Data from over 1,400 coal exploration drill holes, 21 measured sections, and 90 deep mine maps, in conjunction with cursory examination of oil and gas logs and seismic sections, have been used to reconstruct the depositional settings of the Rock Springs Formation in the Green River basin.

From examination of approximately 20 coal seams in the Rock

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Springs Formation, a depositional model was developed to account for areas of variable thickness in coal accumulation. Coals within the formation developed along lower delta plain, upper delta plain-fluvial or on abandoned deltaic lobes and are referred to as Type A, Type B or Type C coals, respectively.

Figure

Depositional regression represented by extensive sheet sandstones are inferred to be delta-front deposits which reflect the cuspate to arcuate geometry of wave-dominated delta deposits. Widespread coal deposits up to 22 ft (6.7 m) thick that occur on top of the deltaic sandstones extend for up to 15 mi (25 km) along depositional dip and 36 mi (58 km) along depositional strike. They accumulated in lower delta plain environments as Type A coal seams. Thick coal seams that were deposited in upper delta plain-fluvial environments are less than 20 mi (32 km) in length and are more variable in thickness (1 to 17 ft, 0.3 to 5.2 m). They are referred to as Type B coal seams. Persistent but thin coals, less than 25 mi (40 km) in length and 1 to 8 ft (0.3 to 2.4 m) thick, that occur on top of d lta plain-fluvial deposits and that are overlain by sheet sandstones are inferred to represent peat accumulation during delta lobe abandonment and are referred to as Type C coal seams. Coal seam discontinuities, represented by areas of reduced coal thickness or by wedges of sediment producing multiple benches or rider coals, are caused by sediment influx from distributary channels, fluvial channels, and splays. Analysis of the geometries and spatial distributions of coal seams is used to develop a detailed geologic model that can serve as a predictive tool for future coal exploration in this region and in other basins with similar depositional settings.

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