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The Hanna basin of south-central Wyoming is one of several Cretaceous and Tertiary Rocky Mountain basins originally part of the broad foreland crustal flexure cratonward of the Sevier thrust belt and later subject to Laramide deformation. In the Campanian (Late Cretaceous), the Mesaverde Group filled the basin by two depositional sequences separated by an unconformity at the base of the locally incised Pine Ridge Sandstone. The unconformity was related to Laramide uplift north of the Hanna basin. Because of highly variable subsidence patterns, the unconformity can be recognized only in the easternmost parts of the Hanna basin, but in the west, an apparently conformable succession occurs. Whereas the lower sequence was sourced from the west, the upper sequence was sourced argely from the north because of the uplift. The two sequences therefore were genetically unrelated. In addition, the Mesaverde Group is underlain and overlain by extensive marine shale units (Steele and Lewis shales, respectively), which represent major paleogeographic reorganizations, perhaps reflecting increased subsidence rates related to major thrust events in the Sevier belt.
The Mesaverde Group in the Hanna basin may be divided into two allostratigraphic units. The lower is bounded below by the Steele Shale and above by the unconformity below the Pine Ridge Sandstone. The upper allostratigraphic unit is bounded below by the unconformity and above by the Lewis Shale. Application of sequence stratigraphy to the basin-fill succession is problematical because of input from several sediment sources of varying direction, superimposed subsidence patterns from Laramide and Sevier tectonism, and the limited lateral expression of valley-fill sandstones resulting from tectonic uplift. Allostratigraphy may be more useful than established sequence stratigraphic models in areas influenced by local tectonism and where influence from several sediment sources cannot be ig ored. However, allostratigraphy, in contrast to sequence stratigraphy, has a more limited predictive potential.
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