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Detrital zircon U–Pb geochronology has enabled advances in the understanding of sediment provenance, transportation pathways, and the depositional age of sedimentary packages. However, sample selection and processing can result in biasing of detrital zircon age spectra. This paper presents a novel approach using in situ detrital zircon U–Pb measurements on thin-sections to provide greater confidence in maximum depositional ages and provenance interpretations. New U–Pb age data of 310 detrital zircon grains from 16 thin-sections of the Triassic Mungaroo Formation from two wells in the Northern Carnarvon Basin, Australia, are presented. Whilst detrital zircon age modes are consistent with previous work, there are some differences in the relative proportions of age modes, which are partly attributed to a lack of small grains in hand-picked grain mounts. The relative sample bias is quantified via grain size comparison of dated zircon (in thin-sections or hand-picked mounts) relative to all zircons identified in bulk-mounts and thin-sections. The youngest age mode (c. 320–195 Ma) is consistent with an active margin to the north. The dated zircons reveal a single grain defined maximum depositional age of 197 Ma for the upper part of the Mungaroo Formation, suggesting deposition may have continued into the Early Jurassic. Supplementary material: Additional figures, tables, datasets and programming scripts are available at https://doi.org/10.6084/m9.figshare.c.5628911 [ABSTRACT FROM AUTHOR]