Nechako River Map-Area

the contact is not sharp or clearly defined. For the most part it is an arbitrarily
chosen contact separating two units in one of which acidic and in the other basic
rocks predominate. They are lithological units primarily and although the andesite
unit is believed to be older than the rhyolite unit, parts of one unit in one place
may be the same age as parts of the other unit elsewhere. The andesite unit
of Fawnie Range may be younger than the rhyolite unit and if this is so the rocks
in question may not be a part of the Ootsa Lake Group but a younger volcanic
group most of which outcrops south and southeast of the map-area (Tipper, 1959).

The Ootsa Lake Group rests with angular unconformity on the Mesozoic
rocks. Along Nechako River it rests on Topley granites, on Mount Davidson on
Late Jurassic granites, and around Lucas Lake on Hazelton Group rocks. Much
of the group occupies depressions in the eroded pre-Tertiary surface and sediments
apparently accumulated mainly in pre-Tertiary valleys. The group is overlain uncon-
formably by the Endako Group.

Age and Correlation

On an island in Chief Louis Bay on Ootsa Lake, one fossil occurrence in the
basal sediments of the andesite unit contains freshwater mollusks and plant
remains. W. A. Bell, Geological Survey of Canada, examined the plant remains
and commented as follows:

The collection consists of imprints of leaves in a rather coarse sandstone which
also carries remains of pelecypods and gastropods. Most of the plant remains are
too fragmentary to be of any value. The few complete or nearly complete leaves
show, at best, only the primary and secondary nervation and hence even a generic
identification is open to question. With this reservation the leaves are referred to the
following:

Alnus sp.

Rhamunites marginatus? (Lesquereux)

Spirodela? scutata? Dawson

Remarks: The doubtful nature of the above identifications precludes any confi-
dent conclusion on the age represented. So far as evidence goes a Paleocene age is
tentatively considered more probable than a late Upper Cretaceous or Post-Paleocene
age.

The freshwater mollusks were examined by E. T. Tozer, Geological Survey of
Canada, who reported as follows:

GSC locality 17180.
Unionid genus indet.
Sphaerium sp. (cf. S. heskethense Warren)
Lioplacodes sp. (cf. L. sanctameriensis (Russell) )

Remarks: Although this faunule is very rich in individuals it is poor in species
and consequently a satisfactory comparison with other faunas cannot be made. No
significance can be attached to the unionid but some of the sphaeriids have the promi-
nent, posteriorly placed umbones characteristic of the Edmonton (upper Cretaceous)
species S. heskethense Warren. The Lioplacodes is closer to L. sanctameriensis a con-
temporary of S. heskethense, than to any other described species.