SEDIMENTOLOGY AND PROVENANCE OF THE UPPER CRETACEOUS (CENOMANIAN TO SANTONIAN) FRONTIER FORMATION, SOUTHWEST MONTANA AND EAST-CENTRAL IDAHO

The Frontier Formation near Lima Peaks and the western Centennial Mountains is characterized as a dominantly non-marine ~800-2000m thick foreland basin deposit. Frontier strata in the western Centennial Mountains are situated in an area of structural overlap between the northwest-trending Sevier-style Idaho-Montana fold-and-thrust belt and the northeast-trending Laramide-style Blacktail-Snowcrest uplift. The aim of this study is to integrate the sedimentology with new multi-proxy provenance data to investigate both of these topographically high regions as potential sediment sources during Frontier time. Our approach is to build a dataset of measured stratigraphic sections, sandstone petrographic data, detrital zircon and detrital apatite U-Pb geochronological data, and biostratigraphical data to construct a chronostratigraphic framework in which to interpret the provenance data. Preliminary results from our sedimentological observations support previous interpretations that the Frontier Formation is a fine-grained fluvial system that accumulated in a structurally complex part of the foreland basin. Detrital zircon analyses show a youngest graphical peak age of ~87-88.5 Ma, a largest graphical peak age of ~94.5-99 Ma and additional minor populations of ~157 Ma, ~1000-1100 Ma, ~1400 Ma, ~1620-1680 Ma, ~1740-1790 Ma and ~2700 Ma. Overall the maximum depositional ages inferred from the youngest graphical peaks are consistent with published ages of 85.81±0.22 Ma and 86.25±0.38 Ma from ⁴⁰Ar/³⁹Ar of sanidine found in volcanogenic units of the middle Frontier Formation. Additionally, palynological data from the middle Frontier indicates a depositional age of Coniacian (89.8-86.3 Ma or younger). In contrast, detrital apatite analyses range from ~80-220 Ma, with peaks around ~82 Ma, ~94 Ma, ~105 Ma, ~115 Ma and ~130 Ma. In summary, provenance signatures based on detrital zircon suggest a predominance of early-Late Cretaceous first-cycle igneous sources and a coeval Coniacian source from the Idaho batholith. The minor older peaks likely represent recycling of the Paleozoic passive margin strata of the fold-and-thrust belt. In comparison, the young nature of detrital apatite populations (<220 Ma) suggests that apatite predominantly records only first-cycle sediment flux from the Cordilleran Arc.