Neoichnological Evidence for Differentiating Active and Abandoned Delta-Plain Deposits, Fraser River Delta, British Columbia, Canada
Shahin E. Dashtgard and James A. MacEachern
Earth Sciences, Simon Fraser University, Burnaby, BC, Canada
The neoichnological character of sediments in the lower-delta-plain of the Fraser River Delta, British Columbia, Canada were evaluated to identify the differences between active and abandoned parts of the system. Variations in the neoichnological expression of these two depositional settings are mainly a consequence of organism responses to various physico-chemical stresses. The main stress experienced by fauna occupying the abandoned portion of the lower-delta-plain is subaerial exposure. By contrast, organisms must contend with exposure combined with regular salinity fluctuations, heightened water turbidity, and elevated deposition rates. The combination of these stresses on the active lower-delta-plain favors sediment colonization by trophic generalists, who can adjust quickly to changing depositional conditions. Such generalists tend to construct simple burrow forms such as horizontal and vertical, simple to branching tunnels and shafts. Burrow densities also tend to be reduced, in response to higher deposition rates.
The reduced stresses experienced by infauna colonizing the abandoned lower-delta-plain permits employment of more specialized feeding strategies, reflected by more complex burrow morphologies. Complex morphologies include spiral burrows, U-tubes, and increased densities of three-dimensional burrow networks. Specialized feeding likewise favors numerous ethologic strategies, leading to elevated ichnologic diversity.
Based on this research, it is apparent that sedimentologically similar delta-plain subenvironments can be reliably differentiated based on their ichnological character. In the subsurface, evidence of similar variability in the ichnological character of deltaic facies may be used to identify active versus inactive delta lobes.
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