THE SHANNON SHELF-RIDGE SANDSTONE COMPLEX, SALT CREEK ANTICLINE AREA, POWDER RIVER BASIN, WYOMING (1984)

Two vertically stacked shelf ridge bar complexes in the Shannon Sandstone member of the Cody Shale designated upper and lowersandstones crop out in the Salt Creek anticline of the Powder River Basin Wyoming The shelf ridge complexes are composed primarily of moderately to highly glauconitic fine to medium grained lithic sandstone and attain thicknesses of over 70 feet The shelf ridge complexes were deposited at least 70 miles from shore at middle to inner shelf depths by south to southwest flowing shore parallel currents intensified periodically and frequently by storms Ridges in each sequence trend north south slightly oblique to current flow A possible source of sediments for the shelf ridges was the Eagle Sandstone shoreline and deltaic deposits of southern Montana 200 miles to the northwest Eleven facies were defined in outcrop on thebasis of physical and biologic sedimentary structures and lithology Vertical and lateral changes in facies are relatively abrupt where observed in closely spaced outcrop sections and in general facies are stacked in coarsening upward sequences with Central Bar Facies commonly immediately overlying Interbar Sandstone Facies Porous and permeable potential reservoir facies include Central Bar Facies a clean cross bedded sandstone Bar Margin Facies Type I a highly glauconitic cross bedded sandstone containing abundant shale and limonite after siderite rip up clasts and lenses and Bar Margin Facies Type 2 across bedded to rippled sandstone These facies were formed by sediment transported and deposited in the form of medium to large scale planar tangential troughs and sand waves on and acrossthetops of ridges by moderate to high energy shelf currents Storm flow deposited Central Bar planar laminated Facies are rare Finer grained non to marginal reservoir quality facies include Interbar Sandstone Facies rippled to ripple form bedded sandstone Bioturbated Shelf Sandstone Facies Bioturbated Shelf Siltstone Facies Interbar Facies interlaminated rippled sandstone and shale Shelf Sandstone and Shelf Siltstone Facies sub horizontally laminated sandstone and siltstone Interbar Sandstone Facies were most commonly deposited lateral to the higher energy portions of the ridges as well as near the base of the shelf ridges during their initial development The two bioturbated facies most commonly occur near the base of the ridge complex and between the two vertically stacked ridge complex sequences and probably represent periods of slow deposition The Shelf Silty Shale Facies is actually a facies of the Cody Shale of which the Shannon Sandstone is a member The most common vertical sequence of sandstone facies is one in which a coarsening upward sequence is formed where Central Bar Facies overlie lnterbar Sandstone Facies this contrasts with thesequence observed at Hartzog Draw Field 25 miles to the northeast where themost common coarsening upward sequence from bottom to top is Interbar Facies Bar Margin Facies Type I and Central Bar Facies Relatively abrupt lateral changes in facies are observed in surface cross sections spaced from one fourth to one mile apart Thickness changes but not facies changes are readily observable on subsurface cross sections constructed using SP resistivity logs The association of two vertically staeked shelf ridge complexes at Salt Creek is atypical compared to other Shannon sequences inthe Powder River Basin in several respects The lowersandstone is correlative withproductive Shannon sandstones in many of the Powder River Basin fields e g Hartzog Draw the upper sandstone sequence is only locally developed in the area of the Salt Creek anticline Also the spacing between ridges and the length to width ratios of the ridges at Salt Creek are much smaller than those in other areas These differences are particularly apparent in the upper sequence wherein the sandstone bodies appear to have oblate geometries very unlike the strongly linear geometries typical of most shelf sandstone ridges These differences are attributed to the presence during Shannon time of an actively growing paleo high in the area of the present day Salt Creek anticline which localized sand deposition and ridge formation Similar early structural growth and its influence on shelf sedimentation has been well documented for the Lost Soldier anticline area by Reynolds 1976 Baculites zones are commonly used for surface correlations in the study area and in Upper Cretaceous units throughout Wyoming Subsurface cross sections paralleling surface sections at distances from one half to three miles away corroborate the surface sandstone correlations in the Salt Creek area Bentonites above and below the Shannon form excellent subsurface correlation datums Foraminiferal data indicate that the shelf ridge complexes were deposited in water depths ranging from themiddle shelf to the outer part of the inner shelf A wide diversity in size orientation and type of burrow fill material suggests a relatively hospitable environment for burrowers in portions of the shelf ridge complexes Rare Teichichnus Thallasinoides Chondrites and plural curving tubes were identified Common Cretaceous shoreline traces such as Ophimorpha Asrerosoma and Rhi zocorallium were not observed The Bioturbated Shelf Sandstone Facies and the Bioturbated Shelf Siltstone Facies range from 75 to 95 burrowed Burrowing in the other facies averages from 5 to 27 Glauconite is present throughout and is most abundant in association with shale rip up clasts and limonite after siderite lenses and rip up clasts in BarBargin Facies Type I