The western part of the Late Cretaceous Nanaimo Basin
is exposed on Vancouver Island at Nanaimo, British Columbia.
The five lowest members of the Nanaimo Group are present
and represent a complete sedimentary cycle. The two lowest
formations, the Comox and Haslam, represent the marine part
of the cycle. The Comox rests with angular unconformity on
the underlying Triassic Karmutsen volcanics, and is composed
of shallow marine deposits of sandstones, conglomerates
and limestones, one of the limestones being an algal
type not previously reported for the Nanaimo Basin. The
Haslam represents a quiet marine environment, possibly
lagoonal, which grades upward into a swampy environment
represented within the lower Extension Formation.
The Extension Formation represents the first of the
terrestrial part of the cycle. Above the Wellington Coal
Member, deposited in a swampy environment, lie channel
conglomerates and sandstones indicative of a braided stream
environment. The Newcastle Formation onlaps the Extension
Formation and is composed of sandstones, siltstones, conglomerates,
and the Newcastle and Douglas Coal Seams. The
environments of deposition of the Newcastle Formation are
postulated to be the upper floodplain of a short headed
stream. Paleocurrent data and composition of the rocks
indicate sources to the west.
The Protection Formation is the uppermost of the formations
in the area, and is composed of thick- and thin-bedded
sandstones. The sandstones indicate a barrier-beach
complex, probably deposited as the paleo-shoreline
migrated west during a transgression.
Subsequent faulting, and fluvial and glacial erosion
produced the present topography of the area.