Synthesis of chronostratigraphic data and methods in the Georgia Basin, Canada, with implications for convergent-margin basin chronology

Abstract

Accurately estimating depositional ages of sedimentary strata is crucial for resolving basin evolution and architecture, and this is particularly important in convergent-margin basins where structural deformation of strata impedes stratigraphic correlations unassisted by age constraints. Depositional ages of strata are commonly determined using biostratigraphy, ash bed chronology, and detrital zircon (DZ) maximum depositional ages (MDAs). Herein, we use DZ methods to estimate depositional ages, and we compare these estimates to biostratigraphically-defined ages. From this comparison we suggest best practices for establishing depositional ages using DZ with the intent of refining the chronostratigraphies of convergent-margin basins. To do this, we review and compare all available age data for the lower Nanaimo Group, a forearc succession which partially comprises the fill of the Georgia Basin, British Columbia, Canada. These data include macrofossils, an absolute age from an ash bed, and large-n DZ datasets; the data span the geographical and temporal range of the lower Nanaimo Group. Using the biochronological dataset, we summarize the age ranges of all index fossils used in the biostratigraphic scheme for the lower Nanaimo Group. Using the DZ dataset, we generate MDAs, a multi-dimensional scaling plot, and a map of DZ age spectra. These data are plotted and compared in stratigraphic context on a recently developed chronostratigraphic scheme for the lower Nanaimo Group.The DZ MDA calculation method which most reliably estimates true depositional age is YGC2σ, which is calculated by taking the weighed average of the youngest grain cluster with overlapping 2σ uncertainties. YGC2σ is reliable because it incorporates most/all measurements originating from the youngest source of a DZ sample. By comparison, MDA methods that employ a limited number of youngest grains, such as youngest 3 grain (Y3Zo), only incorporate grains from the young tail of the youngest source, leading to inaccurately young MDAs. Conversely, YGC2σ is susceptible to incorporating grains from a slightly older source. To detect influence from older sources, we introduce the ‘MDA comparison plot’, a graph that compares the number of grains incorporated into a DZ sample's YGC2σ to the difference between its YGC2σ and Y3Zo MDA. Samples with greater than expected differences between their YGC2σ and Y3Zo MDAs are likely influenced by grains from older sources, and should be processed using DZ unmixing software.As sources for convergent-margin basins tend to evolve in predictable ways, we propose that DZ age spectra can be used to separate strata into temporally-equivalent packages with similar provenance. We observe these changes in age spectra across a documented disconformity in the lower Nanaimo Group. On the basis of this reasoning, we identify a previously unknown disconformity within the Georgia Basin and the existence of older strata. This work serves as an example of how DZ methods can compliment biostratigraphy to accurately estimate the age and architecture of strata in convergent-margin settings.