Geology Reference
In-Depth Information
the metabolism of bioturbating organisms (Van den
Berg
1981
).
In micro- and mesotidal settings, tidal-flat deposits
characteristically display high bioturbation intensities
and homogeneous burrow distributions (Fig.
4.1b, f
).
This is the consequence of high infaunal biomass and
a markedly low sedimentation rate (Gingras et al.
1999
; Dashtgard
2011
). Under increasingly macrotidal
conditions, rapidly shifting sediment and high velocity
currents may counteract the efforts of bioturbators,
such that lamination and bedding are dominantly pre-
served (Klein
1970
; Dalrymple
1984
; Dalrymple et al.
1990
); nevertheless, there are zones in macrotidal set-
tings where sedimentation rates are sufficiently low
and conditions more or less stable, leading locally to
intense bioturbation (e.g. upper intertidal flats).
Notably, it is under these somewhat more energetic
conditions that regular heterogeneous trace-fossil dis-
tributions may develop in dune-associated neap-spring
bundles (Fig.
4.1d
).
The ichnological characteristics of supratidal, inter-
tidal and subtidal bars are summarized in Table 4.1 and
Fig.
4.2
. The ichnological identification of intertidal-
flat deposits should not be under-estimated as an inter-
pretive tool for the rock record. The identification of
such deposits infers the presence of tidal processes and
reveals the depositional base level. Further, these deposits
are sufficiently distinctive that they provide local
correlation levels, provided the tidal flats were broad
and/or prograded long distances. Moreover, the pre-
served thickness of tidal-flat deposits, especially where
subtidal to supratidal aspects of the deposits are dis-
cernible, can act as a proxy for tidal range (discussed
in Mangano et al.
1998
; Gingras et al.
1999
). Tidal-flat
deposits are generally situated above subtidal bars
(Fig.
4.1a
), resulting in a distinctive bioturbation-
increasing upwards signal that is readily observed in
vertical succession.
The lowermost channel/bar deposits display minor
bioturbation, whereas the medial parts of the bars con-
tain seasonally induced regularly heterogeneous trace-
fossil distributions. Trace fossil assemblages are
dominated by either horizontal or vertical trace-fossil
forms in monospecific or bispecific suites (e.g.
Fig.
4.2c-h
). The commonest vertical traces observed
are
Skolithos
and
Cylindrichnus
and more rarely
Arenicolites
,
Ophiomorpha
and
Polykladichnus
(Fig.
4.2b, d, f
). The most common horizontal ichno-
fossils are
Planolites
and
Teichichnus
with subordinate
Thalassinoides
(Fig.
4.2c, e
). Overall, the degree of
bioturbation attenuated compared to overlying inter-
tidal strata; this is owing to higher sedimentation rates
on the subtidal bar. Most commonly, unburrowed
beds are intercalated with burrowed beds (BI 2-3), and
in environmentally hostile conditions (very low salin-
ity, higher depositional energies, very turbid water),
bioturbation is rare throughout and sporadically
distributed.
The top of the bars grade into the intertidal zone,
which displays highly bioturbated (up to BI 6), homo-
geneous to sporadic heterogeneous trace-fossil dis-
tributions (Figs.
4.1b
, f and
4.2g-j
). The intertidal
zone is most commonly characterized by a mixed
assemblage of co-occurring vertical and horizontal
trace fossils (
Planolites, Teichichnus, Thalassinoides,
Siphonichnus, Skolithos
, and
Arenicolites
with subor-
dinate
Cylindrichnus
,
Polykladichnus
,
Ophiomorpha
,
Lockeia
, and stellate interface trace fossils) (Fig.
4.2g-j
).
Mud-dominated intertidal strata are normally highly
bioturbated (BI 4-6) (Fig.
4.2g, i
) with bioturbation
increasing in intensity upwards. Primary bedding, if
preserved, is associated with tidal run-off creeks. Sand-
dominated intertidal flats typically contain more
abundant primary sedimentary structures. This is due
to wave-reworking of the tidal flat, tidal-dune migra-
tion or the presence of variably scaled tidal run-off
creeks (Fig.
4.2h
). Within sandy intertidal flat depos-
its, BI ranges between 0 and 3, with rare beds display-
ing higher intensities of bioturbation.
If the entire vertical succession is preserved,
supratidal deposits overlie the intertidal units. The
supratidal zone is commonly eluviated, locally mas-
sive appearing, and contains rhizoliths (Fig.
4.2k-o
)
and displays rare trace fossils, including
Psilonichnus
,
Scoyenia
and/or
Naktodemasis
.
As a result of variability in the position of the high
and low tide levels, the transition from subtidal
through intertidal to supratidal sedimentation is rep-
resented by gradational sedimentological and ichno-
logical changes. Correspondingly, a precise level
demarcating subtidal from intertidal levels may be
difficult to discern—this is exacerbated with increas-
ing tidal range.
It is notable that the vertical succession outlined
above (i.e. bioturbation intensity increasing markedly
upwards) occurs repeatedly in tidal settings such as the
Lower Cretaceous McMurray Formation, Pleistocene
and modern Willapa Bay, and the present-day Shepody
