Graduate Thesis Or Dissertation
 

Paleoenvironments of the Cretaceous rocks, Gabriola Island, British Columbia

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  • 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 rapid. 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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  • File scanned at 300 ppi (Monochrome, 24-bit Color) using Capture Perfect 3.0 on a Canon DR-9050C in PDF format. CVista PdfCompressor 4.0 was used for pdf compression and textual OCR. Plates: Master map file scanned at 600 dpi, 24-bit color on a Paradigm ImagePRO GxT 42 HD (OEM version of ColortracSmartLF Bx 42). Image manipulated by SmartLF1.3.05.
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