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The Muddy Formation in the northern Powder River basin contains a stratigraphic paradox whereby the oldest basal sandstones appear to be the youngest and vice versa. Subsidence history is the underlying cause of the paradox. Relating depositional events as determined from physical and paleontologic data to the subsidence leads to a simple and consistent depositional history.
On lithology, the Muddy is subdivided into lower and upper units. The lower unit consists of thick basal sandstones and thin contemporaneous siltstones and shales that were deposited by a single southeast-to-northwest regression. These thick sandstones are important Muddy reservoirs and consist of coastal barriers (Bell Creek and Rozet fields) and distributary or estuarine channels (Recluse field).
When the lower Muddy regression had proceeded to a point northwest of the currently producing area, the character of the shoreline changed from a high-energy sandy shoreline to a low-energy muddy and tidal-flat shoreline. The shoreline remained northwest of the productive area throughout the rest of Muddy deposition,
although subsidence was greater there than in the southeast. This differential subsidence caused the southeast thinning of the nonmarine upper Muddy sediments. Muddy deposition was terminated by the rapid eastward advance of the Shell Creek or lower Mowry sea. This transgression reworked the uppermost nonmarine Muddy sediments, producing a thin, widespread sandy zone that constitutes the principal reservoir at Hilight field.
Because the lower Muddy was deposited during a southeast to northwest regression, the sandstones are progressively younger toward the northwest. However, the overlying nonmarine upper Muddy thickens northwestward, causing the underlying regressive sandstone to appear to be stratigraphically lower and, hence, older in that direction. This stratigraphic paradox has caused much confusion and difficulty in resolving Muddy stratigraphy.
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