Date of Award
Doctor of Philosophy (PhD)
F.D. Holland, Jr
The Tongue River-Sentirel Butte contact has been regarded by many workers as a vague color boundary of minor extent within a relatively homogeneous sequence of Paleocene strata. Consequently, the Sentinel Butte has come to be regarded as a subordinate unit of the ”Tongue River Formation. As defined in report, the contact is a distinctive horizon between two discrete lithogenetic units. It characterized by three criteria: a horizon (HT Butte bad) &t the top of the Tongue River sequence; a. basal sandy unit in the Sentinel Butte sequence, and a marked change in color between buff yellow Tongue River sediments below and somber gray Sentinel Butte sediments above.
This contact has been mapped on a scale of 1:250,000 throughout the badlands of the Little Missouri River, and along the Missouri River from the Montana-North Dakota border to the mouth of the Little Missouri River. The contact is concealed in the central part of the Williston basin, but crops out on the eastern flank of the basin about 60 to 80 miles east of the area mapped. The extent of the contact along the eastern margin of' the basin has not been determined, but outcrops in Marton County display lithologic relationships similar to those which distinguish the contact farther west. No evidence was found in support of the Tongue River-Sentinel Butte facies relationship postulated by previous investigators.
Recognition of distinctive stratigraphic relationships at the Tongue River-Sentinel Butte contact and documentation of their regional persistence demonstrate that the Sentinel Butte sequence is a mappable lithostratigraphic unit. It is therefore recommended that in western North Dakota and adjacent areas the Sentinel Butte sequence be assigned formational rank. The name Tongue River Formation should be applied only to beds underlying the Sentinel Butte Formation.
Granulometric analysis of nearly 500 sediment samples from 11 stratigraphic sections show that Tongue River sediments are finer grained and less well sorted than those of the Sentinel Butte For mation. Median diameter and skewness are environmentally sensitive particle-size statistics. CM patter-as illustrate the fluvial origin of these Paleocene deposits and are used to differentiate sediment transport types and depositional environments; channel, floodplain, and backswamp facies are recognized. Significant differences in fluvial regimes are indicated by the relative abundances of .flood plain and backswamp sediments deposited by Tongue River and Sentinel Butte streams. CM patterns give values for maximum suspended-load and minimum bed-load particle sizes which can be used with Hjulstrom type diagrams to approximate paleocurrent velocities. Sentinel Butte streams had higher velocities than those of Tongue River time, but the magnitude of both was small and maximum and mid-depth velocities of 40 to 50 centimeters per second are estimated.
Evaluation of stratigraphic, lithologic, and sedimentologic relationships, types and occurrences of' primary sedimentary structures, and carbonate contents o:f Tongue River and Sentinel Butte strata permit formulation of a sedimentation model for each sequence. Tongue River strata were dispersed eastward across the North Dakota portion of the Williston basin by slow moving streams which drained a low-lying source area to the west. The gradient of the paleoslope was low and sediments were transported primarily in suspension. The fluvial system was stable and protected backswamps developed in which extensive deposits of locally derived organic material accumulated. Basinal subsidence was uniform and controlled the rate of sedimentation during most of the episode. Sediment characteristics indicate that western North Dakota was near base level during Tongue River time. Near the close of the episode, the elevation of the source area was reduced, basinal subsidence exceeded sedimentation, and swamp conditions prevailed throughout much of Western North Dakota.
The episode of Sentinel Butte deposition was initiated by an influx of coarse, basal sediment which spread eastward and south eastward across the late Tongue River swamp. Streams had slightly greater energies than those which previously crossed the basin, but sediment transport was still primarily by suspension. The paleoslope appears to have been variable, both in magnitude and direction, and may reflect changing or multiple sediment source areas created by late Iarmide activity to the west and northwest. The elevation of western North Dakota above base-level increased during Sentinel Butte time, probably as a result of rapid deposition (in excess of basinal subsidence), vertical accretion, and eastward overstepping of the Sentinel Butte sequence.
Royse, Chester F. Jr., "A stratigraphic and sedimentologic analysis of the Tongue River and Sentinel Butte Formations (Paleocene), western North Dakota" (1967). Theses and Dissertations. 250.