CONSTRUCTING A PRECISE TIMESCALE FOR NON-MARINE SEDIMENTARY STRATA USING U-PB GEOCHRONOLOGY OF REDEPOSITED VOLCANIC ZIRCONS

U-Pb zircon geochronology is a powerful tool to develop precise age constraints for sedimentary strata and their constituent fossil and paleoenvironmental records. Traditionally, robust ages required zircons from primary volcanic products such as air-fall ashes/tuffs and pyroclastic flows. However, many sequences lack such volcaniclastic horizons, making radioisotopic dating difficult. Nonetheless, some sediments include abundant volcanic detritus, which has been reworked to varying degrees before deposition and burial. In such cases, U-Pb zircon ages are difficult to interpret because there is an unknown amount of time between the youngest individual zircon ages and deposition, and individual samples can have a complex age inventories. In contrast, if meaningful ages can be extracted from these data, they could provide important maximum age constraints for sedimentary and paleontologic records that otherwise have unreliable age constraints.

We report a successful case study of this approach from the non-marine Upper Triassic Chinle Formation of the southwestern United States. Unambiguous stratigraphic superposition of samples is provided by the 520m long core 1A of the Colorado Plateau Coring Project, recently recovered from Petrified Forest National Park. We separated zircons from 10 sandstone and siltstone samples from throughout the core. ~300 zircons per sample were analyzed using the LA-ICPMS method; the youngest crystals were then individually analyzed using higher-precision CA-TIMS. These data indicated abundant Late Triassic zircon ages from each sample, and importantly the youngest crystals in each sample are consistent with their stratigraphic position. These data suggest that the duration of reworking in the sedimentary system is short, and that the youngest crystal ages from each sample provide a meaningful maximum depositional age constraint. Combined with other chronostratigraphic data (i.e., magnetostratigraphy), a robust exportable timescale can be developed for the Chinle Formation, allowing precise correlation with other Late Triassic sequences worldwide. Thus, we suggest that U-Pb ages from redeposited zircons can provide valuable depositional age constraints when volcaniclastic deposits are lacking.