|Abstract or Summary
- The bedrock of Thetis, Kuper, and seven adjacent islands and
islets of British Columbia's Gulf Islands consists of four Late Cretaceous
(Campanian) formations of the Nanaimo Group. The four, from
oldest to youngest, are the Cedar District, De Courcy, Northumberland,
and Geoffrey Formations.
The formations vary greatly in thickness but a composite section
of maximum thicknesses totals approximately 3,000 feet. These
sections present a record of continuous sedimentation which can be
subdivided into two cycles of deltaic progradaton.
The northwest-southeast outcrop pattern of the islands presents
a cross-sectional view of the northeasterly prograding Cedar
District/De Courcy delta. Thick prodelta muds of the lower Cedar
District Formation are overlain by delta front platform turbidites of
the upper Cedar District Formation. Conformably overlying the
Cedar District Formation is the De Courcy Formation, which in the
northern part of the thesis area may be subdivided into lower arid
upper arkosic marine sands separated by a conglomeratic fluvial
interval. Laterally, toward the southeast and away from the locus of
deposition, the fluvial interval wedges out and the formation thins
markedly. Paleocurrent data and mineralogy suggest that the sediments were derived from the pre-Cretaceous Island Intrusions and
Vancouver Group to the southwest and south on Vancouver Island.
A longitudinal view of the northwesterly prograding Northumberland/
Geoffrey delta is presented by the outcrop pattern. The
vertical succession of prodelta muds, delta front platform turbidites,
delta front sheet sands, fluvial conglomerates, and shallow marine
sands is the same as for the Cedar District/De Courcy cycle. The
second cycle differs from the first in that the fluvial interval is
present the entire length of the thesis area and the prodelta muds and
overlying turbidites thin toward the southeast, suggesting northwest
rather than northeast progradation. An increase in the amounts of
plagioclase and volcanic rock fragments coupled with a decrease in
amount of quartz suggest that the sediments of the second cycle were
predominantly derived from volcanics of the Vancouver Group. A
shift to a more southerly source area may be indicated by the paleocurrent data, direction of progradation, and mineralogical changes.
Tertiary faulting and folding of the deltaic sediments formed the northwest-trending Trincomali Anticline which has been breached and
extensively faulted by two systems of normal faults, one trending
northwest and the other north to northeast.
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