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The Paleocene Hanna Formation was deposited during tectonic events that led ultimately to the development of the Hanna and Carbon structural basins of south-central Wyoming. Early Paleocene uplift prior to deposition of the Hanna Formation resulted in a regional unconformity as observed in the Carbon basin, on the east side of the Hanna basin, and in the area south of these two basins. Subsequent movement of the thrust-fault system on the north side of the present Hanna basin resulted in southward filling of this asymmetrical foreland basin by clastic wedges, which make up facies associations. The Hanna Formation deposited in the Carbon basin has the same facies associations that occur in the southern part of the Hanna basin.
Facies associations in the Hanna Formation of the Hanna basin suggest deposition in alluvial-fan and alluvial-plain environments. The alluvial-fan facies can be subdivided into proximal, medial, and distal subfacies. The proximal and upper-medial subfacies, which have been eroded away on the north side of the Hanna basin, probably consisted of medium to coarse gravels and muds deposited by debris flow and sheet floods. The lower-medial subfacies consists of tongues of conglomerate interbedded with mudstones and was probably deposited by sheet floods. This subfacies grades laterally southward into the distal conglomeratic sandstones and gray mudstones that were deposited by braided streams and sheet floods. In the Hanna basin, these distal-fan subfacies grade southward into an alluvial plain facies. In the area of the Carbon basin, all the fan facies are isolated in the Medicine Bow Mountains and associated mountain front, and only the alluvial-plain facies occur in the basin.
The alluvial-plain facies in the Hanna Formation can be divided into two subfacies on the basis of thickness and the occurrence of coal and carbonaceous shale. One subfacies consists of overbank and backwater-deposited gray shale and claystone and splay sandstone and siltstone interbedded with backswamp deposits of thin carbonaceous shales and thin coals. These deposits lie laterally from sandstone-filled channel systems. In the Hanna basin this subfacies is thickest near the base of the Hanna Formation. The other subfacies is similar in lithology but contains more backswamp deposits of carbonaceous shale and coal beds. Stacked channel sandstones are also more common in this subfacies. This subfacies is thickest in the central part of the Hanna basin, and makes up most of the alluvial plain strata in the Carbon basin.
Analysis of the alluvial-plain sequences in Hanna basin is hampered by poor outcrops and a lack of subsurface correlation. Complete analysis of the coal-bearing subfacies has been possible in the Carbon basin where a stratigraphic framework has been established. From this stratigraphic framework, a sequential strata model has been constructed. Analysis of several coal-bed-bounded sequences in the Carbon basin confirms the sandstone domination of the sequences as shown by the sequential strata model. Also, mapped southeasterly trends of the sandstone bodies indicate the fluvial channel systems of the Hanna basin were continuous into the Carbon basin.
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