General Geology

distinct, in others the rocks are only slightly gneissic. Most of the foliation has a northwest trend.

The diorite commonly is grey or greyish green and consists of abundant hornblende and andesine. In a few places it grades into quartz diorite. They are coarse-grained rocks and, in Nechako River map-area at least, are weathered to a known depth of 3 feet; in the Nulki hills it was difficult to obtain an unweathered specimen.

Structural Relations and Age

Whether all the lithological phases of this group were intruded at the same time or not is a question as yet unresolved. The granites and granodiorites may have had at one time a mode of emplacement but whether the diorites are similar in age and origin is not known. No contact was exposed between these two distinct units and near Finger and Tatuk Lakes they are separated by a band of metamorphosed Takla volcanic rocks.

As mentioned previously, evidence from Nechako River map-area indicated an age of emplacement of the Topley Intrusions different from that stated by Armstrong (1949, pp. 96-97). In discussing the group, he outlined eight reasons for dating the intrusions as pre-Upper Triassic post-Middle Permian, and evidence from the Fort St. James area supported this conclusion. One reason given was that the Topley Intrusions, which were confined to Cache Creek Group rocks, were not known to intrude Upper Triassic Takla rocks and thus were thought to be pre-Upper Triassic. This reason was found to be invalid in Nechako River area (Tipper, 1959a, p. 32). In reviewing the Hazelton and Takla Groups the writer discussed the age of Topley Intrusions and briefly the pertinent evidence is as follows:

(1) Boulders and pebbles of Topley granites occur in the chert-pebble conglomerate unit of the Hazelton Group which is of Middle Jurassic (early Bajocian) age. Arkoses believed to have been derived from Topley granites are dated as Middle Jurassic. Topley granites must therefore have been intruded before Middle Jurassic time.

(2) On the north shore of Frangois Lake, Topley granites intrude the marine part of the Red Bed Unit but are not in contact with the upper part of the red beds. Possibly the Topley granites were emplaced while part of this unit was being laid down; it would then be of latest Triassic age or more probably Lower Jurassic.

(3) Topley granites intrude fresh andesites and breccias southeast of Frangois Lake. These volcanic rocks are believed to belong to the Takla Group because they are not as highly metamorphosed as the normal Cache Creek Group rocks. The possibility does exist that they may be of Middle or Lower Triassic age, although such rocks have never been recognized in central British Columbia.

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