Facies architecture of the Upper Sego member of the Mancos Shale Formation, Book Cliffs, Utah
Robinson, Eric D.
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The Late Cretaceous upper Sego Member of the Mancos Shale exposed in the Book Cliffs of east-central Utah is a 30 m thick sandstone wedge that overlies the Anchor Mine Tongue of the Mancos Shale and underlies coastal plain deposits of the Neslen Formation. Although this sandstone has been interpreted to be comprised of transgressive valley fills, recent detailed facies architecture studies of the underlying lower Sego Sandstone suggest these deposits may instead be regressive deposits of tideinfluenced deltas. This study maps facies associations, the geometry of lithic bodies, and key stratigraphic surfaces in order to define the architecture of a 12 km long cross section of the upper Sego Sandstone. This broadly depositional dip-oriented cross section exposes a vertical stack of three sandy intervals, truncated by a high-relief erosion surface, and capped by a shell rich lag. Sandy intervals are interpreted be an assemblage of forward stepping successions of tide-influenced delta lobes. Interval 1, dominantly highly marine bioturbated sandstones which thin landward over kilometers, is cut locally by an erosion surface overlain by tidal bed sets. It is capped by a localized transgressive shell lag and then a thin continuous marine shale. Intervals 2 and 3 are composed of stacked tidal bar deposits that successively coarsen upward and thicken basinward. Interval 2 is overlain by thin marine shales, whereas interval 3 is capped by a pronounced oyster shell lag ravinement surface. A high-relief erosion surface that extends from the top of the upper Sego sandstone down into the Anchor Mine Tongue Shale, is overlain by coarser-grained amalgamated fluvial channel deposits and is interpreted to be a incised valley fill. Erosion surfaces at the base of sandy intervals, thicken and decrease in marine bioturbation within successive intervals, and the valley cut into this succession reflects episodic forced regression of a deltaic shoreline.