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The AAPG/Datapages Combined Publications Database

AAPG Bulletin

Abstract


Volume: 52 (1968)

Issue: 3. (March)

First Page: 548

Last Page: 548

Title: Sedimentologic Analysis of Tongue River and Sentinel Butte Formations (Paleocene), Western North Dakota: ABSTRACT

Author(s): Chester F. Royse, Jr.

Article Type: Meeting abstract

Abstract:

The contact between the Tongue River and Sentinel Butte sequences of western North Dakota has been regarded by many workers as a vague color boundary within a relatively homogeneous sequence of continental Paleocene strata. Recognition of distinctive stratigraphic relations at the Tongue River--Sentinel Butte contact, and documentation of their regional persistence, demonstrate that the Sentinel Butte interval is a mappable lithostratigraphic unit which merits formational rank. Granulometric analysis of approximately 500 sediment samples from 11 stratigraphic sections amplify textural differences of the units; Tongue River sediments are finer grained and less well sorted than the Sentinel Butte Formation and median diameter and skewness are environmentally sensitive parti le-size statistics.

CM patterns illustrate the fluvial origin of these Paleocene deposits and are used to differentiate sediment-transport types and depositional environment; channel, floodplain, and backswamp facies are recognized. Significant differences in the regimes of fluvial systems which existed during middle and late Paleocene times are suggested by differences in the relative abundance of facies types.

Sedimentation models have been formulated for each sequence. Tongue River strata were dispersed eastward across the North Dakota part of the Williston basin by slow-moving streams which drained a low-lying source area on the west. Paleoslope gradient was low and sediments were transported primarily in suspension. The fluvial system was stable and protected backswamps developed. Basinal subsisdence was uniform and controlled the rate of sedimentation during most of the episode; western North Dakota was near base level during the time of Tongue River deposition. Near the close of the episode, the elevation of the source area was reduced, basinal subsidence exceeded sedimentation, and swamp conditions prevailed through much of western North Dakota.

Sentinel Butte deposition was begun by an influx of coarse sediment dispersed eastward and southeastward, in deltaic fashion, across the last Tongue River swamps. Streams had slightly greater velocities than those of the previous episode, but sediment transport was still primarily by suspension. The paleoslope appears to have been variable, both in magnitude and direction, and reflects mid-Paleocene tectonism on the west and northwest. The elevation of western North Dakota above base level increased during the time of Sentinel Butte deposition, probably due to vertical accretion and eastward overstepping of the Sentinel Butte sequence.

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