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The environment of deposition of the Bullion Creek Formation in western North Dakota has been variously ascribed to lacustrine, meandering fluvial, and marginal marine deltaic environments with the latter three being favored by most workers. The purpose of the present study is to evaluate these previous models through careful field observations on a local scale, more specifically T145 and 146N, R102W of McKenzie County, North Dakota.
The Bullion Creek Formation rests conformably atop the Slope Formation (nonmarine) in the southwest quarter of the state and is conformably overlain by the Sentinel Butte Formation. The Tongue River Member of the Fort Union Formation is the lateral equivalent of the Bullion
Creek Formation in Montana, Wyoming, and South Dakota. The Bullion Creek Formation is composed primarily of flood-plain sediments, including unconsolidated sands, silts, and clays. Freshwater limestones and thin, discontinuous lignites (with the exception of the thick and laterally extensive HT Butte lignite which has been defined as the upper contact of the Bullion Creek) are also present. The carbonates are of two types: most common are the discontinuous pods of laminated lime mud, which are mudcracked at the top and may indicate evaporative ponds; more rarely, a gastropod-pelecypod-fishscale wackestone occurs near the top of the formation, and represents a small (^approx 1 mi2, 3 km2) lacustrine deposit.
Channeling in the Bullion Creek is of special interest. Paleochannels have a low width to depth ratio and appear laterally stable as evidenced by lack of lateral accretion or point bar deposits and vertical stacking. In map view, channels of equal stratigraphic position converge with and diverge from each other.
The preponderant flood-plain sediments, lignites, and carbonates, as well as channel configuration would seem to indicate deltaic sedimentation; however, the deposits contain fauna which are strictly fresh water, thereby making this interpretation tenuous. A possible alternative to this deltaic interpretation is the anastamosing fluvial model. This model, proposed by Smith and Putnam in 1980, has a dominance of vertical accretion, producing channel deposits which are thick, narrow, and laterally stable, as well as extensive inter-channel wetlands and lakes. Hanley and Flores in 1983 proposed a similar depositional picture for part of the Tongue River Member in the Powder River basin in northeastern Wyoming. Anastamosis in the Williston basin is probably the result of greater relative ocal subsidence of the basin than in the downstream reaches.
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