Shannon Sandstone of the Powder River Basin: Orthodoxy and Revisionism in Stratigraphic Thought
Donald J.P. Swift, Brian S. Parsons, 1999. "Shannon Sandstone of the Powder River Basin: Orthodoxy and Revisionism in Stratigraphic Thought", Isolated Shallow Marine Sand Bodies: Sequence Stratigraphic Analysis and Sedimentologic Interpretation, Katherine M. Bergman, John W. Snedden
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The Shannon Sandstone, consisting of cross-stratified, heterolithic sandstone and thin-bedded sandstone-shale alternations, is a sedimentary complex of ambiguous origin. It lies seemingly "encased in marine shale," far from its time-equivalent shoreline. It has been interpreted as a shelf sand ridge complex, as a detached lowstand wedge, and as a low-stand, deltaic valley fill. The Shannon Sandstone has experienced one of the higher rates of "paradigm shift" seen within the field of stratigraphy, as successive generations have tested it for successively popular depositional models.
Interpretation of the Shannon is best undertaken in the context of a three-fold family of Cretaceous marine sandstones. Class I Sandstones cap the progradational tongues of the Cretaceous cliff-forming sandstones. They are cross-stratified with shore-normal paleocurrent orientations, high in angular deviation, have clear tidal signatures, and are of transgressive, estuarine, valley-fill origin. Class II Sandstones exhibit high-angle to hummocky cross-stratification, are amalgamated to discretely bedded, and in the latter case exhibit shore-normal, sole markings with low angular deviations. Class III Sandstones exhibit high-angle cross-stratification, low angular deviation, along-shore (southerly) paleocurrent orientations, and are enriched in intrabasinal components (phosphatic, sideritic, and glauconitic nodules). They are of transgressive, open-marine origin, and in at least some cases were deposited as sand ridges. While the paleontological evidence appears to be contradictory, the highly glauconitic phases of the Shannon provide an insurmountable problem for the lowstand shoreface, and lowstand deltaic models. Both the high glauconite content and the along-shore paleocurrent pattern point to a shelfal origin for the Class III Sandstones as a group, and for the Shannon in particular.
A generic depositional model for the Class III Shannon Sandstone can be salvaged from elements common to the diverse explanations that have been offered, and by abandoning contradictory elements. In this synthesis, the Shannon Sandstone is considered to exhibit lithologies, textures, and structures indicative of transgressive deposition in a shallow open marine setting; both estuarine and outer shelf settings are rejected. The pattern of distribution is an inherited one; it is the consequence of in situ reworking of sediment delivered to the site of deposition by other means. However, the result is not a simple, band-like "stranded lowstand shoreline." In the subsurface the Shannon lenses coalesce into a broad sheet of backstepping sand bodies.