Recognizing Avulsion Deposits in the Ancient Stratigraphical Record

Comparison of avulsion deposits in Palaeogene deposits from the Bighorn Basin, Wyoming and in the Saskatchewan River system of Canada shows common facies and facies architecture that has helped establish a more detailed model for avulsion deposits. Ancient avulsion deposits can be recognized in other alluvial basins using the following criteria: 1 moderately well-developed to well-developed palaeosols or coals underlie and overlie the avulsion deposits; 2 sandstones deposited by the trunk channels locally overlie the avulsion deposits; 3 the avulsion deposits are lithologically heterogeneous and consist of fine-grained deposits that surround ribbon and thin sheet sandstones; 4 the fine-grained deposits show only weak pedogenic modification; 5 many of the ribbon sandstones show palaeoflow subparallel to or in the same direction as palaeoflow in the trunk channels; 6 ribbons generally have width thickness (W/T) ratios of less than 10; 7 some of the ribbon sandstones cluster at particular stratigraphical levels and are laterally connected by thin sandstone sheets to form ‘tiers’ that suggest that networks of crevasse channels once occupied particular areas of the floodplain; 8 the heterolithic avulsion deposits are extensive laterally and form a significant part of the stratigraphical succession, suggesting that they were deposited over large areas of the floodplain. The heterolithic avulsion deposits can be distinguished from ordinary crevasse-splay deposits developed off stable trunk channels on the basis of their spatial scale, the presence of truncating channel sandstones and attributes of the ribbon sandstones. The Palaeogene avulsion deposits are not only extensive laterally, they also dominate the stratigraphical sections, suggesting that they covered much larger areas than is typical for crevasse-splay deposits. Sandstone bodies deposited by the trunk channels are always underlain by the heterolithic deposits, which is consistent with an avulsion interpretation for the heterolithic intervals. The ribbon sandstones in the avulsion deposits have smaller W/T ratios than is characteristic for the feeder channels of crevasse splays, and their palaeoflow trends tend to parallel flow in the main channel rather than being oblique or perpendicular to it. Various features also distinguish avulsion deposits from flood-basin deposits. The ribbon sandstones are an integral part of the avulsion deposits. In contrast, fine-grained flood-basin deposits may be cut by some of the ribbon sandstones; however, those sandstones scour down from the overlying avulsion interval. More obvious is the presence of well-developed, cumulative palaeosols on the flood-basin deposits and only weakly developed, generally compound, palaeosols on the avulsion deposits. The avulsion deposits show only weak pedogenic development because accumulation rates were so rapid. In the field, the sharp contrast between the welldeveloped palaeosols and the weakly developed palaeosols highlights the presence of avulsion deposits. Differences in climatic conditions and proximity to sediment sources produced lithological and pedologic differences in the avulsion deposits and flood-basin deposits among the study areas. The overall thickness of avulsion intervals also varies among the modern and ancient examples. Thickness appears to be influenced by both levee height and local palaeotopography. The time between successive avulsion deposits was estimated to be of the order of 10