Nechako River Map-Area

reasons? these rocks were not mapped as Hazelton Group (1) the group is entirely
sedimentary, unlike the Hazelton Group which includes volcanic rocks; (2) the
sediments occur in a distinct basin whose boundaries can be drawn precisely; and
(3) in Terrace area the group rests unconformably on Hazelton Group volcanic
rocks (Duffell and Souther, 1956).

The rocks were therefore named the Bowser Group, and comprise a sedi-
mentary succession of Late Jurassic and Lower Cretaceous age. This group in-
cludes rocks formerly mapped as Skeena Formation and some rocks formerly
mapped as Hazelton Group. The Hazelton Group is therefore restricted to Middle
Jurassic, probably early Middle Jurassic (Bajocian) strata, but in so restricting
the age, the areal extent is only slightly lessened.

Within Nechako River map-area the Hazelton Group is mapped as two
units, the chert-pebble conglomerate and the Middle Jurassic. These units may
with further study be designated as formations. They are more fully described
elsewhere (Tipper, 1959a).

Chert-Pebble Conglomerate Unit

This unit is characterized by chert-pebble conglomerate, but volcanic rocks
—both flows and pyroclastic—comprise well over 50 per cent. Conglomerate is
present in all sections studied, but in places is of minor importance. Shale and
greywacke are commonly interbedded with the conglomerate and in places predom-
inate, but limestone was never observed.

The lavas of this unit are characteristically green, dark grey, black, and
reddish brown andesite and basalt not unlike those of the Takla Group volcanics
but commonly fresher. Characteristically the flows have numerous white or grey
lath-shaped phenocrysts of plagioclase one sixteenth to one quarter inch long, in
a finely crystalline, felted groundmass of plagioclase, magnetite, chlorite, and
epidote. Amygdules, vesicles, and pillow structures are characteristically absent
from these rocks in this area. Some of the lavas are massive, consisting of a rock
with a microcrystalline texture and a mineral content similar to that of the por-
phyritic lavas. Individual flows are difficult to distinguish but are believed to be
from 15 to 100 feet thick. The best exposures of the lavas are on Cutoff Butte
and in Nechako Range.

The tuffs and breccias of this unit are more easily recognized than those of
the Takla Group. They are interbedded and each contains angular fragments.
They show a fairly distinct sorting but are generally ungraded. The fragmental
rocks appear to be slightly reworked and sorted into distinct beds of similarly sized
fragments. The fragments consist of broken feldspar crystals and volcanic rock
similar in most respects to the contemporaneous volcanic flows. Generally they

'Roddick, J. A. Personal communication.

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