INSIGHTS FOR PROVENANCE ANALYSIS FROM U/PB GEOCHRONOLOGY APPLIED TO MODERN WATERSHEDS – TALKEETNA MOUNTAINS, SOUTHCENTRAL ALASKA

Detrital zircon geochronology is a widely used tool for determining provenance in the sedimentary record, however, two significant problems exist: 1) an inability to differentiate between first cycle and polycyclic grains in detrital zircon populations, and 2) the assumption that detrital signatures of ancient samples are representative of the bedrock geology in the ancient watershed. We are investigating these problems by using U/Pb geochronology of both detrital zircon (DZ) and detrital apatite (DA) from modern river sand collected from 20 watersheds in the Talkeetna Mountains (south-central Alaska). To address the first issue, we apply U/Pb analysis of DA as an emerging tool for distinguishing igneous, or first cycle sediment input, from recycled, or polycyclic, sediment flux. DA rarely survives past the first cycle of erosion and deposition, and thus dominantly represents first cycle sources in a detrital age distribution. By comparing DA signatures with DZ signatures, which contain both first cycle and polycyclic grains, we can differentiate sediment contributions from each potential sediment source. To address the second issue, we use GIS software and the most recent geologic map of Alaska to simulate the hypothetical DZ signature for each watershed through uniform sampling of the bedrock geology proportionally to the relative exposure of each bedrock unit. Our preliminary data show that this method yields simulated DZ signatures similar to those of the modern river sand, suggesting that DZ signatures from ancient strata may indeed closely approximate the bedrock geology of the ancient watershed.