Nechako River Map-Area

From these three factors it is reasonable to conclude that the Topley Intru-
sions are of Lower Jurassic or late Upper Triassic age—a Lower Jurassic age
is preferred.

Correlation

Lower Jurassic granitic rocks are not common in British Columbia. The
Topley Intrusions, as several lithological units, have been extended now from
the type area north of Topley, across Fort St. James and Nechako River areas
and into Prince George map-area for an undetermined distance. Along the east
side of Quesnel map-area (Tipper, 1959) granodiorite, granite and diorite of
similar lithology and possibly the same age, outcrop discontinuously to the south-
east corner of the area. The Guichon batholith of Ashcroft (Duffell and McTaggart,
1952) and Nicola (Cockfield, 1948) map-areas is of Lower Jurassic age but
is not lithologically identical to the Topley Intrusions. All the areas and rocks
mentioned trend southeast and their geological setting is very similar. This belt as a
unit closely follows the distribution of the Cache Creek Group but is not parallel
with it. In the type area of Topley Intrusions the Cache Creek Group is east of the
granitic rocks; in the Quesnel area the granitic rocks in question are in the
centre of the belt of Cache Creek rocks; and the Guichon batholith is to the
east of much of the Cache Creek Group. If all these granitic intrusions rep-
resent a batholithic complex of Lower Jurassic age then the belt has a known
length of more than 360 miles and is comparable in length to that of the Omineca
Intrusions.

Post-Middle Jurassic Intrusions

Rocks of granitic to intermediate composition occur as stocks and bosses
in Nechako Range, Fawnie Range, and the hills north and south of Tetachuck
Lake. On the preliminary map (Tipper, 1957) these rocks were shown as two
units, one of which comprised the batholiths of Fawnie and Nechako Ranges
that cut the Middle Jurassic strata, and the other several smaller masses around
Tetachuck Lake that were only known to intrude Lower Jurassic rocks. In White-
sail Lake map-area (Duffell, 1959), several lithologically similar granitic bodies
that were mapped west of those under discussion were found to intrude Middle
Jurassic rocks. There seems to be sufficient evidence to say that some of the similar
rocks of the Tetachuck Lake area must also be later than Middle Jurassic. With
the reservation that some unspecified granitic bodies of the Tetachuck Lake region
may be older, all these rocks are tentatively mapped as post-Middle Jurassic.

These plutonic rocks as a unit differ from most granitic batholiths of British
Columbia in that they do not parallel structure. Several bodies are irregular but
most are roughly equidimensional. They commonly occur in low terrain or at the
base of mountains. In Fawnie and Nechako Ranges, from the manner in which
they outcrop it would seem that the granitic rocks were just being unroofed.

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