Geology Reference
In-Depth Information
Animals involved in burrowing include a wealth of
organisms living for a lifetime in one place or sporadi-
cally searching for nutrients and protection (e.g. worms,
echinoids, bivalves, arthropods). Burial velocity appears
to be a major control to organisms contributing to bio-
turbation fabrics (Boudreau 1994).
of bioturbation involve semiquantitative categories.
These categories are divisions between two extremes:
0% bioturbation (no evidence of biogenic mixing of
sediments) and 100% bioturbation (complete biogenic
mixing of sediments). The categories between these end
members in a continuous spectrum of bioturbation are
defined by the ranges of biogenic disturbance, e.g. 1-
10% bioturbation, 10-40% bioturbation, and so on.
Names have been given to such categories reflecting
some ordinal scale (e.g. ichnofabric index 2, ichnofab-
ric index 3) or general descriptions that express the rela-
tive progression of bioturbation (e.g. very slightly bio-
turbated, slightly bioturbated).
Francus (2001) described a method of quantifying
bioturbation based on a thin-section image-analysis
technique. The method is consistent with semiquan-
titative methods, but provides higher solutions of bio-
logical processes.
Diagnostic criteria of bioturbation fabrics in limestones
Small- to medium-scaled burrows diameter ranges
from <1 mm (micro-burrows, Das and Rao 1992) to
several centimeters. They are often backfilled with pel-
lets (Pl. 117/2), are lined (Pl. 92/10, have internal sprites,
are often branched or interconnected (Pl. 25/4), and may
have distinct and consistent geometrical patterns and
orientations. Bioturbation is indicated by mottled fab-
rics (see Pl. 19/6, Pl. 139/1), inhomogeneous micrite
texture (Pl. 135/1), variously colored burrow structures
(Pl. 137/6), whorled alignment of shell material (Pl.
19/1, Pl. 23/1) and differences in grain sizes of burrow
infillings as compared with the non-burrowed parts of
the limestone (Pl. 19/4). Small- and large-scale bur-
rows may be infilled by differently textured sediment
(e.g. wackestone textures within packstone, grainstone
within mudstone).
Bioturbated and non-bioturbated parts within the
same limestone bed can exhibit contrasting textures
which are caused by the homogenization and transfor-
mation of former grainstones by bioturbation into wack-
estones or packstones, and the preservation of the grain-
stone texture outside the burrowed areas. Small bur-
rows in grainstones often exhibit organic and aggluti-
nated linings. Linings are necessary in order to ensure
efficient water and oxygen circulation for the organ-
isms.
Secular variations of Phanerozoic bioturbation patterns
Recent work demonstrates that late Neoproterozoic
sea floors were characterized by well developed mi-
crobial mats and poorly developed vertically-oriented
bioturbation, thus providing a fairly stable, relatively
low water content surface and sharp water-sediment
interfaces characterized by firmground substrates. Dur-
ing the Cambrian microbially bound sea floors became
increasingly scarce in shallow marine settings. This is
largely due the evolution of burrowing organisms along
with an increasing vertically oriented and bedding dis-
ruptive component as a result of bioturbation (McIlroy
and Logan 1999). The Phanerozoic-style sea floor is
characterized by a blurry water-sediment interface,
greater water content, and lack of well-developed mi-
crobial mats (Droser and Bottjer 1993; Bottjer et al.
2000).
Description and quantitative classification of bioturba-
tion fabrics
Important criteria for differentiating bioturbation
types are the spatial arrangement and geometry of bur-
rows, the estimated or measured degree of bioturba-
tion (low, medium, high), texture of burrows (consid-
ering degree of reworking, breakage of bioclasts), fill-
ing (same or different material as compared with the
matrix), and differences in diagenetic criteria between
burrows and matrix (cement, dolomite).
Bioturbation structures are typified in terms of
spatial arrangement and geometry of burrows, burrow
density (using a semi-quantitative 'bioturbation index':
Taylor and Goldring 1993; Droser and Bottjer 1986),
burrow abundance and texture and filling of burrows.
These criteria should be studied in the field but also
can be applied to split core samples and large thin-
sections. Most classification schemes for the amount
Large changes in the extent and depth of bioturba-
tion took place in the Late Neoproterozoic and Early
Paleozoic, reflecting the increasing influence of bio-
turbation upon soft sediment fabrics through the Phan-
erozoic (Bottjer et al. 2000), accompanied by morpho-
logical adaptions of organisms living on soft substrates
(Thayer 1983). Deeper oxygenation of the sediment
with increased mixing via infaunal activity would also
have affected shallow burial diagenetic processes of
carbonates.
Significance of burrowed limestones
Bioturbation in carbonate sediments influences the
composition of the sediment, controls diagenetic pro-
cesses and is of importance in evaluating the reservoir
potential.
