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TitleTesting local and extraregional sediment sources for the Late Cretaceous northern Nanaimo Basin, British Columbia, using 40Ar/39Ar detrital K-feldspar thermochronology
AuthorIsava, V; Grove, M; Mahoney, J B; Haggart, J W
SourceGeosphere vol. 17, no. 6, 2021 p. 2234-2261, Open Access logo Open Access
Alt SeriesNatural Resources Canada, Contribution Series 20200757
PublisherGeological Society of America
Mediapaper; on-line; digital
File formatpdf; html
ProvinceBritish Columbia
NTS82; 83B; 83C; 83D; 83E; 83F; 83G; 83K; 83L; 92B; 92C; 92F; 92G; 92H; 92I; 92J; 92K; 92N; 92O; 92P; 93A; 93B; 93C; 93F; 93G; 93H; 93I; 93J; 93K
AreaVancouver Island; Gulf Islands; Canada; United States of America
Lat/Long WENS-125.0000 -111.0000 55.0000 42.0000
Subjectstectonics; sedimentology; geochronology; stratigraphy; Science and Technology; Nature and Environment; Upper Cretaceous; basin evolution; sediment dispersal; provenance; radiometric dating; argon argon dating; detrital minerals; feldspar; biotite; bedrock geology; basement geology; lithology; sedimentary rocks; clastics; depositional history; tectonic history; thermal history; burial history; intrusions; batholiths; paleomagnetic interpretations; Canadian Cordillera; Cordilleran Margin; Nanaimo Basin; Nanaimo Group; Coast Mountains Batholith; Comox Formation; Extension Formation; Cedar District Formation; De Courcy Formation; Harrison Lake Fault; Geoffrey Formation; Spray Formation; Gabriola Formation; Idaho Batholith; Mojave-Salina Margin; North American Margin; Belt-Purcell Supergroup; Intermontane Belt; Insular Belt; Boulder Batholith; Databases; Phanerozoic; Mesozoic; Cretaceous; Precambrian; Proterozoic
Illustrationsgeoscientific sketch maps; stratigraphic sections; photographs; block diagrams; tables; plots; time series
ProgramGSC Pacific Division
Released2021 11 08
AbstractDetrital K-feldspar 40Ar/39Ar thermochronology was conducted on clastic sedimentary rock samples collected from northern exposures of the Upper Cretaceous Nanaimo Group on Vancouver Island and adjacent Gulf Islands of British Columbia to constrain the denudation history of the local Coast Mountains batholith source region and determine the origin of extraregional sediment supplied to the basin. Strata of the northern Nanaimo Group deposited between 86 and 83 Ma (Comox and Extension formations) exhibit a 130-85 Ma age distribution of detrital K-feldspar 40Ar/39Ar ages that lack age maxima. These are interpreted to have been sourced from the southwestern Coast Mountains batholith. Younger strata deposited between 83 and 72 Ma (Cedar District and De Courcy formations) yield a broader age range (150-85 Ma) with an age maximum near the depositional age. These results indicate focused denudation of deeper-seated rocks east of the Harrison Lake fault. The youngest units deposited after 72 Ma (Geoffrey, Spray, and Gabriola formations) primarily yield younger than 75 Ma detrital K-feldspar ages with pronounced age maxima near the depositional age. This sediment was sourced extraregionally relative to the Coast Mountains batholith.
We sought to constrain the origin of the extraregional sediment by measuring the thermal histories of 74 samples of basement rocks from throughout the Pacific Northwest, and by compiling a database of over 2400 biotite 40Ar/39Ar and K/Ar cooling ages from predominantly Cretaceous batholiths along the western North American margin. This analysis focused upon two previously proposed source regions: the Idaho batholith and the Mojave-Salina margin of southern California. The Nanaimo detrital K-feldspar 40Ar/39Ar age distributions favor the peraluminous Late Cretaceous Idaho batholith and its Proterozoic Belt-Purcell Supergroup sedimentary wall rock as the more likely source of the extraregional sediment and disfavor the Baja-British Columbia hypothesis for 2000-4000-km-scale translation of rocks along the margin during the Late Cretaceous.
Summary(Plain Language Summary, not published)
The coastal region of western Canada is geologically complex, characterized by extensive faulting and translocation movements that have taken place over the past 200 million years of Earth history. These movements of geological regions, or terranes, have resulted from the processes of plate tectonics, or the actions that drive the tectonic plates forming the Earth's outer crust. Many of the rocks of Vancouver Island, British Columbia, are hypothesized to have formed at low, tropical latitudes about 200 million years ago, as evidenced by tropical-type fossil organisms that are found within them, and were then transported northward on the tectonic plates. But just when these rocks moved from their equatorial origin to their modern-day high temperate location is a matter of geological debate: some geologists believe that these rocks must have moved north by about 170 million years ago, while others postulate that this movement took place as recently as 65 million years ago. The authors of this contribution have utilized a variety of geochemical techniques to determine that the source of the sediments in rocks about 80 million years of age found along the southeastern region of Vancouver Island can be tied closely to unique geological sources in Idaho and northwest Montana, thus establishing that these Vancouver Island rocks were at their present latitudinal position by at least 80 million years ago. Ancient river systems of that time transported the sediments from their source in Idaho and Montana to the southern Vancouver Island region.

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