REASSESSMENT OF THE BELT SUPERGROUP: A STRATIGRAPHIC ANALYSIS OF THE DEVIL'S FENCE ANTICLINORIUM, SOUTHWEST MONTANA [AN EDMAP PROJECT]

In the Devil's Fence Anticlinorium of southwest Montana, the Middle Proterozoic Belt Supergroup has traditionally been divided into three formations: Greyson Shale (green and gray mudstone with stromatalitic horizons) conformably overlain by Spokane Shale (grayish red and green mudstone with minor limestone and sandstone beds), which is gradationally overlain by Empire Shale (greenish gray siliceous mudstone with minor sandstone interbeds). Previous workers describe a significant unconformity that removed >100's m of section beneath the overlying Middle Cambrian Flathead Sandstone. Detailed geologic mapping and stratigraphic analysis in the Devil's Fence Anticlinorium suggests a revision of Belt Supergroup stratigraphy in the region should be considered. Two key observations support this revision: 1) the contact beneath the Flathead Sandstone is not a sharp, angular unconformity, but rather a gradational, coarsening upward succession; 2) the north to south lateral transition from dominantly green mudstone (“Empire”) to red mudstone (“Spokane”) represents lateral diagenetic variation as opposed to a vertical stratigraphic succession. These observations suggest that the strata beneath the Flathead sandstone are not Middle Proterozoic, but are probably significantly younger. Ongoing detrital zircon analyses will further constrain the age of this subjacent strata.