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(salinity reduction, salinity variation) factors. Thus,
tidally influenced trace-fossil suites typically display
low-diversities with more normally sized burrows in
tidal marine settings, and low diversities coupled
with diminutive trace fossils in tidal settings prone to
brackish waters.
Size and diversity trends proffer a mappable ichno-
logical function that can be conceptually linked to tidal
range. Gunn et al. (
2008
) and Hauck et al. (
2009
)
mapped burrow sizes and their distributions in two
modern estuaries; the lower mesotidal Ogeechee River,
Georgia, USA and the microtidal Kouchibouguac,
New Brunswick, Canada, respectively (Fig.
4.4
). They
used a simple function: the diameter of the largest bur-
row observed at a site (measured in millimeters) was
multiplied by the number of burrow types observed, to
create the Size-Diversity Index (SDI). Results show a
geometric increase in the SDI in an oceanward
direction. For the lower mesotidal Ogeechee, the SDI
is very high near the mouth of the bay (although
the sound itself is approximately SDI 500), but dissi-
pates on a log-linear basis into the proximal tidal
reach where salinities approach zero (Gunn et al.
2008
). At Kouchibouguac, which is wave-dominated
and microtidal, the central basin has a high SDI, but
this decreases abruptly in the landward direction
(Hauck et al.
2009
). Conceptually, SDI trends
should
be
distinctive for bays and estuaries of various tidal
ranges. Although this has not yet been tested exten-
sively in modern environments, an interpretive frame-
work for SDI based on our observations in modern
settings is provided in Fig.
4.5
.
4.3.3
The Significance of Burrow
Linings and Infills
Animals in tidal settings commonly concentrate fine-
grained sediment in or adjacent to their burrows (Zorn
et al.
2010
). This is a result of selective ingestion of
the small-caliber sediment, which is generally compa-
rably rich in refractory organic carbon (Konhauser
and Gingras
2007
). Thus, some burrows associated
with tidal settings display thickened linings composed
of mud. Due to the tidally driven settling of food at the
sediment-water interface, several animals (e.g. Nereid
Fig. 4.2
Examples of subtidal, intertidal and supratidal depos-
its with ichnological content indicated. (
a
) Mud-dominated tid-
ally influenced subtidal pointbar. At this scale of view, trace
fossils are difficult to observe, however, evident are lenticular
sand beds defining characteristic inclined heterolithic stratifi-
cation (IHS). Pleistocene, Willapa Bay, Washington, USA.
(
b
) Closer view of same outcrop as in (
a
). Ichnofossil assem-
blage dominated by invertebrate domiciles including
Skolithos
(Sk),
Arenicolites
(Ar), and
Psilonichnus
(Ps). The trace fossil
Psilonichnus
is more common to intertidal and supratidal depos-
its and may be indicative of relatively low energy conditions in
the subtidal channel. Pleistocene, Willapa Bay, Washington,
USA. (
c
) Sand-dominated IHS with sparse bioturbation consist-
ing of
Planolites
(Pl). Sand (
black
due to heavy-oil content) may
be burrowed as well, but in the absence of lithological definition,
this is difficult to assess. Cretaceous, Bluesky Formation, Peace
River area of Alberta, Canada. (
d
) Sand-mud IHS attributed to
deposition on a tidally influenced subtidal pointbar.
Cylindrichnus
(Cy) and
Planolites
(Pl) are indicated. Cretaceous, McMurray
Formation, Athabasca area of Alberta, Canada. (
e
) Mud-
dominated IHS attributed to deposition on a tidally influenced
subtidal pointbar.
Cylindrichnus
(Cy),
Teichichnus
(Te),
Planolites
(Pl) and
Polykladichnus
(Po) are indicated. Cretaceous,
McMurray Formation, Athabasca area of Alberta, Canada.
(
f
) Bioturbated bottom sets and toe sets attributed to deposition
in tidally influenced subtidal compound dunes.
Cylindrichnus
(Cy), and
Skolithos
(Sk) are indicated. Cretaceous, McMurray
Formation, Athabasca area of Alberta, Canada. (
g
) Bioturbate
texture with vestigial sand laminae locally preserved. The degree
of bioturbation is high, but discrete trace fossils are observable
where sand contributes lithological definition.
Planolites
(Pl), and
Skolithos
(Sk) are indicated. (
h
) Pleistocene, Willapa
Bay, Washington, USA. Intertidal sand flat deposit with tidal
runoff channels crosscut by
Siphonichnus
(Si) and
Thalassinoides
(Th). Pleistocene, Willapa Bay, Washington, USA. (
i
) Intensely
burrowed strata common to intertidal deposits. This example
contains
Planolites
(Pl),
Teichichnus
(Te) and
Thalassinoides
(Th). Cross-cutting the bioturbate texture are pyrite-replaced
rhizoliths
(Rh), evidencing vertical proximity to the supratidal
setting. Cretaceous, Bluesky Formation, Peace River area of
Alberta, Canada. (
j
) Similar to (
i
) with the addition of the char-
acteristic intertidal trace-fossil
Siphonichnus
(Si). Cretaceous,
McMurray Formation, Athabasca area of Alberta, Canada.
(
k
) Intensely rooted supratidal strata (located in outcrop 60 cm
above bioturbated intertidal media). Pleistocene, Willapa Bay,
Washington, USA. (
l
) Similar to (
k
), but at the transition zone
from intertidal to supratidal strata. Pleistocene, Willapa Bay,
Washington, USA. (
m
) Eluviated supratidal strata with rare
rhizoliths
(Rh) and putative insect-larval traces
Nactodemasis
(Na). Cretaceous, Bluesky Formation, Peace River area of
Alberta, Canada. (
n
) Outcrop view of eluviated supratidal media.
Note the ironstone nodules. Cretaceous, McMurray Formation,
Athabasca area of Alberta, Canada. (
o
) Eluviated supratidal
strata with rare
rhizoliths
(Rh), putative insect-larval traces
Nactodemasis
(Na) and
Skolithos
(Sk). Cretaceous, McMurray
Formation, Athabasca area of Alberta, Canada
