A Field-Based Test of Spatial Recurrence of Ichnofabrics and Ichnofacies in Shallow-Marine Successions

James A. MacEachern¹, Kerrie L. Bann², Murray K. Gingras³, Shahin E. Dashtgard¹, George Pemberton³, Cindy D. Hansen¹, Lynn T. Dafoe³, John Lerette⁴, and Nadine J. Pearson^5
1Department of Earth Sciences, Simon Fraser University, Burnaby, BC, Canada
²Ichnofacies Analysis Inc., Calgary, AB, Canada
³Ichnology Research Group, Dept. Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB, Canada
⁴Brazalta Resources Corp., Calgary, AB, Canada
⁵Department of Geological Sciences, University of Saskatchewan, Saskatoon, SK, Canada

Extensive analysis of approximately 35 sections was undertaken from the Early Permian Wasp Head, Pebbley Beach and Snapper Point formations, Sydney Basin, Australia. Facies successions comprise offshore-shoreface, prodelta-distal delta front, shelf-lower offshore and estuarine-embayment deposits. At each section, five pairs of researchers evaluated the succession from an ichnofabric and ichnofacies perspective. Ichnofabric analysis included assessment of Bioturbation Index, tiering relationships and construction of constituent diagrams, following published protocols. Ichnofacies analysis identified trace fossil suites, relative proportions of ichnogenera, Bioturbation Index, uniformity of burrowing and an ethological evaluation.

At each study site, two sections (per research team), typically less than 2 m apart, were measured in the same bedsets. No recurrence of ichnofabrics could be established between section stations, even within the same bed. Furthermore, the evaluations were time-consuming, averaging 2 hours/m. In the case of bioturbated shelfal silty mudstones this was particularly surprising, given the general uniformity of depositional conditions in such settings.

In contrast, ichnofacies analyses were time efficient (20 min/m), permitting full integration of ichnological features with sedimentological characteristics for the purpose of facies interpretation. Trace fossil suites and accompanying ethological evaluations were demonstrably recurrent across all sections at each locale, and for considerable distances along depositional strike.

The lack of spatial recurrence indicates that there is no basis for a depositional interpretation founded solely upon the ichnofabric dataset. Only by combining ichnofabrics into ethological groupings of suites, and assessing them in the context of the ichnofacies paradigm, is it possible to interpret the paleoenvironment.

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