5

TRIASSIC SEDIMENTS

Small areas of crystalline limestone believed to be of Triassic age out-
crop on Thornhill mountain, along Zymoetz river and Williams creek, and
along Skeena river 3 miles west of Amsbury. The limestone is usually
white and occurs as both thinly laminated and thick beds. It is commonly
associated with beds of cherty quartzite, slate, argillite, chert pebble con-
glomerate, and limestone boulder conglomerate. Five miles up Zymoetz
river limestone outcrops along the main trail and appears to be overlain
by andesite flows of the Hazelton group. Fossils collected by Hanson in
1925 from a limestone boulder conglomerate were identified as probably
of Permian age and fossils (Daonella sp.) from an argillite of the series
were stated to be of Triassic age. Considerable detailed work must yet
be done before the age, thickness, and position of the formation can be
stated with accuracy.

HAZELTON GROUP

In Terrace area the Hazelton group comprises a thick assemblage of
volcanic rocks of Lower Jurassic age and a thick upper sedimentary divi-
sion of probable Middle and Upper Jurassic age. The group forms the
country rock of most of the mineral deposits of the district.

The lower volcanic division consists of a thick series of andesite and
andesite porphyry flows with local areas of tuff and volcanic breccia. There
are occasional interbedded argillite and chert beds. The formation out-
crops along Skeena river between Kitsalas and Pitman and extends east
over the area drained by Zyometz river, Kleanza creek, Chimdemash creek,
St. Croix creek, and Legate creek. The mountains comprised of these rocks
rise to heights exceeding 6,000 feet; so that the original thickness of the
formation must have been very great. The volcanics are usually massive
and in most places the structure is difficult to determine. In those places
where strike and dip could be reliably measured, the flows and tuff beds
were found to strike a little north of west, parallel to the orientation of the
tributary streams. Their dips ranged from 50 degrees north to 50 degrees
south. The folding is believed to have occurred during the period of moun-
tain building that accompanied the intrusion of the Coast Range batho-
lith. Long tongues and small stocks from the batholith cut the volcanic
rocks throughout the area.

The upper sedimentary division of the Hazelton group is believed on
the basis of fossil evidence to pass imperceptibly into overlying conform-
able sedimentary rocks of Lower Cretaceous age as described under Skeena
formation. This division consists of well-bedded tuffs, sandstones, argillite,
quartzite, greywacke, slate, and conglomerate. The beds are commonly
thick and extend for great distances. The formation crosses the northern
part of the area north of Dorreen, from Kitsumgallum lake east to Seven
Sisters mountain and beyond. Its passage into the underlying volcanic
division in the vicinity of Dorreen has not been studied, but the transition
is believed to be abrupt. The rocks have been gently folded and are in-
truded in the neighbourhood of Kitsumgallum lake by large apophyses of
the Coast Range intrusives. Small isolated stocks of granodiorite and quartz

1 Hanson; G.: Geol. Surv., Canada, Sum. Rept. 1925, pt. A, p. 103.