HudsonJonPark1975.pdf

Abstract

The bedrock of North and South Pender Islands, the southernmost islands of British Columbia's Gulf Islands chain, is formed entirely by six formations of the Late Cretaceous Nanaimo Group. These six formations are, from oldest to youngest, the Extens ion- Protection, Cedar District, DeCourcy, Northumberland, Geoffrey, and Spray Formations, A composite section of maximum thicknesses for these formations totals approximately 11,600 feet. The formations, however, vary in thickness significantly along strike. These formations represent four cycles of deltaic progradation, the youngest and oldest of which are incompletely exposed. The Extension-Protection Formation is considered to be the upper part of a west- to northwest-prograding delta complex, the lower part of which is not exposed within the thesis area, The fluvial conglomerates of the Extension-Protection Formation, inferred to be topset beds, are overlain on South Pender Island by an interval of shallow marine arkosic and ljthic sandstones, This shallow marine interval pinches out to the northwest and is absent on North Fender Island. The mineralogy of the conglomerates and sandstones suggests that the ExtensionProtection Formation was derived from the pre- Cretaceous rocks of Vancouver Island. The Extension-Protection Formation intertongues with and grades into the Cedar District /DeCourcy delta. Prodelta muds of the lower Cedar District Formation are overlain by distal bar turbidites. The turbid ite deposits are conformably overlain by arkosic delta-front sheet sands or river mouth bars of the lower DeCourcy Formation. Continued basin subsidence, possibly combined with lower sedirnentation rates, caused a transgression over the lower DeCourcy Formation and a return to prodelta deposition of the upper Cedar District Formation. The upper Cedar District and upper DeCourcy Formations follow a depositional cycle similar to that of the lower Cedar District and lower DeCourcy Formations, with the exception that fluvial conglomerates inferred to be topset beds intertongue with and overlie the shallow marine delta-front sheet sands or river mouth bars. Paleocurrent data, lateral thinning and facies changes, and mineralogy suggest that the Cedar District/DeCourcy delta sediments were derived from a source, to the northwest on Vancouver Is land, which was composed of the Vancouver Group and the Island Intrusions. The vertical succession of prodelta muds, distal bar turbidites,. delta-front sheet sands or river mouth bars, and fluvial conglomerates is the same for the Northumberland/Geoffrey deltaic cycle as for the upper Cedar District /DeCourcy deltaic cycle. Paleocurrent data, lateral thinning and facies changes, and mineralogy suggest that the Northumberland/Geoffrey delta sediments were derived from a source area to the northwest, which was either the same source area as that of the Cedar District /DeCourcy delta, or one similar to it, The Spray Formation, the lower part of the youngest deltaic cycle exposed in the thesis area, consists of distal bar turbidites. The Northumberland/Geoffrey delta intertongues with the Spray Formation, although this intertonguing may be related to the overlying Gabriola Formation, At least three episodes of faulting have deformed the rocks of North and South Pender Islands, forming the Kulleet Syncline and other subordinate folds, The major faults trend west-northwest, with minor faults trending northwest to northeast