Print this pageSequence Stratigraphic Framework of the Lower Mississippian Lodgepole Formation, Williston Basin: Carbonate Ramp Sedimentation in a Starved Basin Setting
RIPPLE, ROBERT A., STEPHEN D. STURM, and DAVID F. LIST
A sequence stratigraphic framework developed from an integrated well log, lithologic and petrographic database shows the Lodgepole Formation to be a carbonate ramp complex consisting of five third-order sequences. The sequences are comprised of transgressive systems tract (TST) and highstand systems tract (HST) couplets which infilled the basin from the southern and eastern margins. Stratal interpretations are based on correlation of chronostratigraphic surfaces (transgressive and maximum flooding surfaces) and unconformities (sequence boundaries). Sequences 1, 2 and 3 were deposited during the north and west progradation of the Lodgepole ramp into the pre-existing Elk Point Basin. Sequence 4 filled a tectonically-defined subbasin in southwestern North Dakota. Growth of Waulsortian mounds initiated on the lower ramp during a time interval spanning at least two third-order sequences. Sequence 5 is a composite third-order TST coupled with an overlying HST that represents the final phase of Lodgepole progradation into the basin center. The base and top of the Lodgepole are interpreted as Type 1 sequence boundaries with probable subaerial exposure toward the basin margins.
Analysis of stratal patterns indicates the Lodgepole ramp was dominated by extremely low dips (<1 degree), wide facies bands and low sedimentation rates. Much of the basin was starved during lower Lodgepole deposition and an extensive condensed section formed. Global second-order highstand conditions and high subsidence rates during the lower Lodgepole suppressed the development of lowstand systems tracts, emphasized the development of highstand and transgressive systems tracts and was responsible for widespread anoxic conditions and low clastic sedimentation rates.