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A comprehensive surface and subsurface study of the Upper Cretaceous Lazeart Sandstone Member of the Adaville Formation and the lower coal-bearing part of the Adaville Formation in the southwestern Wyoming thrust belt reveals a complex inter-tonguing of marine and nonmarine strata. During late Santonian and early Campanian time, the Lazeart wave-dominated deltaic system prograded southeastward onto a storm-influenced microtidal shelf. Sediments of the Lazeart Sandstone Member accumulated within storm-dominated lower shoreface, barred fair-weather upper shoreface, foreshore, washover, mouth bar, flood tidal delta, and tidal channel subenvironments. Landward of the strand line, sediments of the Adaville Formation were deposited within active distributary, channel margin, bay-head delta, slough, interdistributary swamp, lake, interdistributary bay, salt marsh, lagoon, interdeltaic bay, and peat-forming swamp subenvironments. Nine transgressive-regressive pulses of this wave-dominated deltaic system were superimposed on the regressive portion of the Niobrara Cyclothem.

Thicknesses of noncoal strata, grain size, and sand percent all decrease away from channels and thickness and lateral continuity of coal increase. Because of differential compaction, areas with thick peat deposits became sites of deposition of active channel, channel margin, and slough facies. Subsequently, many of these sites once again evolved into areas of thick peat accumulation.

Individual seam thickness is limited by the magnitude of the associated transgressive-regressive pulse. During regressions of limited areal extent, coastal swamps remained in one place for a long period of time, and thick peats were deposited. High subsidence rates favored such limited regressions and the development of thick, localized coal seams. Local subsidence rates were highest when shoreface sands prograded into deeper water directly over marine muds. Thickest coal seams overlie and intertongue with thick shoreface, foreshore, and washover fan sandbodies. Subsidence of the sandbodies maintained a high water table and contributed to the development of stable peat swamps. In interdeltaic regions, lower sediment loads and rates of subsidence led to the accumulation of thinner peats.

The distribution and total reserves of coal seams within the Adaville coal field is controlled by the number of transgressive-regressive pulses and their geographic extent. Vertical stacking of the regressive sand bodies maximizes total coal reserves for a given locality. Stacked shallow marine sandbodies and high total coal reserves for the Adaville Formation are a consequence of relative sea-level rise during a period of isostatic compensation to thrust and sediment loads of the Sevier Orogenic Belt.

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