Geology Reference
In-Depth Information
and Buatois
2004
; Rebatah et al.
2006
; Hovikoski et al.
2007, 2008
; Carmona et al.
2009
). However, the inverse
question, “can tides be inferred
from
ichnological data”
has yet to be tested.
By far, the largest volume of work has depended on the
association of brackish-water ichnofossil assemblages
with tidal settings. Through the establishment of a
brackish-water trace-fossil model, and in coordination
with the documentation of physical features ascribed
to tidal sedimentation, several studies have established
that a passive relationship exists between the occurrence
of brackish-water ichnofossils and the presence of
tides (Beynon et al.
1988
; Pemberton et al.
1982
;
MacEachern et al.
1999
; Gingras et al.
2002a, b
;
McIlroy
2004
; Bann et al.
2004
; Mangano and Buatois
2004
; Buatois et al.
2005
; Rebatah et al.
2006
;
MacEachern and Gingras
2007
; Hovikoski et al.
2007
;
Carmona et al.
2009
). However, it is fair to say that the
identification of brackish-water settings has been con-
flated with the identification of tidally influenced set-
tings and that the presence of a brackish-water
trace-fossil assemblage, in and of itself, should not be
taken as an indicator of tidal conditions.
Some effort has been directed toward establishing
an animal response to variable, but not necessarily
rhythmic sedimentation rates. In particular, the ichno-
logical response and recovery to event sedimentation
has been carefully documented from numerous sys-
tems, most notably with respect to tempestite and tur-
bidite deposition (Crimes et al.
1974, 1981
; Seilacher
1982
; Wanless et al.
1988
; Pemberton and Frey
1984
;
Pemberton et al.
1992a
; Pemberton and MacEachern
1997
; Savrda and Nanson
2003
). Storm- and turbidity-
current events, and perhaps also river floods, however,
are characterized by recurrence intervals of months to
centuries, and are not analogous to tidal sedimentation.
Perhaps more germane to this paper are studies of bio-
turbation under conditions of persistent but irregular
sedimentation in colonized subaqueous dunes (Bromley
1996
; Taylor and Goldring
1993
; Savrda
2002
;
Dashtgard et al.
2008
). However, the potential of iden-
tifying
rhythmic
distributions of bioturbation levels in
association with short-period, rhythmic sedimentation
remains essentially unexplored (Gingras et al.
2011
).
Several workers have identified trace-fossil morphol-
ogies that recur in some tidal environments. Interface-
feeding behaviors that consist of foraging for food
from a central burrow, for example, produce stellate
traces that radiate outwards from the burrow aperture.
Such burrows are commonly, but not exclusively found
on tidal flats (Aitken et al.
1988
; Dashtgard et al.
2008
):
the importance of such trace fossils is discussed below.
Detailed analyses of burrow-infill and burrow-lining
characteristics have recently been investigated for poten-
tial tidal associations (Gingras et al.
2011
); this work is
also discussed below.
4.3
Process Sedimentological
Importance of Some Selected
Ichnological Characteristics
Tidal processes and the sedimentary structures they
generate are understood to a far greater degree than are
the ichnological responses to them. Rhythmic varia-
tions in current strength, direction and duration
generate characteristic and predictable sedimentary
structures that are most easily interpreted in the con-
text of tidally influenced sedimentation. In contrast,
with the exception of rhythmic infilling of trace fossils
and perhaps regular variation in bioturbation intensity,
both of which are rare (see below), the morphologies
of ichnofossils are not directly influenced by the pres-
ence of tidal currents. On the other hand, some aspects
of trace-fossil occurrence and distribution are directly
impacted by the tidal depositional processes. These
include parameters that can be related to sedimentation
rates (e.g. trace-fossil distribution and diversity),
altered water chemistry due to tidal mixing (e.g. trace-
fossil size and diversity), variations in sediment calibre
at the bed (e.g. burrow linings and infills), and the
details of food allocation (e.g., characteristic feeding
behaviors).
In marine and marginal-marine deposits, trace-fossil
size and diversity are generally taken to reflect the
degree of physico-chemical stress in the depositional
environment. Highly diverse and robust suites corre-
spond to optimal environmental conditions, whereas
low-diversity suites and those composed of diminutive
structures are regarded to be indicative of environmen-
tal duress. As such, it is useful to understand that,
for the deposit under scrutiny, the dataset ideally
should be compared to an ichnological baseline (cf.
MacvEachern et al.
2010
). This helps to identify
intervals that have been influenced by elevated sedi-
mentation, brackish water, overall low oxygenation
concentrations, biased larval recruitment, and/or vari-
ous substrate stresses (such as anomalously firm or
