Regionally preserved in a structural basin, rocks of
the Cretaceous Nanaimo Group nonconformably overlie metavolcanic
and metasedimentary rocks of the Permian and older
Cretaceous rocks of the thesis area occur in three
formations, the Comox, Extension-Protection, and Cedar Dist;
and, during two cycles, were deposited in environments which,
as a result of transgressions and regressions, ranged from
fluvial to marine.
Paleocurrent and rock analyses indicate that sediments
were derived from an uplifted volcanic and metamorphic
terrane southwest of the study area, suggesting a possible
Sicker Group source.
The Benson Number of the Comox Formation is the basal
unit of the Nanaimo Group and consists of gravels, cobbles,
and boulders deposited by highly competent streams during
the initial phase of the first depositional cycle. Detritus
was rapidly transported from a nearby source and quickly
buried as channel lag deposits in alluvial fans.
After an erosional episode and/or a period of non-deposition,
sands and gravels of the Extension-Protection
Formation were transported by high velocity, low sinuosity
streams and deposited on nonmarine delta plains and marine
Grading laterally, seaward of the sands and gravels,
the silt- and mud-sized particles of the Cedar District
Formation, the youngest Mesozoic sediments to be preserved
in the thesis area, were transported to a prodelta
environment where they then settled from suspension or
were deposited by turbidity currents.
Existing Cretaceous deposits were preserved in large
blocks tilted by post-Crectaceous faulting.