parameters controlling the distributions of the tracemakers
tend to change progressively with increasing depth.
Because these bathymetrical relationships are potentially
very valuable for paleoenvironmental reconstruction, the
ichnofacies sequence has long been regarded as a relative
paleobathymeter. Today, it is well known that ichnofacies
are essential for the reconstruction of depositional settings,
but paleobathymetry constitutes only one aspect because
the distribution of tracemakers is controlled by a number of
interrelated ecological/sedimentological parameters, including the sedimentation rate, substrate grain size, salinity, oxygen level, turbidity, light, temperature, and water energy
(Pemberton et al., 1992). Because these parameters may
occur at specific water depths, it should not be surprising
to find nearshore assemblages in offshore sediments, and
vice versa. For example, the Skolithos ichnofacies, which
is typical of nearshore settings, may occur in offshore
tempestites or deep-marine turbidites, and the Cruziana
ichnofacies, which is typical of lower shoreface to offshore
deposits, may also be present in shallower settings, such as
intertidal flats on tide-influenced shorelines (Miller, 2007).
In recent decades, ichnologists have proposed many new
ichnofacies from continental and marine environments,
some of which are considered well founded, some are
retained as mutually equivalent, and still others are considered invalid categories (see Buatois and Mangano, 2011
for a detailed discussion). In a recent paper, Knaust and
Bromley (2012) recognized 14 formally defined ichnofacies
among those that conform to Seilacher’s paradigm. Five of
them encompass the marine to marginal-marine softground
substrates: Psilonichnus, Skolithos, Cruziana, Zoophycos,
and Nereites. Three are regarded as substrate-controlled
(omission) ichnofacies and are very useful for delineating
surfaces, with sequence-stratigraphic implications:
Glossifungites, Trypanites, and Teredolites. Six ichnofacies
encompass the continental realm: Scoyenia, Mermia,
Coprinisphaera,
Termitichnus,
Celliforma,
and
Octopodichnus–Entradichnus.
Ichnology and estuarine systems
According to Dalrymple et al. (1992), an estuary is “the
seaward portion of a drowned valley system which
receives sediments from both fluvial and marine sources
and which contains facies influenced by tide, wave, and
fluvial processes. The estuary is considered to extend from
the landward limit of the tidal facies at its head to the seaward limit of the coastal facies at its head.” All of these
environments are characterized by rapid perturbations
and typically by salinity changes, but also other ecological
controls may generate stressful conditions that strongly
affect the benthic biota. Ichnology has provided
a powerful tool with which to identify these depositional
settings by recognizing anomalous ichnofaunas (typical
of marginal-marine brackish conditions), which display
less variety and a lower abundance of forms than are
found in fully marine environments (Buatois and
Mangano, 2011).
Dalrymple et al. (1992) also classified estuaries into
two main groups: wave-dominated and tide-dominated
systems. In the former, there is a well-structured spatial
distribution of energy. Three main zones are recognized:
(1) the bay-head delta, a high-energy inner zone
dominated by river processes; (2) the central basin, characterized by the mixing of marine energy and fluvial currents; and (3) the estuary mouth, dominated by marine
processes.
Bay-head deltas are strongly stressful environments
with unbioturbated or sparsely bioturbated deposits showing very low ichnodiversity, which is dominated by the
dwelling structures of suspension feeders. In terms of
ichnofacies, this zone mainly contains the Skolithos
ichnofacies, followed by an impoverished Cruziana
ichnofacies. Central basin settings show a combination
of stress agents (brackish water, water turbidity, and oxygen depletion) associated with a low degree of bioturbation, although bioturbation may be moderate in some
beds. The ichnofauna reflects the dominance of
unspecialized deposit feeders and is characterized by the
depauperate Cruziana ichnofacies, with minor contributions from the Skolithos ichnofacies. Although the
estuary-mouth complex is highly variable, in terms of both
trace concentrations and depositional settings, the bioturbation intensity and ichnodiversity generally range from
moderate to intense (higher than in the previous zones),
reflecting near-normal marine salinities; mixed depauperate Cruziana and Skolithos ichnofacies are present. In
summary, trace fossil distributions along wave-dominated
estuaries are mainly controlled by the salinity gradient,
varying from the brackish waters of the inner zone to the
near-open-marine salinity of the outer estuary.
Tide-dominated estuaries are characterized by a less
pronounced distribution of energy along the estuarine valley because of the migration of intertidal runoff channels.
Nevertheless, the following zones are recognized: (1) the
upper estuary, a fluvio-estuarine transition zone characterized by freshwater conditions; (2) the middle estuary,
meandering to straight tidal channels, tidal flats, and salt
marshes; and (3) the lower estuary, comprising the outer
zone with elongate subtidal sandbars, channels, and tidal
flats (Figure 2).
Arthropods are the dominant tracemakers in the typical
freshwater/terrestrial biotas of upper estuaries, and their
activities are recorded in tidal rhythmites, which display
a mixture of the elements of continental depauperate
Scoyenia and Mermia ichnofacies. Farther towards the
sea, the middle estuary commonly has brackish-water
conditions. To different degrees in a number of
settings, tidal flat deposits are dissected by a network of
meandering tidal channels and creeks that migrate across
the intertidal zone, producing lateral accretions in
point bars (Dalrymple, 1992); the substrate-controlled
Glossifungites ichnofacies may occur, corresponding
to coplanar surfaces (incision during a sea-level fall
and subsequent transgressive erosion), whereas mixed
impoverished Cruziana and Skolithos ichnofacies record
BIOGENIC SEDIMENTARY STRUCTURES
79
tend to change progressively with increasing depth.
