Lithofacies control in detrital zircon provenance studies: Insights from the Cretaceous Methow basin, southern Canadian Cordillera

High-frequency sampling for detrital zir- con analysis can provide a detailed record of fine-scale basin evolution by revealing the temporal and spatial variability of de- trital zircon ages within clastic sedimentary successions. This investigation employed detailed sampling of two sedimentary suc- cessions in the Methow/Methow-Tyaughton basin of the southern Canadian Cordillera to characterize the heterogeneity of detrital zircon signatures within single lithofacies and assess the applicability of detrital zir- con analysis in distinguishing fine-scale provenance changes not apparent in litho- logic analysis of the strata. The Methow/ Methow-Tyaughton basin contains two dis- tinct stratigraphic sequences of middle Albian to Santonian clastic sedimentary rocks: submarine-fan deposits of the Harts Pass Formation/Jackass Mountain Group and fluvial deposits of the Winthrop For- mation. Although both stratigraphic se- quences displayed consistent ranges in de- trital zircon ages on a broad scale, detailed sampling within each succession revealed heterogeneity in the detrital zircon age dis- tributions that was systematic and predict- able in the turbidite succession but unpre- dictable in the fluvial succession. These results suggest that a high-density sampling approach permits interpretation of fine- scale changes within a lithologically uni- form turbiditic sedimentary succession, but heterogeneity within fluvial systems may be too large and unpredictable to permit ac- curate fine-scale characterization of the evolution of source regions. The robust composite detrital zircon age signature de- veloped for these two successions permits comparison of the Methow/Methow- Tyaughton basin age signature with known plutonic source-rock ages from major plu- tonic belts throughout the Cretaceous North American margin. The Methow/ Methow-Tyaughton basin detrital zircon age signature matches best with source re- gions in the southern Canadian Cordillera, requiring that the basin developed in close proximity to the southern Canadian Cor- dillera and providing evidence against large-scale dextral translation of the Meth- ow terrane.