Graduate Thesis Or Dissertation
 

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  • 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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