INVESTIGATING THE SEQUENCE BOUNDARY THROUGH DETRITAL ZIRCON GEOCHRONOLOGY: INITIAL DETRITAL ZIRCON RESULTS FROM PROTEROZOIC AND PALEOZOIC ROCKS OF WESTERN MONTANA

We report initial detrital zircon geochronologic results from a collection of siliciclastic sandstone samples of Meso-Proterozoic and Paleozoic rock units in western Montana. This work is part of a larger program aimed at understanding the nature of stratigraphic sequence boundaries by documenting and analyzing detrital zircon geochronologic 'bar-codes' on suitable stratigraphic deposits bracketing different sequence boundary types. To this end, we have sampled several stratigraphic units within the uppermost portion of the Meso-Proterozoic Belt Supergroup and across the stratigraphic boundary/boundaries separating the Belt Supergroup from younger Paleozoic units. As of the deadline for this abstract, we have sampled and recovered a workable zircon separate from rocks as old as the Mount Shields Unit #2 within the Meso-Proteozoic Missoula Group, and rocks as young as the Devonian Maywood Formation. In addition to these two units, we have recovered workable zircon separates from the Bonner, McNamara, Pilcher, and Garnet Range Formations, all part of the Missoula Group, and we have recovered workable separates from the overlying Cambrian Flathead. Our sample collections are from the northern flank of the Flint Creek Range (Porters Corner area) and the southern flank of the Garnet Range, both in western Montana. LA-ICP data collection is scheduled for March, 2014 at Boise State University. We anticipate presenting preliminary detrital zircon geochronlogy spectra for each of the stratigraphic units listed above and interpreting these data in the context of published tectonic models for terminal evolution of the Belt Basin. In particular, we expect detrital zircon data from the Garnet Range Formation to be notably different from those of underlying Belt units, as suggested by a unique framework grain composition for this unit which is lithic-rich and highly micaceous relative to underlying Proterozoic and overlying Paleozoic sedimentary rock units.