General Geology

but are hard, finely banded, black argillite. On Nechako River, 2 miles south of Big
Bend Creek, one 15-to-20-foot bed of black mudstone contains large, black,
calcareous concretions up to 10 inches in diameter. These are nearly spherical and
each contains one well-preserved ammonite.

Structural Relations and Age

This group of rocks is not in contact with any other group, their general
northerly strike does not conform to the general northwest strike of the nearby
Hazelton Group, and some dips recorded were vertical, unlike the gentler dips of
the Hazelton Group. Moreover these rocks are sheared and, in places, badly con-
torted. From the foregoing, it is reasonable to assume that the group attained its
isolated position by being downfaulted into the older rocks. Exposures are too few
for any certain explanation.

Two fossil localities were noted very close together along Nechako River and
were examined by Hans Frebold, Geological Survey of Canada, who commented as
follows:

GSC locality 2188. One-half mile upstream from Big Bend on Nechako River.
Estheria sp.

This locality was found by G. M. Dawson in 1876 (Dawson, 1878, p. 71).
GSC locality 21885. On Nechako River, 2 miles upstream from Big Bend Creek.
Fossils occur in concretions embedded in finely bedded, black, limy shale.

“This collection consists exclusively of well-preserved ammonites. They belong
to Lilloettia lilloetensis Crickmay which is a typical Lower Callovian ammonite,
which is also known from the Lower Callovian of Harrison Lake area and Alaska.”

Ootsa Lake Group

The name Ootsa Lake Group was given by Duffell (1959, p. 67) to a group
of volcanic rocks, mainly rhyolitic and dacitic, that outcrop in the northeast quarter
of Whitesail Lake map-area. Although mainly acidic, andesitic and basaltic flows
and breccias do occur, commonly near the base, sedimentary rocks form a minor
part of the group.

Similar outcrops within the Nechako River map-area have been assigned to
Duffell’s group, the important difference being that whereas no subdivision was
possible in Whitesail Lake area, in Nechako River area the group has been divided
into two units—a rhyolite and an andesite.

Andesite Unit

This unit outcrops mainly from Uncha and Binta Lakes eastward to Hallett
Lake and from Cheslatta Lake northward into Fort St. James map-area; a few
outcrops occur along Ootsa Lake near Marilla. The rocks on the northeast side

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