INFLUENCE OF THE ANCESTRAL SWEETGRASS ARCH ON SEDIMENTATION OF THE LOWER CRETACEOUS BOOTLEGGER MEMBER, NORTH-CENTRAL MONTANA

In north-central Montana the Bootlegger Member of the upper Lower Cret aceous Blackleaf Formation is well exposed near Great Falls and at one locality in the Sweetgrass Hills. In both areas, Bootlegger strata co mprise four stratigraphic successions. In the Great Falls study area e ach 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 surfac e and capped in many places by a conglomeratic transgressive lag depos it. The stacking of these successions is attributed to episodic change s of relative sea level, probably related to episodic: reactivation of the ancestral Sweetgrass Arch, During episodes of arch uplift accommo dation space was reduced and caused the Bootlegger shoreline to progra de eastward, Subsequently, because of arch subsidence and/or the ongoi ng basinwide eustatic rise, the shoreline migrated westward, Erosion a ssociated with transgressive shoreface retreat was most intense near t he crest of the arch, As a result, in the western study area, which li es above the arch crest, successions are thin and typically consist on ly of inner-shelf strata (Lithofacies 1); strata deposited in shallowe r 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 teased o r simply could no longer keep pace with the eustatic rise. Consequentl y, the shoreline continued to transgress westward, and only fossilifer ous silt and mud accumulated in a distal shelf environment, This marke d a eustatic drowning and termination of rhythmic Bootlegger sedimenta tion.