REVISED INTERPRETATIONS OF DETRITAL ZIRCON POPULATIONS IN THE MESOPROTEROZOIC BELT AND PURCELL SUPERGROUPS OF MONTANA, IDAHO AND BRITISH COLUMBIA
Multidimensional scaling of catalogued detrital zircon ages reveals significant heterogeneity of zircon populations. Komogorov-Smirnov tests show a low internal consistency of ~30% within the Belt-Purcell Basin, indicating high spatial and temporal grain heterogeneity. As locations become more separated, they become more dissimilar. A “good fit” is surpassed beyond 1000 km of geographic separation, and grains become completely dissimilar beyond ~2000 km. We attribute this geographic variance in ages to predominantly proximal rather than mixed distal sources.
These local source areas question the premise that Belt units can be lithically and temporally correlated across the basin on the basis of their zircon populations. We hypothesize that the southern sources were less than ~500 km away throughout Belt deposition, and contributed sediment that interfingered with the Ravalli and Piegan groups. As erosion to the south accelerated high in the Belt, southern sediments overwhelmed eastern and western sources, with minimal evidence for rifting or tectonic reorganization.