CONSIDERATIONS FOR THE CALCULATION OF U-PB DETRITAL ZIRCON MAXIMUM DEPOSITIONAL AGES: INSIGHTS FROM CRETACEOUS FOREARC AND FORELAND BASINS IN NORTH AND SOUTH AMERICA

The calculation of U-Pb detrital zircon maximum depositional ages (MDA) has become a common tool in sedimentary basin analysis and can yield key insights into a variety of geological problems. By their nature, MDAs can only provide a maximum limit on the age of a stratigraphic unit and the true depositional age of the unit may be significantly younger. However, in some cases, MDAs approximate the true depositional age of a stratigraphic unit; in these cases, MDAs can be used to constrain the depositional timing of sedimentary systems, as well as compute the rates of sedimentary processes within them. In all applications, careful consideration of confounding geological and analytical limits is required for appropriate application of MDAs and selection of a calculation method that generates reliable results. The proportion of near-depositional ages, primarily a function of the tectonic setting of the basin, is a first-order control on the utility of MDA methods. Where near-depositional age zircon represents a small fraction of the total zircon population, large-n geochronology may be required to identify sufficient grains to calculate a reliable MDA. Conversely, where near-depositional age grains represent a large fraction of the total population, analytical limitations introduced by the measurement method, specifically the measurement of grains near the extremes of the method’s error distribution, can lead to erroneous results for some calculation methods. Additionally, the study objectives also influence the choice of acquisition and MDA calculation method. Where depositional rates are the primary focus or where chronostratigraphic correlations are sought, more precise and reproducible MDAs may be preferred over their absolute accuracy. Conversely, correlation of a sedimentary deposit to an independently dated event or sedimentary unit may require greater accuracy. In all cases, the integration of existing independent age constraints (i.e., index fossils, ages from volcanic ash beds) can guide the selection of MDA calculation method and validate the results. Here, we combine case studies from three basins (Magallanes Basin, Chile; Nanaimo Basin, Canada; Western Canada Foreland Basin; Canada) with numerical modelling results to illustrate how flexibility in MDA method can yield insightful results.