ABSTRACT

Extrachannel sediments of the lower Eocene Willwood Formation, which were subdivided into different pedofacies or developmental stages, exhibit two well-developed orders of pedofacies sequences, over which a grosser scale cyclicity is superposed. Simple sequences, with thicknesses measured in meters, are comprised of one or more soil profiles that are bounded above and below by pedogenically unmodified crevasse-splay deposits. On a larger scale, compound pedofacies sequences are tens of meters thick and consist of several simple sequences sandwiched between channel sandstones. Between these sandbodies, pedogenic maturity of the multistorey paleosols progressively increases and then decreases upwards in response to episodic avulsion. Formation of both compound and simple pedofacies seq ences was largely controlled by local patterns of deposition and erosion that produced vertical variability in both rate of sediment accumulation and parent material.

Because paleosol maturity also reflects the effects of extrabasinal factors, a pedofacies megasequence, which is hundreds of meters thick, developed in response to changes in tectonic activity through Willwood time. Comparison between Willwood deposits in the northern and central parts of the Bighorn Basin also reveals that areally differing sediment accumulation rates and thereby basin subsidence rates can be interpreted from large-scale pedofacies sequences. Consequently, paleosols are useful in determining changes in relative rates of sediment accumulation that are controlled by both intrabasinal and extrabasinal factors.