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The Role of a Diversity Index (DI) in Ichnological Analysis

Dashtgard, Shahin E.1; MacEachern, James A.1
1 Earth Sciences, Simon Fraser University, Burnaby, BC, Canada.

Ichnological analysis historically has been based upon empirical observations. Methodologies for semi-quantifying ichnological data has permitted comparison of independently generated datasets, and has minimized researcher subjectivity. This is the case for the bioturbation index (BI), which has been used effectively since its inception. Unfortunately, the bioturbation index is commonly treated as a proxy for marine influence rather than as an indicator of the balance between faunal abundance and deposition rate. Ichnogenera diversity is a superior assessment of the degree of marine influence. The development of a diversity index (DI) will enable the ready assessment of marine influence in facies. Further, comparisons of DI with BI may be used to quantify the ichnological characteristics of marine subenvironments.

At the most basic level, absolute numbers of identified ichnogenera can be plotted. This method would be limited by a worker’s ability to identify ichnogenera. Further, assessing total numbers of ichnogenera fails to address the diversity of behaviour, which is essential to ichnological interpretation. A diversity index defined by “grades” or ranges, as has proven successful in the BI framework, would reduce variability between workers. The DI grades would also be more effective if they closely reflect behavioural diversity. Weaknesses in assessing such ethological diversity, though not insurmountable, lie in its greater subjectivity, behavioural overlap in ichnogenera, and discerning the behaviours of greatest significance. The integration of BI values with DI values promises to further reduce interpretative bias between ichnological workers, while enhancing interpretation of ichnological datasets and its integration with sedimentary facies characteristics.

 

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009