THE BEAR GULCH CONGLOMERATE (SERPUKHOVIAN) BEAR GULCH LIMESTONE, CENTRAL MONTANA: INFORMING A STRATIGRAPHIC DEBATE

The Serpukhovian Bear Gulch conglomerate is a newly recognized limestone conglomerate in central Montana. It overlies and contains clasts of the Bear Gulch Limestone, a world renowned plattenkalk lagerstätte. The Bear Gulch conglomerate is up to 2 meters thick, markedly lensoid, and laterally discontinuous in its outcrop for a distance of 2 km but likely extends beyond the study area. Well logs indicate its presence in the subsurface between the underlying Bear Gulch Limestone and overlying Tyler Formation. This conglomerate provides new information regarding the transition of the marine-dominated Bear Gulch Limestone to the overlying fluvial Tyler Formation. Features of the Bear Gulch Conglomerate include subaerial/fluvial features such as lateral bars and floodplain paleosol deposits.

Freshwater exposure early in the post-depositional history of Bear Gulch Limestone explains the unconformities that contribute to the debate of the exact stratigraphic position, however, also suggests the need for a new nomenclature. Further analysis of these unconformities, and the unique fauna, clarifies the Bear Gulch Limestone’s position in time.