A stratigraphic and sedimentologic analysis of the lower 200 m of the non-marine Two Medicine Formation (Campanian) near Choteau, Montana, permits correlation between the northern (type area) and southern (Choteau area) exposures of the formation. The Two Medicine Formation records a variable alluvial response to the Late Cretaceous Telegraph Creek-Eagle Regression (R7) and the Clagget Transgression (T8). The type Two Medicine Formation contains an erosional disconformity, marked by an oxidized intraclast lag. This disconformity is located ~75-80 m above the base of the formation and contains up to 5 m of erosional relief. It has been interpreted to reflect a relative drop in sea-level at ~80 Ma. The erosional disconformity is overlain by ~30 m of transgressive systems tract shoreface deposits and fine-grained strata that preserve a brackish water molluscan fauna. A comparable disconformity is not apparent in the southern part of the Two Medicine Formation near Choteau. Instead, non-marine facies there apparently record the R7/T8 transition by a decrease in the sandstone/shale ratio. Alluvial strata at the base of the Two Medicine are interpreted to reflect valley incision during R7 and are characterized by thick, tabular fluvial sandstones. Approximately 70-80 m above the base of the Choteau section, this channel-dominated succession gives way to ~40 m of bentonitic overbank mudstone with several intercalated bentonite beds and thin sandstone splay deposits. These finer-grained strata likely correlate with T8 and are interpreted to reflect pending of sediment due to rise in base level. Unlike the type Two Medicine, there is no evidence for a marine incursion into the Choteau area. The absence of a R7/T8 disconformity in the southern exposures of the Two Medicine Formation may reflect higher rates of basin subsidence in the Choteau region during R7. Alternatively, the absence disconformity may simply reflect lateral changes in paleogeography and paleoenvironment across the alluvial plain.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah