The Teapot Sandstone forms the upper member of the Upper Cretaceous Mesa Verde Formation in the Powder River basin. Previous interpretations of the Teapot based on outcrop or subsurface data range from nearshore marine to fluvial. Integration of outcrop data with subsurface data from cores provides a more complete interpretation of depositional

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environment because of the rapid facies changes in the Teapot. The Teapot Sandstone has a complex diagenetic history and diagenetic patterns are facies controlled.

Lithofacies of the Teapot Sandstone are analogous to the modern Nile delta. The Teapot Sandstone is interpreted as a regressive, wave/fluvial-dominated deltaic sequence which prograded eastward into the Cretaceous seaway. Marine lithofacies coarsen upward from bioturbated offshore siltsone to nearshore sandstone with large, pellet-lined ophiomorpha and overlying well-sorted, horizontally laminated foreshore sandstone exhibiting ridge and runnel topography. Marine foreshore sandstone is overlain by complexly interbedded sandstone and carbonaceous shale in stacked fining-upward sequences of the delta plain. Rootlets and contorted beds are common. Fining-upward units are interpreted as abandoned channels, whereas coarsening-upward sequences are interpreted as interdistributary bay or lag onal deposits. Capping the sequence is a thick, cross-bedded fluvial section consisting of levee, point bar, and channel sand deposits. Slumped beds, intraformational basal conglomerates, and minor eolian ripple laminations are present in fluvial sandstone.

The Teapot Sandstone has a complex diagenetic history. Siderite and framboidal pyrite formed early in the diagenetic sequence at shallow depths of burial under anaerobic conditions. Pore-filling kaolinite, chlorite, and quartz overgrowths formed coevally following dissolution of relatively unstable framework grains. Poikilotopic calcite cement is locally abundant and extensively replaces framework grains. Depositional facies exert strong control on diagenetic patterns. Kaolinite occurs predominantly in fluvial sandstone. Chlorite is restricted to marine facies, and calcite is further restricted to well-sorted foreshore marine sandstone. Quartz overgrowths occur only in relatively well-sorted sandstone, whereas pyrite and siderite are common in shaly sandstone and siltstone.

Nearshore marine and fluvial sandstone are potentially hydrocarbon reservoirs, although authigenic clays have significantly reduced permeability. Reservoir potential of well-sorted foreshore marine sandstone was destroyed by pore-filling calcite cement. However, tightly cemented sandstone forms a potential diagenetic trapping mechanism.

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