Geology Reference
In-Depth Information
Destruction of original structures:
Stratification is
frequently devastated by deep burrowing (Enos and
Perkins 1977; Bromley 1990). Vertical bioturbation of
shelf sediments is largely responsible for the homog-
enization of the sediment and can cause considerable
stratigraphical contamination of skeletal grains (Sarn-
thein 1972), which should be considered in using mi-
crofossils seen in burrowed limestones as biostrati-
graphic markers. Bioturbation may speed up or slow
down the degradation of carbonate grains. A turnover
of sediment may cause a decrease in grain size through
breakage which in turn affects potential dissolution of
grains. Burrows and sediment mixing can bring previ-
ously buried grains to the surface or can preserve grains
in deep and sheltered burrows. Burrowing will strongly
influence the taphofacies (Brashaw and Scoffin 2001).
tical burrows are more frequent in intertidal areas. Anas-
tomosing burrows are more common in subtidal sedi-
ments. Deep-marine burrows differ from shallow-ma-
rine by ichnotaxa association and spatial distribution.
Sedimentation rates:
The type and abundance of bio-
turbation fabrics can be used as evidence of high or
low sedimentation rates. The ecological stratification
of trace fossils within the taphonomically active zones
(infaunal tiering) aids in explaining discontinuity
surfaces and lack of sedimentation (Monaco et al.
1996).
Impact of bioturbation and burrowing on diagen-
esis
: Burrowing and bioturbation create inhomogene-
ities in permeability and porosity of the sediment, con-
trol migration paths of fluids and gases (Libelo et al.
1994), and accelerate elemental cycles (e.g. of carbon,
sulphur: Berner and Westrich 1985).
Bioturbation has a strong impact on the nitrogen
cycle in marine sediments and the distribution of or-
ganic matter contaminations. The physical network of
burrows within the sediment and the increased rough-
ness of the surface caused by burrow openings are likely
to be responsible for increased transfer of dissolved
and particulate material between the sediment and the
water column (Meadows and Meadows 1994).
Differences in permeability and porosity in turn in-
fluence the style and course of dolomitization. The finer
crystal size of burrow fills as compared with micritic
matrix may be one factor controlling the often observed
selective dolomitization of burrow fills (Zenger 1992,
Pl. 19/6). Bio-retexturing profoundly controls diage-
netic processes because it can selectively increase in-
dividual bed permeability where coarse burrow back-
filling is dominant. Conversely, locally it can inhibit
early sea floor cementation by admixing micritic sedi-
ment into grain support fabrics. Feeding, burrowing and
irrigation increase solute transport and solid phases re-
action rates, thus leading to rapid carbonate dissolu-
tion (Green et al. 1992). This may explain fluctuations
in the frequency of preserved micro- and macrofossils
in bioturbated limestones as compared with non-bio-
turbated carbonates.
Substrate conditions:
Burrows are important in rec-
ognizing marine substrate types (Sect.12.1.3). The con-
sistency of bioturbated sediment may be found in five
states (Goldring 1995): Soupground (incompetent, wa-
ter saturated sediment), softground (muddy sediment
with substantially compacted burrows, see Pl. 19/6),
looseground (silt or sand grade sediment with grain-
or mucus-stabilized walls of burrows, Pl. 19/1, Pl. 92/
10), firmground (stiff substrate with minor burrow com-
paction, Pl. 19/4, Pl. 147/1) as well as hardground,
rockground and shellground (best recognized by en-
crusting and boring biota and mineralized crusts, bor-
ings cut evenly across grains and matrix alike).
Oxygenation:
Burrowing is used to evaluate the oxy-
gen distribution and chemical conditions within the
sediment and at the sediment-water interface (Sect.
12.1.5). The degree and intensity of bioturbation is an
excellent indicator of level-bottom oxygenation and for
the estimation of ancient oxygen-deficient environ-
ments.
Bioturbation tends to increase the subduction of or-
ganic matter below the sediment-water interface. In-
faunal benthic animals have critical requirements for
oxygen in their respiratory processes. A progression of
trace fossils with specific life habits exists ranging from
reducing settings to well-oxygenated substrates.
Particle flux:
Bioturbation is an indication for the
flux of organic matter and nutrients near the sea bot-
tom, because the intensity of biological mixing in ma-
rine sediments is related to the flux of organic matter
to the sea bottom.
Economic significance:
Bioturbation influences the
distribution of permeability and porosity in limestones
(Dawson 1981). Burrowing acts on reservoir proper-
ties, e.g. as in Early Tertiary limestones offshore Cen-
tral Tunisia (Hauptmann 2000). The tight network of
branching burrows resulting from the activity of echi-
noids, crabs and bivalves is responsible for the com-
plex fractal distribution patterns of dissolution versus
Depositional setting:
Bioturbation and burrowing
are common in inner and mid-shelf environments. Ver-
