Late Cretaceous sedimentary marine and deltaic rocks of
Gabriola Island were mapped and studied during the summer of 1971.
Cretaceous strata constitute all the exposed outcrops on the island,
which is the northernmost of the Gulf Island chain of British Columbia.
The upper four formations of the Nanaimo Group are recognized,
totalling 2707 feet in composite section, The thinly laminated marine
mudstones and siltstones of the Northumberland and Spray Formations
are separated by deltaic sandstones and conglomerates of the Geoffrey
Sandstone. The younger Gabriola Sandstone lacks the conglomerate
units. Although unconformities between the formations may exist on
a regional basis, locally the major contacts are sharp and planar.
The marine strata are characterized by fine-grained, thinly
laminated beds, and an abundant ammonite fauna, indicative of a low
energy, open marine environment. The deltaic units are mediumto
coarse-grained arkosic wackes and pebble-cobble conglomerates
representing a nearby source, and a high energy, nearshore environment
in which little sediment reworking occurred and burial was
The minerals and lithic clasts of the deltaic units exhibit practically
no variation throughout the sections. Source areas are considered
to be the older formations on Vancouver Island.
Channel axis measurements of festoon trough cross-bedding,
plus orientation of sole marks and parting lineations, indicate a predominant
northwest current transport direction for the upper two
formations. Soft-sediment slumping and flame structures support the
probability of a northerly to northwesterly declining paleoslope.
The Cretaceous sequence forms the broad, open, east trending
Gabriola syncline. Intersecting the axis of the syncline on the north
end of the island is the tight, asymmetrical, northeast-trending Lock
Bay anticline. Northwest-trending vertical faults occur parallel to
the axis of the Gabriola syncline. Cross-cutting the syncline axis at
both ends of the island are northeast-trending vertical faults. Except
for that involving the steep limb of the Lock Bay anticline, no folding
of the strata was observed.
Although further work is needed, it is postulated that the Sp-ay
Formation marks the end of basinal downwarping of the Georgia Seaway and the Gabriola Sandstone marks the close of Cretaceous
tectonic activity within the coastal ranges of Vancouver Island.
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