Influence of the Ancestral Sweetgrass Arch on Sedimentation of the Lower Cretaceous Bootlegger Member, North-Central Montana

Abstract

ABSTRACT In north-central Montana the Bootlegger Member of the upper Lower Cretaceous Blackleaf Formation is well exposed near Great Falls and at one locality in the Sweetgrass Hills. In both areas, Bootlegger strata comprise four stratigraphic successions. In the Great Falls study area each succession consists of an upward-coarsening (-shoaling) package of interbedded very fine sandstone and shale grading upward into fine to medium sandstone, abruptly overlain by a transgressive erosion surface and capped in many places by a conglomeratic transgressive lag deposit. The stacking of these successions is attributed to episodic changes of relative sea level, probably related to episodic reactivation of the ancestral Sweetgrass Arch. During episodes of arch uplift accommodation space was reduced a d caused the Bootlegger shoreline to prograde eastward. Subsequently, because of arch subsidence and/or the ongoing basinwide eustatic rise, the shoreline migrated westward. Erosion associated with transgressive shoreface retreat was most intense near the crest of the arch. As a result, in the western study area, which lies above the arch crest, successions are thin and typically consist only of inner-shelf strata (Lithofacies 1); strata deposited in shallower environments were removed mainly by later transgressive erosion. By contrast, in the eastern area, farther from the arch crest, the upward-shoaling part of each succession is more completely preserved. At the end of Bootlegger time, however, movement of the arch either ceased or simply could no longer keep pace with the eustatic ri e. Consequently, the shoreline continued to transgress westward, and only fossiliferous silt and mud accumulated in a distal shelf environment. This marked a eustatic drowning and termination of rhythmic Bootlegger sedimentation.