Mudrock‐dominated fills formed in avulsion splay channels: examples from the Willwood Formation, Wyoming

Mudrock‐dominated channel fills are common features of the fluvial Palaeogene Willwood Formation of Wyoming. These fills are small, on average 30 m wide and 3 m deep, and they vary from simple plugs to more lithologically complex fills with internal scour surfaces. Some fills preserve plant material, and others show pedogenic modification. All fills are located within metre‐thick intervals of weakly pedogenically modified mudstones that surround ribbon sandstones. The intervals have previously been attributed to channel avulsion; the fine‐grained channel fills are interpreted as crevasse‐splay feeder channels within these avulsion deposits.Variation in fill type, particularly the presence of pedogenic features, appears to be related to floodplain drainage. Possible factors that influenced drainage were basin position of the fills and Palaeogene climate fluctuations, although the role of either is not unequivocal. In general, channel fills with sombre colours and preserved plant detritus, indicating poorly drained conditions, dominate locations that were proximal to the Palaeogene basin axes. Fills that contain root traces, red and yellow‐brown matrix colours and mottling, which indicate pedogenic modification and episodic subaerial exposure of the sediment, dominate locations that were more distal to the Palaeogene basin axes. Eocene climate fluctuations, particularly episodes of global warming, may have been a secondary control on the kind of fill that developed. Mudrock‐dominated channel fills have not been described previously from ancient avulsion deposits. Their common presence in the Willwood Formation suggests that they are a typical component of avulsion belts and that similar channel fills in other stratigraphic units should be considered with regard to channel avulsion. The variety of fills observed in the Willwood Formation indicates that avulsion belt channels can experience very different fill histories. The descriptive details of fills provided here should help in their recognition elsewhere. In addition, the nature of the channel fill provides important information on ancient drainage conditions in the avulsion belt. Pedogenically modified channel fills have been recognized only rarely from the stratigraphic record. Their presence throughout the Willwood Formation suggests that they should exist in other alluvial successions and may have been overlooked previously. They indicate that periods of sediment exposure alternated with episodic influxes of water and sediment in an active avulsion belt.