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Jurassic horizon. In the McConnell Creek map-area to the west, the Takla
group contains Upper Jurassic beds, which, however, were not observed to
be cut by the Omineca intrusions. But as no important unconformity has
been recognized within the Takla group, and as the whole group appears
to have been deformed as a unit prior to the intrusion of the major part of
the Hogem batholith, it seems probable that all of the Omineca intrusions
are younger than the entire Takla group; that is, not older than late
Jurassic.

Bodies of these intrusions were unroofed to supply abundant cobbles
and pebbles to the Uslika formation and to the Sustut group strata. The
Uslika formation is tentatively assigned a late Lower Cretaceous age. The
Sustut group contains early Upper Cretaceous fossils. The Omineca
intrusions were, therefore, emplaced between late Triassic and late Cretace-
ous time, and probably between late Jurassic and early Cretaceous time.

USLIKA FORMATION
GENERAL STATEMENT

The Uslika formation includes a single body of conglomerate exposed
east and north of Uslika Lake, together with faulted, contiguous blocks
south of Conglomerate Mountain and in the lower part of Vega Creek
Valley. Beds of black argillite and chert-pebble conglomerate that out-
crop on Vega Creek, just west of, and in apparent conformable relations
with, the main mass of conglomerate, are also tentatively assigned to this
formation.

LitrHoLocy

The main conglomerate mass consists of well-rounded pebbles and
cobbles up to 10 inches in diameter in a heterogeneous, grey-green to brown,
sandy to gritty, greywacke matrix. The most abundant stones are of
voleanic rocks, mainly grey-green and green, finely porphyritic andesites,
with lesser purple andesite or basalt, and andesite breccia. Somewhat
less plentiful, but forming almost all the cobbles, and the predominant
material in zones whose aggregate thickness is about one-third of the total,
are rounded fragments of a great variety of plutonic rocks. All of the types
of plutonic rocks found in this conglomerate are similar to rocks of the
various bodies of the Omineca intrusions, although, in general, diorites,
syenodiorites, and quartz-poor porphyritic granodiorite are proportionally
more abundant in the conglomerate than in the Omineca intrusions now
exposed. Leucocratic, relatively quartz-rich granodiorite and adamellite-
granite, which are the most abundant rocks in the Hogem batholith at the
present level of erosion, are relatively rare as boulders in the Uslika
formation.

Other rock types widely distributed as pebbles in the conglomerate
include: fine-grained, well-bedded andesitic tuffs; pale grey, in part micace-
ous, quartzite; white vein quartz; and blue-grey chert. Less abundant
rock types, found mainly in small (less than 1 inch) pebbles, include:
black and grey argillaceous limestone; blue-grey cherty limestone; red
chert or jasper; quartz-mica schist and quartz-mica-feldspar gneissic rock;
and fine-grained brown sandstone. Almost all of the fragments except
those of flakv rocks, such as the argillaceous limestones and the schists.
are well rounded.

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