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Sequence
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.