REVISITING A REGIONALLY SIGNIFICANT TERRESTRIAL BOUNDING SURFACE IN THE UPPER CRETACEOUS (CAMPANIAN) TWO MEDICINE FORMATION, NORTHWESTERN MONTANA

New data are brought to bear on a fluvial disconformity previously interpreted to represent a sequence boundary in fully terrestrial strata of the Upper Cretaceous (Campanian) Two Medicine Formation in northwestern Montana. The disconformity is embedded in a relatively flat-based fluvial sandstone sheet ~75 m above the base of the formation, and is marked by several meters of internal erosional scour, a thick and laterally persistent intraclast lag facies, and pervasive oxidation. Physical stratigraphic evidence and associated age data indicate that this fluvial disconformity correlates with widespread erosion and base level fall in more distal parts of the basin that transpired at ~81 Ma. Data indicate that the fluvial disconformity in the lower Two Medicine Formation reflects a negative base-level adjustment that occurred at the end of the Telegraph Creek–Eagle regression (R7). Paralic deposits of the subsequent Claggett transgression (T8) overlie the disconformity. Recognition of this disconformity is significant because it is one of very few well-documented examples of a nonmarine sequence boundary. Here we delve deeper into the nature of this terrestrial sequence boundary by documenting its detrital mineralogy and cement/framework grain ratio in relation to underlying and overlying sandstone beds. Mineralogical trends were documented via 400-grain point counts. Preliminary data indicate that there are no significant mineralogical distinctions that transpire across the sequence boundary. Sandstones spanning the disconformity can be classified as litharenites, and all yield abundant volcanic rock fragments. There is no apparent change in provenance across the sequence boundary. In contrast, there is a distinct diagenetic signature associated with the sequence boundary, as evidenced by changes in the cement/framework grain ratio. Abundant calcite cement and loose grain packing characterize the sandstone body hosting the sequence boundary. Less cement and tighter grain packing characterize fluvial sandstones intercalated below and above the sequence boundary. Early cementation, which inhibited compaction, can be added to the list of features that characterize the disconformity.