Interpreting Sediment Dispersal in Western North America from Detrital Zircon Ages
Dickinson, William R.
U-Pb ages for detrital zircons (DZ) augment paleocurrent, petrofacies, and deposystem analysis for provenance studies but DZ constraints require shrewd orchestration. The exact sources of DZ grains are eroded away, with only analogues or deeper horizons remaining in the provenance. Sediment dispersal systems mix DZ from multiple sources to generate integrated provenance signals that must be deconvolved to understand. Since zircon U-Pb ages are not reset by any sedimentary processes and zircon is resistant to weathering, DZ ages define ultimate bedrock sources but not necessarily proximate sources because recycling of DZ from older sedimentary rocks is common. As there are only a finite number of basement provinces and volcanic belts of discrete ages in North America, DZ populations cannot readily contain grains of other ages. Volumetric yields of DZ from basement rocks are controlled by varying zircon fertility of different source granitoids, and proportions of DZ do not necessarily equate to proportions of total sand. Mafic volcanics yield few DZ grains of sand size, but polymodal volcanic suites contribute abundant DZ to derivative sands. Detecting DZ grains derived from airborne ash clouds is challenging because suspension transport in streams can carry fine sand for long distances without rounding the grains. Despite inherent caveats, DZ studies provide advantages over other provenance tools. Intrabasinal paleocurrent trends cannot delineate extrabasinal dispersal paths but DZ can. Petrofacies analysis largely founders for quartzose sands but DZ does not, and provenance signals from DZ are geographically more specific than those from petrofacies. Facies analysis of deposystems can mistakenly relate stratal assemblages that DZ shows had different provenances. Salient contributions of DZ studies to Cordilleran sedimentary assemblages include the appreciation that transcontinental paleorivers loom large for sediment dispersal across cratons, and that tectonically transverse and longitudinal transport paths can deliver sediment jointly to basinal depocenters.
AAPG Search and Discovery Article #90162©2013 Pacific Section AAPG, SPE and SEPM Joint Technical Conference, Monterey, California, April 19-25, 2013