A new Cenozoic forminiferal biostratigraphy, paleoecology (biofacies) and strontium isotope of Shell Canada exploration wells from Tofino Basin, offshore Vancouver Island, British Columbia
Date
2003
Authors
Narayan, Yogeeta Roshni
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Abstract
During the past thirty years hydrocarbon exploration has been suspended within the coastal waters of British Columbia by a moratorium imposed by the Province of British Columbia and Government of Canada. During the late 1960's, Shell Canada Ltd. drilled and abandoned six exploratory wells (Shell Canada Apollo J-14, Zeus I-65, Zeus D-14, Prometheus H-68 and Cygnet J-100 wells) in the Tofino Basin (TB). Currently, with the possibility of the moratorium being lifted, there is renewed interest in reassessing energy resource potentials and in understanding the geology and tectonic evolution of this region.
Shell Canada' s large database of drill cutting (microfossil) subsamples were previously studied to develop preliminary stratigraphic ages, but no detailed taxonomic studies were carried out. This present micopaleontological study of Cenozoic foraminifers was initiated by the SSHRC/NSERC Coasts Under Stress Project. Through reassessment of the existing Shell Canada well samples, made available by the Geological Survey of Canada, Pacific, this study examines the thick Cenozoic elastic marine sequence in TB and provides: l) the first detailed taxonomic descriptions and SEM illustrations of Cenozoic TB foraminifers, 2) clarification and refinement to the stratigraphic ages and paleoenvironments, 3) new strontium (Sr) isotope age data and 4) correlation potentials with Vancouver Island and other North American sequences. A database of over 2000 Shell Canada drill cutting subsamples of foraminifer microfossil slides, from the six Shell wells and several Eocene and Oligocene (Carmanah Group) surface samples from western Vancouver Island, were examined. Detailed taxonomic study of 159 foraminifer species (141 benthic and 18 planktic) and their distributions supports a diverse Northeast Pacific faunal assemblage and contributes to the first TB offshore biostratigraphy. Eleven new and/or redefined foraminifer interval zones are defined, based on last appearance datums (LADs) of short-ranging taxa and range from the late Eocene/Oligocene to the Holocene. These are the Cassidulina galvanensis, Turrilina alsatica, Rectuvigerina branneri, Baggina washingtonensis, Bolivina wissleri, Uvigerina hootsi, Bulimina subacuminata, Elphidiella hannai, Uvigerina peregrina and Cassidulina reniforme/Cribroelphidium excavatum f. clavatum zones. Correlations of the late Eocene/Oligocene strata within the TB with Vancouver Island (Carmanah Group) Eocene and Oligocene strata (C. galvanensis and T alsatica zones) are possible.
The diverse assemblage of foraminiferal taxa within the TB wells, represents biofacies that range from the inner neritic (0-50 m) to the lower bathyal 2,000 m+). Paleobathymetric estimates from biofacies changes indicate a shoaling upwards trend throughout the Oligocene with deep biofacies encountered during the early Miocene and early Pliocene, reflecting a major transgression. Late Pliocene-Pleistocene inner to outer neritic biofacies, cool-water faunal assemblages and coarser-grained lithology indicate a regressive phase and regional onset of glaciation.
Initial Sr isotope analysis of 3 I onshore and 44 offshore taxa-samples resulted in successful offshore Sr ages ranging from 37.3 Ma (million years; late Eocene) to 1.3 Ma (Pleistocene). Results nearly complement the foraminiferal biostratigraphy and provide improved resolution. Age reversals within Oligocene and Miocene strata indicate apparent stratigraphic reversals. Strontium isotope studies were successful in the TB but proved unsuitable for onshore samples, suggesting possible contamination and/or overprinting by pore-fluids from Jurassic and Pleistocene strata. Only a single onshore sample yielded a Late Eocene age of 37.8 Ma.
Since the Eocene, the TB has undergone episodic uplift, tectonic underthrusting, climatic cooling and eustatic fluctuations resulting in complex facies changes and stratigraphic breaks and structural complexities. New and/or updated biostratigraphic and combined Sr isotope results provide significant tools for constraining regional geological events and in providing improved/high resolution analysis, correlations and paleoenvironmental interpretations. These results provide baseline data for future taxonomic and biostratigraphic studies in regional basins, and provide essential information for basin modeling and energy resource assessment.