DETRITAL ZIRCON PROVENANCE OF THE LOWER-UPPER CRETACEOUS BLACKLEAF FORMATION (UPPER INTERVALS) IN SOUTHWESTERN MONTANA, USA: IMPLICATIONS FOR REGIONAL TECTONICS AND SEDIMENTATION PATTERNS

The Lower-Upper Cretaceous (Albian-Cenomanian) Blackleaf Formation in southwestern Montana preserves fluvial to shallow-marine depositional environments associated with sediment flux from the Sevier thrust belt into the North American Cordilleran foreland basin and the Cretaceous Interior Seaway. Some previous work has suggested that parts of the Blackleaf Formation record the presence of earlier-than-expected intra-basinal Laramide-style uplifts typically associated with flat-slab subduction. Previous models exhibit significant temporal and spatial variations, which warrants a more detailed investigation of the sediment dispersal patterns and tectonic development of the region during this time. We present new high-n U-Pb detrital zircon geochronologic data and mixture models from upper Blackleaf Formation sandstones that allow for quantitative provenance determination and testing of previous interpretations. Age distributions vary widely; some contain up to 45% Mesozoic grains with ages of ~104, ~115, and ~166 Ma, whereas others exhibit up to 97% older ages including notable ~420, ~1060, ~1780, and ~2700 Ma peaks. Examination of detrital zircon age spectra and mixture modeling identify two distinct sediment source groups: 1) primary sourcing from Mesozoic arc-rocks in British Columbia and Idaho and 2) recycling of Triassic, upper Paleozoic, lower Paleozoic, Cambrian, and Neoproterozoic units from thin-skinned east-central Idaho thrust sheets. No sediment input from the Belt Supergroup was detected. In conjunction with previous sedimentological work, our detrital zircon data document more input from thrust sheet-derived detritus in dominantly fluvial depositional systems compared to Blackleaf-equivalents to the north and south, indicating a tectonically-derived, prograding sediment wedge in southwestern Montana during Blackleaf time. In addition, the inference that volcaniclastics in the uppermost Blackleaf Formation resulted from syndepositional volcanism of the Idaho or Boulder batholith is inconsistent with our youngest zircon age peaks (102-108 Ma) that suggest older, non-syndepositional arc sources.