Chapter III

GENERAL GEOLOGY

The geological history of the map-area, as expressed by the bedrock
formations, began in Mesozoic time. Except for a few small areas of
post-Oligocene basalt and some probable early Tertiary rhyolites, all other
rocks belong to the Mesozoic era and largely to the Jurassic period.

Upper Triassic breccia, tuff, flows and sedimentary rock outcrop in the
northeast corner of the map-area and may be correlated with the Takla
group, as May some green andesites along the eastern border in the vicinity
of Tetachuck Lake.

Metamorphosed volcanic and sedimentary rocks of mainly Middle
Jurassic age underlie by far the greater part of the area. No Upper or Lower
Jurassic beds were defined by fossils but volcanic rocks of these epochs
may occur.

Most of the sedimentary and volcanic rocks may be correlated with the
Hazelton group. No Cretaceous rocks included with the Hazelton group in
other areas were observed in Whitesail Lake area, though some do occur
immediately north of the north boundary near Anzac Lake.

The relationship of the Takla and Hazelton groups has never been
very clear. However, Tipper (1955), working in Nechako River map-area
to the east of Whitesail Lake area, found an erosional unconformity at the
base of the Middle Jurassic. This unconformity is expressed by an abundance
of chert pebble conglomerate and coarse clastic material which outcrop only
in the northeast corner of Whitesail Lake map-area. This unconformity
which Tipper followed down the whole western side of Nechako River
map-area forms a logical horizon, in this part of the country, for the
separation of the Takla and Hazelton groups. In Whitesail Lake area, as no
known Upper Jurassic rocks are present and the Lower Jurassic strata are,
according to the above principle, included with the Takla group, the Hazelton
group is confined to rocks of Middle Jurassic age.

Late Lower Cretaceous, fossiliferous shale, arkose, greywacke, and
voleanic rock, equivalent to the Haida formation of the Queen Charlotte
Islands, occur on Swing Peak and Laventie Mountain. These rocks outcrop
along the south shore of Tahtsa Lake but were not found on the north shore.
The relationship between these rocks and those of the Hazelton group is
certainly nonconformable and they may be in fault contact.

The plutonic rocks of the Coast Intrusions, which form the second
largest group of rocks within the map-area, intrude rocks of the Hazelton
group and to some extent the late Cretaceous rocks mentioned above. The

Pad