MIDDLE ALBIAN PROVENANCE, SEDIMENT DISPERSAL, AND FORELAND BASIN DYNAMICS IN SOUTHWESTERN MONTANA, NORTH AMERICAN CORDILLERA

The Lower Cretaceous Blackleaf Formation in southwestern Montana records sedimentation in the Idaho-Montana sector of the North American Cordilleran foreland basin system. Contrasting paleogeographic reconstructions have been presented during Albian time; some workers have interpreted topographic highs, which were actively shedding sediment into the foreland. In contrast, regional-scale sedimentology suggests that southwestern Montana was flooded by an early marine incursion of the Western Interior Seaway. Additionally, previous detrital zircon provenance suggests a transcontinental river system with headwaters in the Appalachian region fed sediment as far west as the Bighorn basin. We use sandstone petrography, large-n (n=600) detrital zircon geochronology, and mixture modeling to determine the provenance of the lower Blackleaf Formation. Our analysis suggests three distinct provenance groups: Group 1 sandstones occur in the eastern region of the study area, are quartz-rich, and have U-Pb ages of ca. 110, 160, 420-450, 1050, and 1160 Ma; these sandstones were delivered to the foreland by transcontinental rivers with headwaters in the Appalachian region. Group 2 sandstones occur in the western region of study area, are lithic-rich, and have prominent U-Pb ages of ca. 110, 160, 1780, 1840, 1920, 2080, and 2700 Ma; these sandstones were sourced from exhumed lower-middle Paleozoic strata from the Idaho sector of the Sevier belt. Group 3 sandstones occur in the western region of the study area, are lithic-rich, and have prominent U-Pb ages of ca. 115, 170, 430, 600, 1085, 1170, 1670, and 1790 Ma; their sources were exhumed Triassic-upper Paleozoic strata from the Idaho sector of the Sevier belt. Based on detrital zircon analysis of the lower Blackleaf Formation, we document a sharp change in provenance that coincides with the western shoreline of the Albian Western Interior Seaway. Sediments in the eastern region of the study area were sourced from transcontinental rivers and reworked by marine waters as the epicontinental seaway encroached from the north. Our analysis suggests the presence of an irregular, submarine forebulge that may have influenced sediment dispersal patterns, rather than topographic highs within—and providing sediment to—the southwestern Montana sector of the foreland basin.