ONSET OF AN ARC: DETRITAL ZIRCONS IN EOCENE SEDIMENTS FROM THE OREGON COAST RANGE

Detrital zircon studies provide an important means to track the provenance of sedimentary detritus in sandstone and other sedimentary formations, and to trace changes in provenance through time in response to tectonic development. With the development of LA-ICP-MS technology it is now possible to date large numbers of zircons in relatively little time, and with precision and accuracy that is suitable for sedimentary provenance studies. We have applied LA-ICP-MS to study zircon provenance of the Tyee Formation, a thick sequence of clastic rocks deposited in the Eocene forearc of western North America, and of overlying sediments. Comparison between detrital zircon ages and the existing understanding of provenance taken from other geochemical and petrological indicators in these units provides the means for evaluation of provenance tracing techniques, and to further understand the response of sediment provenance to the tectonic development of western North America. To date, the majority of zircons within the Tyee Formation have ages from 40-60 Ma. These grains were derived from arc volcanism in the east that was ongoing during deposition in the forearc to the west. There is a small population of grains older than 100 Ma, likely derived from erosion of intrusive rocks from the continental interior. The overlying Nestucca Formation contains many more of these older grains, which may indicate that a combination of tectonic and drainage reorganization occurred between deposition of the two units. Comparison of standard analyses suggests that the LA-ICP-MS protocol used is accurate to within 2-3%. Initial results are encouraging, and this method will be applied to additional Tyee Formation samples in order to better constrain the age populations, their temporal variation, and the degree to which grain ages track volcanic arc evolution.