TRACKING THE TERRESTRIAL “80 MA SEQUENCE BOUNDARY” IN THE SURFACE AND SUBSURFACE OF NORTHWESTERN MONTANA (TWO MEDICINE FORMATION) AND SOUTHERN ALBERTA (BELLY RIVER GROUP)

The nonmarine expression of the “80 Ma sequence boundary” was initially identified in outcrop during a regional study of the sedimentology of the Upper Cretaceous (Campanian) Two Medicine Formation in northwestern Montana. Numerous features identified in surface exposures suggested the presence of a regionally significant surface in a widespread sheet-like fluvial sandstone body, hereafter referred to as the “sequence boundary sandstone” (SBSS), positioned ~75 m above the base of the formation. These features, which were interpreted to reflect negative/paused accommodation, characterize the SBSS throughout its outcrop belt, and include: (1) pervasive oxidation, (2) unusually thick intraclast lag deposits (including large bank collapse blocks), and (3) internal erosion surface(s) with up to 5 m of relief. While some of these features might be expected in fluvial strata in general, comparison with the overall section, which spans ~550 m, indicate that the SBSS facies is distinct from surrounding Two Medicine strata. Two of the most notable aspects of the SBSS are its unusually thick nature and its geographic persistence. Electric and gamma-ray logs help track the SBSS in the subsurface and supplement our observations on thickness and lateral extent. Subsurface data indicate that the SBSS averages ~16 m in thickness in Montana and southern Alberta. Its southern terminus appears to fall just south of the Two Medicine River, where it was first identified in outcrop. Well log coverage is patchy within Montana to the north of the town of Cut Bank, but the SBSS is present in northernmost Montana, and it can be tracked ~40 km northward into southern Alberta, confirming its widespread regional development. Lastly, throughout its outcrop belt, the SBSS marks a significant shift in facies indicative of regional transgression. It is abruptly overlain by paralic deposits along the Two Medicine River and shallow marine sandstones near Cut Bank. A new high-precision U-Pb date of ~80.2 Ma (early Campanian) from a bentonite in the overlying paralic deposits on the Two Medicine River suggests that the transgressive surface capping the SBSS likely correlates with the Milk River shoulder (=Eagle shoulder in Montana), a key stratigraphic surface used for regional correlation in equivalent marine deposits.