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Coal-bearing strata of Late Cretaceous age in the western part of the Wind River Basin show transitions in depositional environment from coastal marine deltaic (neritic and paralic) to nonmarine intermontane (limnic). Earliest peat accumulation in the Frontier Formation coincided with several extensive marine regressive cycles. Deposition of terrestrial strata in this deltaic environment ended with encroachment by the sea from the east, which resulted in an extensive period of marine-dominated deposition of the thick Cody Shale.

The overlying Mesaverde Formation was initially deposited as a prograding delta shoreline complex of sand bodies on which coastal swamps were established. Peat accumulation in the Mesaverde was terminated by deposition of the widespread marine transgressive white sandstone member of Troyer and Keefer (1955). During deposition of the overlying Meeteetse Formation, local emergence to the north and subsidence to the east transformed this area into a developing intermontane basin. Extensive airfalls of volcanic ash also accompanied sedimentation throughout Cretaceous and Tertiary time. Many of these events and changes in the environment during these periods are recorded in the coal geochemistry.

Analysis of coal samples from two thick (⩾305 cm [120 in]) coal beds, the Signor in the Mesaverde Formation and the Welton in the Meeteetse Formation, shows significant differences in trace element content and coal quality, which are interpreted to reflect contrasting paleodepositional environments. Higher iron and sulfur content in the Signor coal bed is attributed to marine and brackish influences on syn- and post-depositional sedimentation. Elevated ash values and silica content in the Welton coal bed resulted from the increased influx of volcaniclastic sediment in a developing intermontane basin undergoing rapid subsidence and structural deformation. The depositional history of coal-bearing rocks in this basin is linked to a series of tectonic events and sedimentary cycles that began during deposition of the Frontier Formation within or west of the Green River Basin. This pattern of sedimentation and deformation continued to evolve in an eastward direction through the present area of the Wind River Basin and culminated in the development of enormous coal deposits in the Powder River Basin.

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