Geochemical fingerprints of dated bentonites to constrain correlations in Cretaceous fluvial strata, Dinosaur Provincial Park, Alberta, Canada

ABSTRACT Correlation of meandering fluvial strata is difficult because of the highly variable lateral extent and vertical thickness caused by the erosion of the surrounding floodplain from channel downcutting and meander migration. This variability often results in a lack of confidently traceable marker beds leading to errors with stratigraphic correlation, determination of sedimentation rates, and quantification of stratigraphic completeness in fluvial strata. Bentonites are ideal marker beds for resolving fluvial issues of stratigraphic correlation because they preserve either a single or multiple short volcanic events deposited over widespread areas and can be radiometrically dated. Additionally, they preserve other signals that can be used to construct geochemical fingerprints for individual beds, allowing correlation of fluvial strata despite fragmentary preservation. To test the utility of geochemically fingerprinted bentonites as fluvial marker beds, we examined the Campanian-aged meandering fluvial strata of the Dinosaur Park Formation (DPF), which crops out mainly as badlands in the UNESCO World Heritage Site Dinosaur Provincial Park (DPP), Alberta. The DPF lacks a well resolved stratigraphic framework; this is problematic because the DPF in DPP preserves some of the world’s most abundant and diverse dinosaur fossil assemblages, which have been used as a basis for many hypotheses related to evolution of dinosaurs and other terrestrial vertebrates. A poorly resolved stratigraphic framework for the DPF in DPP leads to substantial uncertainty regarding stratigraphic placement of dinosaur fossil sites, and therefore potential errors with any evolutionary hypotheses supported by that biostratigraphic data. We sampled 14 bentonite deposits from DPP, five of which have been dated previously and nine of which were previously unsampled. Stratigraphic sections were measured at bentonite collection sites, and elevation of all bentonite beds was recorded to ±0.1 m. Geochemical data for sampled bentonites was obtained using electron-probe micro-analysis on preserved biotite, alkali feldspar, and plagioclase phenocrysts. Our results show that bentonites in DPP have unique geochemical fingerprints and physical traits that can be used for their identification and correlation, without the need to be dated. We identify three previously unrecognized and currently undated bentonites in the DPF that will increase geochronological resolution in DPP. We demonstrate that the Plateau Tuff bentonite can be correlated over a 12 km distance across DPP, demonstrating the potential utility of bentonites for both local- and regional-scale fluvial stratigraphic correlation. These results offer a promising first step for resolving stratigraphic correlation issues for the DPF in DPP and provide a correlation framework model that can be applied to other fluvial stratigraphic systems that preserve bentonites.