Porosity and Permeability in Bioturbated Sediments - Trace Fossils as Indicators of Sedimentary Environments - page 821

a particular geological level. Discrete heterogeneities are sharply demarcated
from the matrix by their texture, whereas cryptic heterogeneities are not. Dia-
genetic heterogeneities represent recrystallized cement accompanying burrow
fabrics. Below, the permeability classifications are expanded upon case stud-
ies from the literature and are summarized with each example.

4.3.1 Surface-Constrained Discrete Heterogeneities
This class of biogenically enhanced flow media is characteristic of trace-fossil
assemblagesassociatedwiththe Glossifungites Ichnofacies.The Glossifungites Ich-
nofaciescomprisesunlinedburrows thatdescendfromasinglesurface.Theburrows
commonly exceed 10 cm in diameter and show little or no evidence of post-
depositional compaction.Thesecharacteristicsareassociatedwith thedevelopment
of Glossifungites assemblages, which dominantly result from animals burrowing
into firm sediment (i.e., compacted substrates). The most common expression of
the Glossifungites Ichnofacies is sand-filled burrows within a mud matrix (e.g.,
Gingras et al., 2001; MacEachern et al., 1992; Pemberton and Frey, 1985 ).
Trace-fossil assemblages that constitute the Glossifungites Ichnofacies typ-
ically penetrate as little as a few centimeters and as much as approximately
70 cm below their surface of origin. Some commonly associated ichnogenera
include isolated burrows such as Skolithos , Diplocraterion, and clavate bivalve
burrows (e.g., Gingras et al., 2001 ), and burrows that potentially interpenetrate
one another such as Thalassinoides (e.g., Pemberton and Frey, 1985 ) and
Zoophycos ( MacEachern and Burton, 2000 ). Deeply emplaced burrows that
interpenetrate, such as Thalassinoides -dominated Glossifungites assemblages,
can constitute flow units that are, broadly speaking, isotropic in character.
Owing to the presence of sand-filled burrows within the muddy matrix, SCD
normally represent dual-permeability flow media ( Fig. 5 ). The proportion of
burrowing ranges from 10% to 80% of the volume. Notably, horizontal connec-
tivity is limited until burrow volumes approach 20-30% (depending on the
burrow architecture; La Croix et al., 2012 ).
Glossifungites -associated SCD are typically limited in their distribution.
Due to their association with a single surface, the thickness of the permeability-
enhanced zone is typically 1 m (e.g., Gingras et al., 2001; MacEachern et al.,

1992 ). Additionally, the colonization of firmgrounds depends upon a number of
factors, so their stratigraphic expression is discontinuous (cf. Gingras et al.,
2001; Pemberton and Gingras, 2005 ); SCD are generally limited in scale to
some hundreds of square meters, with a maximum areal extent of about a square
kilometer ( Pemberton and Gingras, 2005 ).
Surface-constrained permeability enhancement is probably not limited
to Glossifungites -Ichnofacies-demarcated surfaces. For example, short-term
colonization windows in deep-water environments (e.g., a turbidite fan) may
promote the emplacement of sand-filled burrows along singular bedding planes.
The porosity and permeability distributions in such occurrences are likely
similar, but no case studies have as yet been conducted on such media.

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