Chapter IIT

GENERAL GEOLOGY

The rocks in the Nechako River map-area range in age from late Palzozoic to Recent. The most widespread sedimentary and volcanic formations are of Upper Triassic and Lower Jurassic, Middle Jurassic, and Tertiary age; the principal intrusive rocks are those of the Lower Jurassic Topley Intrusions and intrusions of post-Middle Jurassic age.

Table of Formations

Era Period or epoch Formation Lithology Recent Stream and lake deposits, talus, soil Pleistocene Glacial and glacio-fluvial deposits Cenozoic Erosion interval Miocene and (?) later Endako Group Basalt, andesite; related tuff and breccia; ibe minor shale and greywacke ES = NY) Angular unconformity Mesozoic Upper Cretaceous to Ootsa Lake Group Rhyolitic and dacitic tuff and breccia; and Lower Miocene shale, sandstone, conglomerate Cenozoic ¥ o- ty 5 : @ Rhyolite, dacite, trachyte, andesite; minor basalt; related tuff and breccia Basalt, andesite; minor rhyolite, sand- stone, and conglomerate Erosion interval Post-Middle Jurassic- Biotite granite, granodiorite, quartz pre-Upper Cretaceous diorite, diorite; minor gabbro loo -tgo Mesozoic Not in contact

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Upper Jurassic (Callovian)

+o

Argillite, argillaceous limestone