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Paleosols formed on overbank deposits of the lower Eocene Willwood Formation in the Bighorn Basin, Wyoming, can be differentiated on the basis of pedogenic maturity. The least mature soils generally formed close to the channel margin where sediment accumulation rates were rapid, whereas the most mature soils developed on distal flood plains where accumulation rates were significantly slower. The term pedofacies is introduced to delimit laterally contiguous bodies of sedimentary rock that differ in their ancient soil attributes as a result of distance from areas of relatively rapid sedimentation.

Vertical successions of Willwood overbank deposits show three orders of pedofacies sequences. Simple sequences consist of one or more paleosols bounded below and above by crevasse-splay deposits. They were generated by slow and sporadic alluviation and soil modification that were periodically interrupted by more rapid crevasse-splay deposition. On a larger scale, compound pedofacies sequences are composed of multi-story paleosols sandwiched between channel sandstones. Pedogenic maturity of the paleosols progressively increases and then decreases upward in response to episodic channel avulsion. Development of compound and simple pedofacies sequences was largely controlled by local patterns of deposition that produced vertical variability in the rate of sediment accumulation.

Superimposed on these smaller-scale cycles are pedofacies megasequences that are hundreds of meters thick. Megasequences show a distinct upward change in the overall maturity of their constituent compound sequences. They provide evidence for changes in the rate of sediment accumulation produced by allocyclic processes including varied tectonic activity. Comparison between Willwood deposits in the northern and central parts of the Bighorn Basin reveals that areally differing sediment accumulation rates, and thereby basin subsidence rates, can also be interpreted from large-scale pedofacies sequences.

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