flows, or possibly necks or stocks that were the
feeders for the flows. Pillow lavas were seen only south of Skins Lake. Vesicular
and amygdaloidal flows rarely occur, and columnar flows were not seen.

Associated with the lavas, and generally in lesser amounts, are volcanic
breccias and tuffs that commonly are well bedded, poorly to well sorted, and
graded. The fragments are derived almost entirely from contemporaneous vol-
canism but occasionally chert, limestone, or quartzite fragments from the Permian
strata are incorporated either by erosion of the older rocks or by eruption through
the older strata. Most of the coarse breccias are unsorted and show no water
action, but finer breccias and tuffs show various degrees of sorting, stratification,
grading, and water-working. Many tuffs should be called argillaceous tuffs, in-
dicating that an important fraction of their constituent material is derived from
normal erosion. The well-bedded tuffs commonly grade into well-bedded, black
argillite.

The tuffs are evenly bedded and commonly graded. The thickness of in-
dividual beds ranges from a fraction of an inch to 18 inches, and commonly
decreases upward in any one section. Sections totalling more than 300 feet
have not been observed. The breccias seem to be more abundant than the tuffs,
but this may not actually be so, as the tuffs are softer and more likely to be
covered. At several places the tuffs become coarser downward and pass into fine
breccias; upward they grade into argillites. This relation was common but not
everywhere apparent or invariable.

Argillites and shales occur associated with the tuffs or as distinct sedimentary
units interlayered with flows. Limestone and greywacke are rare.

The argillites are black, hard, siliceous and fine grained, and display ex-
cellent bedding, but crossbedding is rare and ripple-marks were not observed.
Some of the beds are limy and a weathered surface has a ribbed appearance
caused by alternating resistant and non-resistant beds. Although some of these
beds may be chert, none was definitely recognized as such. Graded bedding is
common and is abundantly visible in hand specimens and commonly in thin-
section. Even the coarsest component however would probably be classed as
fine sand.

Limestone or limy beds are rare, but as mentioned in the previous paragraph,
some argillites are limy. Presumably, in some beds the lime content predominates,
but such rocks are uncommon. A shell limestone bed 15 feet thick is interbedded
with argillites on Tetachuck Lake near Bryan Arm, but is only of local extent.

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