STRUCTURAL FRAMEWORK AND DETRITAL ZIRCON PROVENANCE OF CLAYTON, IDAHO: DEFINING THE WESTERN LAURENTIAN RIFT MARGIN NORTH OF THE SNAKE RIVER PLAIN

Neoproterozoic-Ordovician passive margin sediments of Central Idaho play a pivotal role in understanding the rift geometry of western Laurentia. Yonkee et al. (2014) defined rift geometry south of the Snake River Plain (SRP), but geometry north of the SRP is poorly defined and sediments are discontinuous or absent. One exception is near Clayton, Idaho, where a thick package of Neoproterozoic-Ordovician clastic sedimentary rocks are exposed in the footwall of the Copper Basin thrust, which places the Salmon River Assemblage (M€sr) over the Saturday Mountain Formation (Osm). Previous mapping here interpreted several younger-on-older faults as terrane-bounding thrusts. Geologic mapping in the summer of 2015 near Clayton resulted in reinterpretation of several of these thrusts as normal faults, allowing combination of a number of previously split stratigraphic units. The Clayton units total 4,620 m in thickness and were deposited west of the Lemhi Arch, where Middle Ordovician Kinnikinic Quartzite rests unconformably on Mesoproterozoic Belt Supergroup-equivalent rocks. In contrast to rift-related rocks in northern Utah and southeastern Idaho, the Ordovician part of the Clayton section is ~3900 m thick, whereas the Neoproterozoic and Lower Cambrian section is only ~720 meters, and may be overlain by an unconformity. Detrital zircon provenance data displays significant variation in Grenville, Yavapai-Mazatzal, and Archean populations, indicating drastic drainage changes within the Cambrian to Middle Ordovician rift-drift succession. Zircon populations of 665 ± 10 Ma in the Neoproterozoic-Cambrian quartzite of Boundary Creek (Cbc) and 685 ± 26 Ma in quartzite-siltite of upper Cash Creek (OCq) are likely sourced from magmatic rocks related to the Edwardsburg-Pocatello volcanics. A strong 1785 Ma peak in the Middle Cambrian Cash Creek Quartzite (Cc) suggests a correlation with the Flathead Sandstone of Montana. A strong Grenville-age peak (1170 Ma) in the Ordovician Ramshorn Slate (Or) is surprising, given the lack of Grenville-aged zircons in post-Middle Cambrian rocks in the Stibnite and Pocatello area stratigraphic sections.