Porosity and Permeability in Bioturbated Sediments - Trace Fossils as Indicators of Sedimentary Environments - page 819

4.2.2 Non-Destructive Methods of Spatial Analysis
Non-destructive methods can be used to resolve the three-dimensional nature
of sedimentary structures at the hand-sample scale. The most common of
these are X-ray and X-ray CT. X-rays provide only two-dimensional data.
CT scans are three dimensional. Neither X-ray nor CT scans are particularly
sensitive to slight variations in density. Nevertheless, CT scans have the
potential to yield high-resolution data that can be used to establish the
three-dimensional nature of biogenic fabrics ( Herringshaw et al., 2010;
Pierret et al., 2002 ).
Microtomography (Micro-CT) is beginning to be used for ichnological
applications (e.g., La Croix, 2010 ). It permits the resolution of micron- through
to millimeter-scale density heterogeneities in geological media and is appropri-
ate for the delineation of small to microscopic borings, burrows, and fecal
pellets. Importantly, the technique can resolve the pore distribution in a small
volume of sediment.
Unlike the aforementioned radiographic techniques, MRI does not measure
the density of a rock directly. Rather, it is sensitive to fluids imbibed into the
pore space of a rock. Therefore, MRI allows for three-dimensional mapping
of a magnetic resonance signal, providing a tool that geologists can use to
map the pore-space distribution in rocks (e.g., Gingras et al., 2002a,b ). MRI
can also be used to visualize the imbibition of porosity by gas or fluids, a true
advantage in permeability studies (e.g., Chen et al., 2003 ).
4.3 Mechanisms and Styles of Burrow Permeability
In modern sediments, bioturbating animals, including earthworms, crustaceans,
amphipods, polychaetes, and meiofauna, have been shown to enhance perme-
ability and porosity primarily by creating macropores within the sediment (e.g.,
Aller, 1983, 1994; Katrak et al., 2003; Lavoie et al., 2000; Meadows and Tait,
1989; Pedley, 1992; Pierret et al., 2002 ). Although this type of flow-media
enhancement can be a factor in the later developing of permeability fabrics, bur-
rows do not remain open and unfilled in their passage to the rock record, and
they are not preserved as macropores. In the rock record, the preserved patterns
of permeability and porosity are strongly influenced by the character of burrow-
filling sediment, the nature of burrow backfilling, compaction, and cementa-
tion. Recognizing this, Pemberton and Gingras (2005) classified biogenic flow
media on the basis of their sedimentological, ichnological, diagenetic, and strat-
igraphical contexts. Pemberton and Gingras (2005) proposed that biogenic flow
media could be separated into five classes ( Fig. 6 ), including (1) surface-
constrained discrete heterogeneities (SCD), (2) non-constrained discrete
heterogeneities (NCD), (3) weakly defined heterogeneities, (4) cryptic heteroge-
neities, and (5) diagenetic heterogeneities. In this context, “surface-constrained”
implies that the bioturbationdescends froma single surface and “non-constrained”
suggests that the burrows are distributed vertically and not associated with