The Book Canyon Conglomerate: A Sequence Boundary Between the Bear Gulch Limestone and Tyler Formations of Central Montana

Abstract

The Serpukhovian Book Canyon Conglomerate of central Montana is a newly recognized limestone conglomerate. It overlies and contains intraformational clasts of the Bear Gulch Limestone, a world-renowned Carboniferous plattenkalk lagerstätte containing exquisite hard and soft-bodied fossils. The Book Canyon Conglomerate varies in thickness up to 2 meters over 2 km of laterally discontinuous outcrop. Well logs and cores indicate its presence beyond the study area 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 Tyler Formation, a meandering alluvial valley-fill within the Big Snowy Trough. Features of the Book Canyon 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 variations in the contacts between units that contribute to the debate of stratigraphic position. Modern slumping and lack of datable fossils within the Tyler have complicated studies in the past. Fauna within the Bear Gulch Limestone are Mississippian, but the Tyler had been Abstracts are organized as listed in Technical Session (page 27) Rocky Mountain Section – AAPG: 2019 Annual Meeting 36 previously interpreted as Pennsylvanian. Recent workers with updated stratigraphic techniques and additional core have confirmed that the Mississippian-Pennsylvanian boundary is located within the Tyler Formation, removing previous concerns over possible age discrepancies. The Book Canyon Conglomerate clarifies the Bear Gulch Limestone’s position in time as a brief marine invasion between the deposition of the lower and upper Tyler Formations. It represents channels cutting through the Bear Gulch Limestone due to deepening and shallowing of the Big Snowy Trough, which in past studies confused its relationship to the Heath and Tyler formations. The Conglomerate observed in outcrop is in contact with the Tyler Formation and represents a sequence boundary that confirms core and well log work that places the Bear Gulch Limestone within the Tyler Formation.

AAPG Datapages/Search and Discovery Article #90357 ©2019 AAPG Rocky Mountain Section Meeting, Cheyenne, Wyoming, September 15-18, 2019