CONTROLS ON HETEROLITHIC DEPOSIT THICKNESS BELOW AVULSION CHANNELS

In the ancient record, avulsion deposits are generally recognized by heterolithic overbank deposits that are genetically related to the avulsion and lie stratigraphically below or adjacent to the avulsion channel. Stratigraphic data from the Lance Fm., Fort Union Fm., Willwood Fm., and Ferris Fm. indicate that the thicknesses of heterolithic deposits can vary from ~ 0 to 4 multiples of the channel depth. Currently we know little about what controls this first-order variable of avulsion stratigraphy. Toward this end we have initiated a field and modeling study to determine the controls and variability on the thickness of heterolithic deposits that precede avulsion channels. Our hypothesis is that heterolithic avulsion deposit thickness is controlled by an avulsion parameter, A_p, which is a ratio between a characteristic overbank depositional flux during an avulsion and floodplain erosional flux. When A_p > 1 (overbank flux dominates) there will be a thick succession of overbank deposits, whereas when A_p < 1 (floodplain erosion dominates) the overbank deposits will be preferentially eroded by floodplain channels, if any are even deposited. To test this hypothesis we conducted a series of morphodynamic modeling experiments using Delft3D. We start with a single channel carved into floodplain and prescribe a flood hydrograph. At the upstream boundary we specify a sediment size distribution, which controls the overbank flux via the Rouse number, and on the floodplain we specify a slope and critical erosion threshold, which control the rate of floodplain erosion. Modeling results confirm our hypothesis that at high A_p an avulsion channel is preceded by thick, laterally persistent overbank deposits. At low A_p, overbank deposits are still produced, but because floodplain incision outpaces overbank deposition, the avulsion channel is captured by upstream migrating floodplain channels before it can prograde downstream via overbank deposition. The percentage of the avulsion channel that is underlain by heterolithic deposits depends on how close A_p is to 1 and how close the avulsion channel is to the parent channel.