Because these bathymetrical relationships are potentially
very valuable for paleoenvironmental reconstruction, the
ichnofacies sequence has long been regarded as a relative
paleobathymeter. Today, it is well known that ichnofacies
are essential for the reconstruction of depositional settings,
but paleobathymetry constitutes only one aspect because
the distribution of tracemakers is controlled by a number of
interrelated ecological/sedimentological parameters, including the sedimentation rate, substrate grain size, salinity, oxygen level, turbidity, light, temperature, and water energy
(Pemberton et al., 1992). Because these parameters may
occur at specific water depths, it should not be surprising
to find nearshore assemblages in offshore sediments, and
vice versa. For example, the Skolithos ichnofacies, which
is typical of nearshore settings, may occur in offshore
tempestites or deep-marine turbidites, and the Cruziana
ichnofacies, which is typical of lower shoreface to offshore
deposits, may also be present in shallower settings, such as
intertidal flats on tide-influenced shorelines (Miller, 2007).
In recent decades, ichnologists have proposed many new
ichnofacies from continental and marine environments,
some of which are considered well founded, some are
retained as mutually equivalent, and still others are considered invalid categories (see Buatois and Mangano, 2011
for a detailed discussion). In a recent paper, Knaust and
Bromley (2012) recognized 14 formally defined ichnofacies
among those that conform to Seilacher’s paradigm. Five of
them encompass the marine to marginal-marine softground
substrates: Psilonichnus, Skolithos, Cruziana, Zoophycos,
and Nereites. Three are regarded as substrate-controlled
(omission) ichnofacies and are very useful for delineating
surfaces, with sequence-stratigraphic implications:
Glossifungites, Trypanites, and Teredolites. Six ichnofacies
encompass the continental realm: Scoyenia, Mermia,
Coprinisphaera,
Termitichnus,
Celliforma,
and
Octopodichnus–Entradichnus.
Ichnology and estuarine systems
According to Dalrymple et al. (1992), an estuary is “the
seaward portion of a drowned valley system which
receives sediments from both fluvial and marine sources
and which contains facies influenced by tide, wave, and
fluvial processes. The estuary is considered to extend from
the landward limit of the tidal facies at its head to the seaward limit of the coastal facies at its head.” All of these
environments are characterized by rapid perturbations
and typically by salinity changes, but also other ecological
controls may generate stressful conditions that strongly
affect the benthic biota. Ichnology has provided
a powerful tool with which to identify these depositional
settings by recognizing anomalous ichnofaunas (typical
of marginal-marine brackish conditions), which display
less variety and a lower abundance of forms than are
found in fully marine environments (Buatois and
Mangano, 2011).
Dalrymple et al. (1992) also classified estuaries into
two main groups: wave-dominated and tide-dominated
systems. In the former, there is a well-structured spatial
distribution of energy. Three main zones are recognized:
(1) the bay-head delta, a high-energy inner zone
dominated by river processes; (2) the central basin, characterized by the mixing of marine energy and fluvial currents; and (3) the estuary mouth, dominated by marine
processes.
Bay-head deltas are strongly stressful environments
with unbioturbated or sparsely bioturbated deposits showing very low ichnodiversity, which is dominated by the
dwelling structures of suspension feeders. In terms of
ichnofacies, this zone mainly contains the Skolithos
ichnofacies, followed by an impoverished Cruziana
ichnofacies. Central basin settings show a combination
of stress agents (brackish water, water turbidity, and oxygen depletion) associated with a low degree of bioturbation, although bioturbation may be moderate in some
beds. The ichnofauna reflects the dominance of
unspecialized deposit feeders and is characterized by the
depauperate Cruziana ichnofacies, with minor contributions from the Skolithos ichnofacies. Although the
estuary-mouth complex is highly variable, in terms of both
trace concentrations and depositional settings, the bioturbation intensity and ichnodiversity generally range from
moderate to intense (higher than in the previous zones),
reflecting near-normal marine salinities; mixed depauperate Cruziana and Skolithos ichnofacies are present. In
summary, trace fossil distributions along wave-dominated
estuaries are mainly controlled by the salinity gradient,
varying from the brackish waters of the inner zone to the
near-open-marine salinity of the outer estuary.
Tide-dominated estuaries are characterized by a less
pronounced distribution of energy along the estuarine valley because of the migration of intertidal runoff channels.
Nevertheless, the following zones are recognized: (1) the
upper estuary, a fluvio-estuarine transition zone characterized by freshwater conditions; (2) the middle estuary,
meandering to straight tidal channels, tidal flats, and salt
marshes; and (3) the lower estuary, comprising the outer
zone with elongate subtidal sandbars, channels, and tidal
flats (Figure 2).
Arthropods are the dominant tracemakers in the typical
freshwater/terrestrial biotas of upper estuaries, and their
activities are recorded in tidal rhythmites, which display
a mixture of the elements of continental depauperate
Scoyenia and Mermia ichnofacies. Farther towards the
sea, the middle estuary commonly has brackish-water
conditions. To different degrees in a number of
settings, tidal flat deposits are dissected by a network of
meandering tidal channels and creeks that migrate across
the intertidal zone, producing lateral accretions in
point bars (Dalrymple, 1992); the substrate-controlled
Glossifungites ichnofacies may occur, corresponding
to coplanar surfaces (incision during a sea-level fall
and subsequent transgressive erosion), whereas mixed
impoverished Cruziana and Skolithos ichnofacies record
BIOGENIC SEDIMENTARY STRUCTURES
79