Whitesail Lake Map-Area

abundant; contacts are sharp and the disturbance of older rocks is greater.
It would appear that the plutonic rock in these instances was emplaced as a
magma forcing its way into the older rocks rather than as a quiet replacement.

Some of the contacts along the ‘main mass’ are faults, and probably
the contacts of some of the outlying bodies are also faults, although no
evidence was observed to show that whole bodies have been faulted into place.

Lower Cretaceous Rocks (11)

A series of interbedded black shales and fawn to grey sandstones and
arkoses occurs on Swing Peak and Laventie Mountain, which is lithologically
and palzontologically similar to the Haida formation of the Queen Charlotte
Islands. Overlying these sedimentary strata is a series of volcanic rocks,
but whether the contact is conformable or unconformable is not definitely
known. However, the two series appear to be conformable and the similarity
of their attitudes and relationship to the intrusive rocks suggest they are
a single unit. They have, therefore, been mapped together.

Distribution

Rocks included in this group occur only in one locality, mainly on Swing
Peak and Laventie Mountain. They extend westward from the small creek
immediately west of Kasalka Butte along the whole length of Swing Peak
ridge. They underlie all of Laventie Mountain and outcrop along the south
shore of Tahtsa Lake as far west as Laventie Creek. They were not observed
on Mount Baptiste but occupy the northern extremity of Mount Bolom. No
other rocks with a similar fossil content were noted elsewhere in the area but
a small patch of sandstone on Tahtsa Range was similar lithologically. Except
for fossil content these rocks are very similar to parts of the Hazelton group
and have been mapped with that group in the past (Marshall, 1925, p. 48).
Other small isolated patches may have been included with the Hazelton group
in the present mapping.

Lithology

The lower half of the group consists entirely of sedimentary rocks of
marine origin. Dark grey to black fossiliferous mudstones are conspicuous
near the base, occurring in beds 75 to 100 feet thick. Minor thin beds of grey
to fawn arkose separate the mudstones in this part of the sequence. Higher in
the sequence the arenaceous material increases and the mudstones are inter-
bedded with more and more sandstone and arkose. Still higher in the sequence

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