SEDIMENTOLOGY AND DETRITAL GEOCHRONOLOGY OF THE LATE CRETACEOUS BEAVERHEAD GROUP, SOUTHWEST MONTANA

The Beaverhead Group of southwest Montana is synorogenic; to the northeast of the study area it unconformably overlies and is deformed by Archean basement rocks of the Laramide Blacktail-Snowcrest uplift. To the west, it is deformed by and structurally juxtaposed against Paleozoic passive margin strata in the Sevier fold-thrust belt. The Beaverhead Group records coarse-grained, conglomeratic deposition in the northwestern foredeep of the Cordilleran foreland basin. Exposures of the Beaverhead Group are isolated, making it problematic to interpret depositional ages and relationships between exposures. The only age constraints for the Beaverhead were obtained from a limited sample of palynomorphs, which suggest Coniacian-Campanian (~89-72 Ma) depositional ages. Based on clast compositions, previous work suggests that Mississippian-Triassic limestone and Precambrian Belt Supergroup quartzite shed from thrust sheets to the west were the primary sediment sources. This study aims to better define the depositional age and provenance of the Beaverhead Group through the use of U-Pb dating on both detrital zircon and detrital apatite grains. Eight samples from a wide geographic and stratigraphic range within the Beaverhead were collected and analyzed using LA-ICPMS. Stratigraphic sections were measured where possible and detailed sedimentologic descriptions were recorded for each field site. The samples can be divided into three groups, where group 1 is quartzite conglomerate and interbedded sandstone; group 2 is limestone conglomerate and interbedded sandstone; and group 3 is distal, laterally equivalent sandstones. In our preliminary dataset, based on either the youngest single grain or youngest graphical peak age of the zircons the maximum depositional ages range from ~67.5-87.5 Ma (Coniacian-Maastrichtian). Zircon age distributions vary between samples; however, all groups have peaks between ~340-560 Ma, ~900-2100 Ma, and ~2350-3000 Ma, as well as a few single grains >3000 Ma. The primary difference is that the peaks in group 1 are relatively equal, whereas in group 2 there are large peaks at ~1440, 1720, 2490, and 2706 Ma. Group 3 differs in that it yields no Mesozoic zircons. Only four of the eight samples yielded detrital apatite, with a wide range of ages between ~70-560 Ma and ~1300-2000 Ma.