Semi-Quantitative Analysis of Bioturbation – Implications for Core Studies
Modeling of simple burrow fabrics shows that it is problematic to estimate the volume of media that is burrowed based upon a cross-sectional view of core. Therefore, expressing bioturbation as a percentage may be invalid as the visible burrowed area can differ significantly from the volume occupied by the same burrows (both those visible and those not cross-cut by the slabbed core). Four types of burrows were modeled based on their idealized geometries Planolites, Skolithos, Phycosiphon and Zoophycos. In addition, idealized burrow suites with varying proportions of each of the four burrow types were flow modeled to gain an understanding of burrow interconnectedness.
Where horizontal burrows are dominant, the cross-sectional assessment of burrow area (i.e. % area burrowed) closely approximates the percent volume occupied in the third dimension. Counter intuitively, when a suite of dominantly vertical burrows is considered, 2-D views greatly over estimate the burrow volume (approximately 2 times). Thus, caution should be used when applying the Bioturbation Index or Ichnofabric Index to characterize bioturbate texture, especially at higher intensities. Particularly for permeability studies, we recommend the use of a modified bioturbation index, which presents the volume occupied by burrows from a 2-D view.