Geology Reference
In-Depth Information
produced by infaunal deposit feeders (e.g. worms and
crabs living on tidal flats) during feeding and burrow
excavation leading to vertical size sorting of the sedi-
ment (Meldahl 1987). This bedding is distinct from
bio-
genic graded bedding
(Rhoads and Stanley 1965) com-
monly formed in intertidal sediments by the feeding
activity of polychaete worms in that the transition from
the lower coarse-grained interval is sharp and not
graded.
fabrics (Ekdale et al. 1984; Bromley 1996; Donovan
1994; Fig. 5.4). These organisms occur in niches at dif-
ferent levels above and within the substrate. Tiering of
infauna is reflected by vertically changing ichnofos-
sils. Ichnofabrics are recorded by both discrete trace
fossils as well as complicated structures, e.g. bioturba-
tion fabrics. Burrowing is a prominent process in mod-
ern subtidal environments, often causing a lack of in-
ternal layering. Burrows and bioturbation provide in-
formation on conditions of life in sediments (Curran
1991) and, thus, about the depositional environment.
They are also valuable indications of synsedimentary
and diagenetic deformations (Gaillard and Jautee 1987).
Compaction can be directly measured from deformed,
originally cylindrical bioturbation tubes using the com-
paction law developed by Ricken (1987), see Sect. 7.5.1
and Pl. 49/3. Burrows are formed within soft uncon-
solidated sediments (muds, sands, firmgrounds) by the
activity of animals during feeding, resting or migra-
tion. The size of burrow structures ranges from milli-
meters to a few centimeters and up to decacentimeters.
The distinction between burrows and borings (biogenic
structures drilled into hard sediment or shells) is not
always easy to discern (Pl. 23/1). Roots molds and casts
(caliche, tidal flat, subtidal settings) can be mistaken
for burrows (Pl. 20/6). Most distinctive is the down-
ward branching pattern where there is a marked and
systematic decrease in tube diameter at junctions. Roots
may have carbonaceous linings and exhibit branching
out into clusters of root hairs.
Diagenetic controls
are revealed by parallel disso-
lution seams (Pl. 37/6), horse tail structures (Pl. 37/8),
stylolite sets (Pl. 37/3) and significant differences in
carbonate and clay contents. Stylobedding and stylo-
lamination are common features of shallow and deep
marine limestones (Sect. 7.5.2). Other criteria of di-
agenetically caused or at least enhanced bedding are
cemented layers (Pl. 50/6), stratified cementation, hard-
grounds (Sect. 5.2.4.1, Pl. 23) and selective compac-
tion.
Diagenetic bedding is a common feature in chalks
(Ekdale and Bromley 1988) and in argillaceous lime-
stone and marly sequences of open marine platforms
(Bathurst 1987). Diagenetic controls are of major im-
portance in models explaining the origin of marl-lime-
stone alternations in terms of repeating zones of diage-
netic carbonate dissolution and cementation (Ricken
1986). These processes considerably enhance primary
carbonate variations and can thereby generate rhyth-
mic bedding (Sect. 16.1.1.1).
Bioturbation, burrowing and bio-retexturing
The term
bioturbation
was proposed for 'all kinds
of displacement within sediments and soils produced
by the activity of organisms and plants' (Richter 1936).
The term designates the churning and stirring of sedi-
ments by organisms resulting in the destruction of sedi-
mentary structures (e.g. bedding). Because bioturba-
tion is often very loosely used, a new term has been
Significance of depositional and biogenic lamination
Depositional laminae, produced by variations in
sediment input and primary production, are character-
istic of hemipelagic sediments and occur in shallow
and deep-water settings. Sediment is supplied season-
ally and/or episodically. The preservation and compo-
sition of laminations of fine-grained carbonate rocks
are excellent proxies for paleoclimatic conditions, oxic
or anoxic bottom and pore waters, and ecologic con-
straints of benthic organisms. Thin-section image analy-
sis is a common method of identifying and quantifying
sedimentary laminae in the context of paleoclimatic
studies of both marine (particularly lagoonal carbon-
ates, Park and Fürsich 2001) and lacustrine sediments
(Francus 2001, Trichet et al. 2001).
5.1.4 Burrowing and Bioturbation Fabrics
Organisms that travel across, burrow into, rest upon
or ingest sediment create trace fossils exhibiting char-
acteristic shapes and patterns and well-defined ichno-
Fig. 5.4.
Distinct burrow in a mudstone
destroying the
dark organic-rich layers. Siliciclastic components enriched
within the burrow. Late Devonian: Menorca. Scale is 2 mm.
