Structural and Historical Geology

Middle Jurassic Time

By the beginning of Middle Jurassic time a northwesterly trending sedimen-
tary trough or basin was formed, bounded on the north and northeast by a
landmass in which the Topley Intrusions were beginning to be exposed, and on
the southwest by a volcanic landmass or series of islands (see Fig. 3). The
southeastern limit of the basin is not known. To the northwest the basin was
constricted and may have had narrow connections with a marine area to the west.
In this basin was deposited detritus derived from the erosion of the exposed
Permian Cache Creek Group and Topley Intrusions on the northeast, detritus
from the volcanic islands on the southwest and flows, tuffs, and breccias of
contemporaneous volcanism. EDs Of ate

In the earliest part of the Middle Jurassic, chert-pebble conglomerates accu-
mulated along the shores of this basin and interfingered with sandstones and
shales. Volcanism continued intermittently in different parts of the basin. The
sediments deposited were characteristically coarse and included little volcanic
material. In later Middle Jurassic time finer sediments accumulated and there was
much admixing with volcanic tuffs and fine breccias.

Apparently this basin underwent a change during Middle Jurassic time.
The volcanic island arc on southwest margin continued to be uplifted so that the
basin was shifted to the east and the former northeast margin was inundated.
The northeastern borderlands were being rapidly eroded, as arkoses were formed
nearby.

Late Middle Jurassic time has no known representative in Nechako River
map-area.

Late Jurassic Time

Limy shales were deposited in a basin in the central part of the area in
early Upper Jurassic time. Fossils collected from these shales represent the earliest
zone of the English Callovian (Upper Jurassic), but fossils of this era have
not been recognized north of the area. The occurrence of similar fossils and
rocks of closely comparable age to the south Suggest that the two marine basins
may well have been connected.

Late Jurassic and Early Cretaceous Time

During this period or even earlier, regional deformation, intrusion, uplift,
and erosion began, and probably continued until Late Cretaceous time. The
granitic intrusions of the south and central parts were emplaced then or possibly
in late Middle Jurassic time. During this period also the sea was completely and
finally excluded from Nechako River area.

Late Cretaceous to Late Oligocene Time
By Late Cretaceous time most batholiths and stocks were emplaced, the
area had been regionally deformed, and distinct ranges of northwest-trending

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