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Sedimentology and Ichnology of Mixed Wave- and Storm-Dominated Deltaic Deposits: Examples from the Permian of Australia

Kerrie L. Bann1, Christopher R. Fielding2, James A. MacEachern3, and Stuart C. Tye4
1 Simon Fraser University, Calgary, AB
2 University of Nebraska-Lincoln, Lincoln, NE
3 Simon Fraser University, Burnaby, BC
4 Husky Energy, Calgary, AB

Differentiating strandplain shoreface deposits from those deposited within proximity to wave-dominated deltas is a formidable task. The lithological characteristics of wave- and storm-dominated delta front deposits are similar to those of wave-formed shorefaces in strandplain settings. Archetypal facies models are currently inadequate to describe and distinguish between such deposits. Deltaic conditions influence infaunal organisms in subtle but significant ways. Ichnological signatures in deltaic deposits clearly reflect innate differences in physico-chemical and paleoenvironmental conditions.

Early Permian successions in the Sydney/Bowen Basin, Australia, contain similar sedimentological characteristics to strandplain shoreface deposits. Ichnological differences, however, demonstrate that the successions were deposited under the influence of anomalous paleoenvironmental stresses consistent with riverine influence and deltaic conditions. Fair-weather beds closely resemble strandplain shoreface deposits, containing diverse trace fossil assemblages, comprising a mixture of Cruziana and Skolithos ichnofacies elements. Variations in ichnological signatures, in the form of sporadic bioturbation levels and reduced diversities, suggest intermittent environmental stresses. Tempestites contain characteristics that reflect river influx, such as soft-sediment deformation structures, mudstone drapes, unbioturbated carbonaceous mudstone interbeds with synaeresis cracks, abundant phytodetrital material, allochthonous wood, large logs, and sandstone beds with stressed suites attributable to the Cruziana Ichnofacies, where ordinarily suites representative of the Skolithos Ichnofacies would be expected.

Such characteristics reflect ambient wave shoaling during fair-weather, punctuated by event-style deposition associated with increased river input during storms. The persistent temporal and genetic association of river discharge and storm activity demonstrates the proximity of distributary systems and highlights wave- and storm-dominated deltaic conditions rather than purely strandplain settings.