Geology Reference
In-Depth Information
sistency and stability of soft and hard substrates have
been designated by the terms soupground, softground,
looseground, firmground, hardground, rockground
(rocky sea bottom), and shellground (Ekdale 1985;
Goldring 1995). Softground refers to a muddy sediment,
looseground to a silt or sand grade sediment, the first
type with indefinite burrows with irregular outlines, the
second type with well-defined burrows. The last three
names describe hard bottoms characterized by en-
crusting and boring biota, and frequently a mineralized
crust.
Firmgrounds
are stiff carbonate substrates in which
the sediment has begun to consolidate through initial
compaction, physical cohesiveness of fine particles,
biological agglutinants (e.g. biofilms, organic mucus),
or from early cementation. Firmgrounds may be initial
stages of hardgrounds; they require some effort to be-
come eroded, but can be ripped up by storms resulting
in the formation of lithoclastic limestones.
Hardgrounds
are formed at times of extremely slow
or intermittent sedimentation by lithification of the non-
depositional surface prior to the deposition of the over-
lying bed. The hard surface requires substantial me-
chanical effort to break down and can be destroyed only
by boring or by chemical processes (Sect. 9.3; Pl. 52).
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Shell bed bottoms:
Autochthonous (biological) or
allochthonous (physical) concentrations of the skeletons
of shelly organisms. Hydraulic accumulations may be
enhanced by storm events, tidal currents and turbidity
currents. Common in inter-supratidal and shallow sub-
tidal environments. Known since the Cambrian.
•
Carbonate sand to gravel bottoms:
Mobile and un-
stable substrates, formed by bioclastic and non-bioclas-
tic carbonate grains. Redistribution of the grains by cur-
rents and waves and winnowing of the fines are com-
mon. Burrowing deposit feeders as well as vagile epi-
bionts may be abundant. Oolitic bottoms are known
since the Precambrian, bioclastic sand bottoms since
the Cambrian.
•
Coral-sponge-bryozoan-algae-dominated sub-
strates:
Muddy to sandy carbonate substrates in quiet
waters, covered by basal skeletons of 'coralline' organ-
isms, most of which are clones of the first individual
resulting from initial larval settlement. Platy, hemi-
spherical and cylindrical growth forms act in protect-
ing and baffling sediment. The skeletons form hard sub-
strates for epibionts and cryptic organisms. Known
since the Ordovician.
•
Hard bottoms
are substrates that are firm enough to
become encrusted and bored and that are resistant to
normal erosion (include firm grounds, hardgrounds, and
conglomeratic to skeletal and rocky grounds). Sedimen-
tation rates are low to very low. Known since the Cam-
brian.
•
Rocky bottoms:
Hard substrates formed by rocks
exposed at the sea floor, large boulders and bedrock
surfaces; predominantly in shoreline environments. The
specialized biota is characterized by abundant borers
and encrusters. Known since the Cambrian but more
records known since the Late Mesozoic.
12.1.3.1 Carbonate Substrate Types and
OrganismSediment Interactions
Copper (1993) differentiated seven types of common
soft and hard bottoms.
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Muddy soft bottoms:
Formed by sedimentation of
microorganisms or breakdown of larger skeletons. Bot-
tom dwellers are predominantly epibenthic deposit feed-
ers because of high mud fractions associated with or-
ganic matter. Species diversity is often low. Fine lami-
nation may be common. Muddy soft bottoms often
occur at the base of sedimentary cycles, in protected
quiet-water environments, in lagoons, and in deep-water
environments. Known since the Cambrian.
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Mangrove-sea grass muddy bottoms:
(a) Man-
groves: Fine-grained muds, formed in low-energy in-
tertidal-supratidal tropical or temperate coastal envi-
ronments by baffling of sediment, and deposition of
epibionts and organic matter (Plaziat 1995). The man-
grove ecosystem is also called 'mangal'. Known since
the Late Cretaceous. (b) Sea grass: Fine-grained muds,
formed by baffling, and deposition of epibionts. Car-
bonate grains are stabilized by sea grasses. The sea grass
ecosystem dates back to the Late Eocene.
Interactions between organisms and carbonate substrate
Benthic organisms require the organic matter within
substrates as food resources and the sediment as larval
settlement sites, shelter and for anchoring and fixation.
Suspension-feeders (e.g. many bivalves, sponges, cor-
als, bryozoans, brachiopods, crinoids) favor firm and
hard or coarse-grained substrates whereas deposit feed-
ers (e.g. worms, echinids, some bivalves, many gastro-
pods) prefer silty and muddy bottoms, probably because
of the higher organic content of muds. Organisms in-
fluence substrates through bioturbation and by provid-
ing skeletons, soft organic tissues and fecal pellets.
Organisms may have a strong impact on (1) grain size
and grain textures (by digestive activity and burrow-
ing), (2) grain composition (by providing different types
of bioclasts and by selective feeding), (3) sedimentary
structures (by destroying bedding through bioturbation;
