A relationship between alluvial backswamps and avulsion cycles: an examplefrom the Willwood Formation of the Bighorn Basin, Wyoming

Stratigraphically limited intervals from the Lower Eocene Willwood Formatio n contain laterally extensive carbonaceous shales and ribbon sandstone netw orks associated with channel avulsion. We present data from one such interv al that documents the avulsion sequence. Vertical sections measured along t he outcrop of this interval are similar and comprise a basal carbonaceous s hale overlain by fine-grained deposits on which weakly developed, hydromorp hic paleosols formed. The paleosols enclose and are locally incised by ribb on sandstones, some of which cut down to and partly through the carbonaceou s shale. The ribbons have width/thickness ratios between 3 and 13. Some rib bons cluster at a particular stratigraphic level, which, together with pale ocurrent trends, suggests that they formed channel networks. Sections are c apped by yellow-brown paleosols showing moderate pedogenic development. We suggest that the carbonaceous shales developed in low-lying topogeneous swa mps in distal portions of the floodplain far from the trunk channel. Such a location set limits on the sediment that they received. The mudrocks with weakly developed paleosols and associated ribbon sandstones are interpreted as crevasse-splay complexes resulting from avulsion of the trunk river. Th e ribbon sandstones represent ancient feeder channels of the avulsion compl ex. The rapid influx of avulsion deposits appears to have been crucial to p reserving the organic material, and this study reveals an important and as yet uncharacterized link between trunk channel processes and the accumulati on of organic rich deposits in distal alluvial swamps. Similar deposits are found in other stratigraphic units in the Rocky Mountain region, and the d evelopment of these and other organic-rich deposits should be reassessed in terms of channel avulsion. (C) 2001 Elsevier Science B.V. All rights reser ved.