Nechako River Map-Area
Structural and Stratigraphic Relations

The strata of the Hazelton Group accumulated in a central northwest-
trending basin. The shape of this basin cannot be drawn precisely; it probably
was changing rapidly as the group was being deposited. Although there is no
apparent widespread break in deposition between that of the chert-pebble conglom-
erate unit and that of the Middle Jurassic unit, there was a broad tilting or
shifting of the basin from west to east, so that in Fawnie Range non-marine sedi-
ments rest on marine, on Kuyakuz Mountain marine sediments rest on marine,
and east of Nechako Range, marine sedimentary strata rest on non-marine and
apparently overlap onto the exposed Topley Intrustions.

The Hazelton Group rests with angular discordance on the Takla Group
in Fawnie Range but elsewhere may be conformable. The change in the nature
of the sediments from the fine clastic rocks of the Takla Group to the coarse
clastic rocks of the Hazelton Group suggests a change in sedimentary environ-
ment. Both units of the Hazelton Group received debris from the Topley In-
trusions, and there is no doubt that the Hazelton Group rests unconformably on
older strata. Whether this unconformity is an angular discordance, a discon-
formity, or an erosional interval cannot be stated with certainty. All three pos-
sibilities may be true with the added possibility that in some deeper part of the
basin, not within Nechako map-area, there is no unconformity at all. There is
no evidence to indicate an appreciable time gap—possibly there was none.

Age of the Hazelton Group

The age of the Hazelton Group can be defined fairly closely from fossil evi-
dence within the group and from relations with other stratigraphic units. Of the
twenty fossil localities found in the two units of the group, only two yielded
diagnostic fossils. The other localities yielded collections of pelecypods, gastropods,
brachiopods, and belemnites that either were of no stratigraphic significance or
were unidentifiable.

The Middle Jurassic unit is dated closely by two collections of ammonites,
one from south slope of Kuyakuz Mountain, the other from Euchiniko River, a
mile west of the mouth of Taiuk Creek. These collections were examined by H.
Frebold, Geological Survey of Canada, who comments as follows (see also
Frebold, 1957, p. 16):

1. Collections from Kuyakuz Mountain.

This collection contains poorly preserved ammonites, probably belonging to
the genera Sonninia and Witchellia, and some pelecypods (Trigonia sp., Pleuromya
sp.). This fauna is of early middle Bajocian (early Middle Jurassic) age. Its
stratigraphic position is probably between the somewhat older Tmetoceras and
the younger Stephanoceras beds in the adjacent Whitesail area. Beds with Sonninia
that form part of the Hazelton Group near Hazelton, British Columbia, and some
beds of the Takla Group in McConnell Creek area that contain Witchellia are
probably of the same age.

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