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Improved age constraints on the Wahweap Formation indicate that the middle and upper members were deposited during the eustatic Claggett transgression (T8 of Kauffman 1977) in the adjacent Western Interior Seaway.Additionally,facies analysis of the Wahweap Formation has revealed an increase in both the sand:mud ratio and the degree of sandstone amalgamation from the upper part of the middle member to the base of the capping sandstone,suggesting a gradual reduction in generation of accommodation in the basin.Following recent alluvial sequence stratigraphic models,the middle member is interpreted as the isolated fluvial facies tract,while the upper member represents the tidally influenced and highstand facies tracts.Maximum transgression occurred during deposition of the lowest part of the upper member,synchronous with the putatively eustatic Claggett highstand in other parts of the Western Interior Basin.The sequence boundary is placed at the base of the overlying capping sandstone member,diagnosed by a major shift in petrography and paleocurrent direction,as well as up to 4 m of fluvial incision into the underlying upper member.The capping sandstone member is interpreted as the amalgamated fluvial facies tract,and the sequence boundary at the base of the capping sandstone is regarded as tectonically induced.Improved geochronology in the formation now permits high-resolution correlation with marine units to the east and along-strike correlation with contemporaneous alluvial units up and down the western margin of the Western Interior Basin.
Improved age constraints on the Wahweap Formation indicate that the middle and upper members were deposited during the eustatic Claggett transgression (T8 of Kauffman 1977) in the adjacent Western Interior Seaway.Additionally,facies analysis of the Wahweap Formation has revealed an increase in both the sand:mud ratio and the degree of sandstone amalgamation from the upper part of the middle member to the base of the capping sandstone,suggesting a gradual reduction in generation of accommodation in the basin.Following recent alluvial sequence stratigraphic models,the middle member is interpreted as the isolated fluvial facies tract,while the upper member represents the tidally influenced and highstand facies tracts.Maximum transgression occurred during deposition of the lowest part of the upper member,synchronous with the putatively eustatic Claggett highstand in other parts of the Western Interior Basin.The sequence boundary is placed at the base of the overlying capping sandstone member,diagnosed by a major shift in petrography and paleocurrent direction,as well as up to 4 m of fluvial incision into the underlying upper member.The capping sandstone member is interpreted as the amalgamated fluvial facies tract,and the sequence boundary at the base of the capping sandstone is regarded as tectonically induced.Improved geochronology in the formation now permits high-resolution correlation with marine units to the east and along-strike correlation with contemporaneous alluvial units up and down the western margin of the Western Interior Basin.
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