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In-Depth Information
soft/soupy consistencies). If possible, an ichnological
baseline should be drawn from the marine part of
the depositional system that was characterized by uni-
form salinity, fully marine waters, variable substrate
consistencies, generally reduced deposition rates, and
variable but abundant food resources such as suites of
the
Cruziana
Ichnofacies occurring in proximal off-
shore / inner shelf locales (MacEachern and Bann
2008
; MacvEachern et al.
2010
). If any of these
parameters are compromised—as is common in tidal
settings—a downward shift in the size and diversity of
the ichnogenera present will likely be observed.
As a result of tidal influence on epifaunal and infaunal
organisms, three styles of trace-fossil distribution are
expected: (1) owing to low overall sedimentation rates
and commonly abundant food, homogeneous distribu-
tions probably characterize most microtidal and
mesotidal intertidal flats; (2) regular heterogeneous dis-
tributions probably result from neap-spring tidal pro-
cesses; and (3) seasonal / annual regular heterogeneous
distributions can result from seasonal interactions
between the tidal and fluvial waters. Sporadically het-
erogeneous distributions can also occur in tidal settings,
but in other settings as well. Being less diagnostic, such
fabrics are not discussed here further.
Perhaps counter to intuition, regular heterogeneous
ichnofossil distributions are not typically observed in
most (neap-spring) tidal rhythmites. This is due to the
high sedimentation rates that characteristize deposi-
tion (several milimeters to a few centimeters per week)
of (diurnal/semidiurnal) tidalites. Neap-spring bundles
may contain laterally regular, heterogeneous trace-
fossil distributions in the distal toe- and bottom-sets of
migrating tidal dunes (Fig.
4.1d, e
); however, this rela-
tionship is difficult to observe in cored vertical succes-
sions, and is most readily observed with good lateral
exposure (Savrda
2002
; Gingras et al.
2002a, b
,
Pearson and Gingras
2006
).
Seasonal variations in sedimentary conditions are
strongly associated with regular heterogeneous distri-
butions of burrowing, especially in marginal-marine
environments. Several examples of such distributions
are associated with inclined heterolithic stratification
(IHS) common to tidal-bar deposits (Gingras et al.
1999
; Pearson and Gingras
2006
; Hovikoski et al.
2007
; Lettley et al.
2007
). Bioturbation within IHS
dominantly points to seasonal colonization (Fig.
4.1c
).
Such successions consist of beds that accumulated
under conditions of lower energy, intercalated with
beds deposited during high fluvial-discharge events
(Gingras et al.
2002a
; Pearson and Gingras
2006
),
which, during times of lower sedimentation and higher
salinity, favors predictable periods of recolonization.
Regularly but non-uniformly distributed burrowing in
IHS has been closely associated with deposition in
bays, estuaries and, to a lesser degree, deltas.
Additionally, Dalrymple et al. (
1991
) reported sea-
sonal bioturbation (summer layers bioturbated / winter
layers unburrowed) recorded in macrotidal intertidal
deposits of the Bay of Fundy: this is dominantly a
reflection of strong seasonality and its effect on
4.3.1
Vertical Spatial Distribution
of Trace Fossils in Tidal Settings
At the core and outcrop scale, the vertical spatial dis-
tribution of trace fossils primarily reflects the degree of
stability and temporal persistence of physico-chemical
conditions in a sedimentary environment. Trace fossils
can be distributed: (1) homogeneously; (2) regularly
heterogeneously; or (3) sporadically heterogeneously.
Thorough, homogeneously distributed bioturbation is
generally associated with readily available food and
oxygen, coupled with slow sedimentation. In such set-
tings, tidal effects mainly revolve around the regular
resupply of food materials to the setting and replenish-
ment of marine to brackish waters. Regularly hetero-
geneous trace-fossil distributions result from recurrent
(rhythmic) variability in local physico-chemical
parameters. Depositional environments characterized
by such regular but uneven colonization show a
response to tidal as well as seasonal to annual rhythms.
Regular heterogeneous distributions associated with
tidal settings most commonly are expressed as simi-
larly burrowed horizons of approximately recurring
composition and thicknesses, interbedded with unbur-
rowed or sparsely burrowed media, also of regular
thicknesses. Such distributions are exemplified by
migrating tidal dunes and within sets of Inclined
Heterolithic Stratification (IHS).
Sporadically heterogeneous distributions, on the
other hand, are the result of persistent spatio-temporal
variability in physico-chemical conditions. Sedimentary
environments that are characterized by episodic ero-
sion and/or sediment deposition, such as estuaries and
deltas, are particularly susceptible to such sporadically
heterogeneous distributions of bioturbation.
