A STRATIGRAPHIC APPROACH TO INFERRING DEPOSITIONAL AGES FROM DETRITAL GEOCHRONOLOGY

With the increasing use of detrital geochronology for provenance analyses, we have also developed new constraints on the ages of otherwise undateable sedimentary deposits. Because a deposit can be no older than its youngest mineral constituent, the youngest defensible detrital mineral age defines the maximum depositional age of the sampled bed. Defining the youngest 'defensible' age in the face of uncertainty (e.g., analytical, geological, sample contamination) is challenging. The current standard practice of finding multiple detrital minerals with indistinguishable ages provides confidence that a given age is not an artifact, but we show how requiring this overlap reduces the probability of identifying the true youngest component age. Here we formalize an additional common constraint on deposit ages: relative stratigraphic position. We present two approaches for constructing a Bayesian model of deposit ages, first solving directly for the ages of deposits with the prior constraint that the ages of units must obey stratigraphic ordering, and second describing the evolution of ages with a curve that represents the sediment accumulation rate. Rather than seeking out clusters of young ages to characterize a single maximum depositional age, we assign a decreasing weight to sequentially older detrital mineral ages and construct a mirror image of the cumulative density function adjusted by these weights. With this function, we intend to capture the spirit of maximum depositional age analysis. A deposit is necessarily younger than its youngest mineral age, but if we are not confident in the accuracy of that mineral’s age we may suspect that depositional ages could be slightly older than the youngest measured mineral age. Our approach also accounts for the increased confidence provided by the clustering of other young ages. In two examples, one from the Magallanes Basin of Patagonia and one from the Denver Basin of Colorado, we demonstrate how introducing other age information from the stratigraphic section (e.g., fossil assemblages or radiometric dates) and formalizing the stratigraphic context of samples provides additional constraints on depositional ages or derived quantities (e.g., sediment accumulation rates) than maximum depositional age analysis alone.