STRATIGRAPHIC ANALYSIS OF THE UPPER CRETACEOUS (CAMPANIAN) JUDITH RIVER FORMATION, NORTH-CENTRAL MONTANA: LITHOSTRATIGRAPHIC UPDATES AND REFINED REGIONAL CORRELATIONS

Despite the longstanding significance of the Judith River Formation (JRF) in the annals North American stratigraphy, and its continuing role in paleobiological pursuits, key aspects of the unit remain poorly understood. For example, almost all previous descriptions of the JRF in Montana maintain that the unit is predominantly, if not wholly, terrestrial in origin, whereas approximately half of the JRF in the eastern sector of its type area consists of shallow marine deposits. In addition, the existing composite type section fails to capture the range of lithologies that characterize the formation and, although regional correlations are understood in general, precise correlations with richly fossiliferous units in western Montana and Canada remain elusive. We re-evaluate JRF stratigraphy in its type area in north-central Montana and propose new reference sections that illustrate the full range of lithologies developed in the unit. The first reference section from the western edge of the type area spans the contact with the underlying Claggett Formation. Previously described as gradational and conformable, new data indicate that this contact is an erosional surface consistent with negative accommodation and forced regression of the Claggett Sea. The second new reference section positioned in the middle of the type area spans the entire JRF from its basal contact with the Claggett Formation to its upper boundary with the Bearpaw Formation, and hosts a lithologic discontinuity that marks a regional reorganization of terrestrial and marine depositional systems. Framework grain composition documented via 400-grain point counts of fluvial sandstones shows a significant influx of volcanic rock fragments and feldspar grains above the discontinuity: the average abundance of K-feldspar roughly doubles and the plagioclase content jumps nearly fourfold. New subsurface data extend the geographic range of this discontinuity more than 240 km along depositional strike to the plains of southern Alberta and Saskatchewan. A third reference section in the eastern reaches of the type area intersects the coastal marine portion of the JRF, and a new member will be proposed to accommodate these distinctive facies.