24-1 Lessons Learned Through the Calculation and Application of Maximum Depositional Ages to Constrain the Timing of Clastic Deposition Across the North America Cordillera

Session: Advances and Applications in Geochronology for Interpreting Stratigraphic and Basin Records, Part I

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The youngest detrital zircon age component of a clastic sedimentary rock can be used to constrain its maximum depositional age (MDA) and is applied widely in studies where volcanic ash layers or fossils are absent. There are at least 10 different statistical methods used to determine the MDA of a detrital zircon U-Pb dataset, with no community consensus on workflow. This study works through an approach for calculating MDAs from two datasets within the Jurassic-Cretaceous North America Cordillera forearc and foreland basin systems. For both studies, the systematic error was quadratically added to MDA dates calculated from available software programs. In addition, a minimum of five MDA methods were applied to each dataset to provide the basis for comparison and interpretation. First, the MDA of a sandstone from the Great Valley forearc basin, California was determined using LA-ICP-MS and subsequent CA-ID-TIMS methods. This sample yields a unimodal primary age mode but also contains a scattering of younger dates. Three grains dated with LA-ICP-MS, yield dates of ~133, ~102, and ~94 Ma, but have corresponding CA-ID-TIMS dates of ~146, ~145, and ~145 Ma. The weighted mean of the CA-ID-TIMS dates is 145.58 ± 0.09/0.51, within error of the Youngest Statistical Peak (YSP) of 144.62 ± 1.36. Second, a sandstone from the Frontier Formation from the Cretaceous foreland basin system in Montana was collected to constrain the depositional age of newly discovered turtle material. The three youngest grains dated are 4-5 Ma younger than an overlying dated porcellanite bed. Following the same workflow employed by others on this unit, these young grains are excluded from all the MDA calculations, except the Maximum Likelihood method which requires all grains to be used. All five MDA methods reported agree within error. The YSP age was used for interpretation, owing to the high number of grains used in the calculation (n = 32), a MSWD of 1.1, and overlap with other methods. Modifying the dates input into MDA calculations is justified to honor existing geochronologic and geologic age constraints. These MDA methods highlight the criticality required in using zircon U-Pb dates to calculate and interpret depositional ages.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

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