ESTABLISHING A SEQUENCE STRATIGRAPHIC FRAMEWORK OF THE DEVONIAN-MISSISSIPPIAN BAKKEN-EQUIVALENT SAPPINGTON MEMBER OF THE THREEFORKS FORMATION IN CENTRAL MONTANA
Similar to the Bakken Formation, the Sappington member records two packages of cyclic sedimentation that are unconformably capped by the upper organic-rich mudstone if present. The first transgression is represented by the basal organic-rich mudstone unconformably overlying the ThreeForks Formation, overlain by carbonate reef and fore-reef depositions, followed by a coarsening upward sequence of sandy siltstone. Initiation of the second transgression is represented by mudstone interlaminated with sandy siltstone, gradationally fining upward into mudstone, which transitions back to the interlaminated mudstone-siltstone facies, followed by a coarsening upward sequence of sandy siltstone.
Isopach map based on well log shape analysis showed a presence of Central Montana Trough as an active depocenter over the time of Sappington deposition. During the first cycle, it deposited in the semi-isolated, west-east trending Central Montana Trough. By the end of the first cycle, the Antler forebuldge appeared to become more active and dominant feature in the trough, separating it into two sub-basins in the west and east. During the late stage of the second cycle, paleo-lineaments in the eastern portion of the trough appeared to be reactivated, implied by creations of high accommodation in the eastern portion of the trough and another sub-basin in northwest of the trough.