Nechako River Map-Area

were only briefly examined as they underlie such a small part of the map-area;
for a full discussion see Armstrong’s report (1949, pp. 32-51).

Takla Group

The Takla Group was named by Armstrong (1949, p. 51) from its occur-
rence in the vicinity of Takla Lake and was defined as “an apparently conform-
able succession of interbedded volcanic and lesser sedimentary rocks ranging in
age from Upper Triassic to Upper Jurassic”. Although rocks of this age and
general description occur within Nechako River map-area, they have not been
assigned to the Takla Group and a revision of the Mesozoic Hazelton and Takla
Groups has been made (Tipper, 1959a). Briefly, the reasons for this revision are:
(a) the age assigned to the Takla Group overlapped the age of the Hazelton
Group, (b) evidence of unconformities within the Mesozoic rocks exists, (c) there
was a marked change from fine clastic sediments in the Upper Triassic to coarse
conglomerates and sandstones of the Middle Jurassic, a situation with several
parallels elsewhere, and (d) the Topley Intrusions had cut the Upper Triassic
rocks but were undergoing erosion by Middle Jurassic time.

In the type area of the Takla Group the fossils collected were of Upper
Triassic and Lower Jurassic age only. No strata with Middle or Upper Jurassic
fossils were found in Takla map-area. To the northwest, in McConnell Creek map-
area, Lord found such younger rocks (Lord, 1948, pp. 15-28), and extended the
Takla Group to include them, although this was not apparently justified from
fossil evidence or from a comparison of estimated thicknesses in the type area.

In summary, the Talka Group was re-defined as an Upper Triassic and
Lower Jurassic marine assemblage of grey to green andesite, basalt, and associated
tuffs and breccias, with interbedded argillite, shale, limestone, minor greywacke,
and rarely conglomerate. The group rests unconformably on the Cache Creek
Group and is overlain unconformably by the Hazelton Group. Its greatest thick-
ness is more than 10,000 feet in Aiken Lake map-area (Roots, 1954, p. 159),
and considerably less in Nechako River map-area. The Takla Group occurs mainly
in the eastern part of British Columbia although unnamed Upper Triassic rocks
are distributed along the eastern margin of the Coast Mountains. For the present
it is well to restrict the geographic distribution of this group.

The Takla Group is typically eugeosynclinal; that is to say it was formed
in an eugeosyncline which is defined as “a surface that has subsided deeply in a
belt having active volcanism. . .” (Kay, 1951, p. 4). Practically all the rocks are
volcanic flows and breccias, or rocks derived directly or indirectly from active
volcanism. Only locally are important sections derived wholly or mainly from the
erosion of earlier groups; the Red Bed Unit of the Takla Group in the Nechako
River area is one such local unit. Typically the rocks are dark green to dark grey
or black, andesitic and basaltic flows and breccias occurring in thick sections

18