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polychaetes, the bivalve
Macoma balthica
, and the
amphipod
Corophium volutator
; Gingras et al.
1999
;
Dashtgard et al.
2008
) selectively feed at this inter-
face, thereby allowing interment of mud (as burrow
linings), even if the mud tends to hydraulically bypass
the depositional locale. In marginal-marine settings,
thickened linings are exemplified in the ichnogenera
Ophiomorpha
and
Cylindrichnus
(Fig.
4.6
). Of course,
similar mud stowage may also be observed in other
depositional settings (e.g. lower shoreface, prodelta,
inner shelf), and of itself is not diagnostic. However,
low-diversity assemblages that can be ascribed to
brackish-water settings and which
also
display bio-
genic stowage of fine-grained sediment are best inter-
preted as ichnological features related to rhythmic
availability of mud.
Burrow infill may also be affected by the presence
of tidal currents. Specifically, sediment that is moving
at or near the seafloor can be trapped in burrows that
possess a large, open aperture, or in burrows where a
narrowed aperture has been breached by erosion. In
tidal settings, open burrows may be rhythmically
infilled by tidal processes forming “tubular tidalites”
(Gingras et al.
2007
) (Fig.
4.7
). The most common fea-
ture observed in tubular tidalites is physically gener-
ated sedimentary couplets. However, apparent
neap-spring bundles also have been observed in large
examples of
Thalassinoides
and
Arenicolites
(e.g.,
Miocene-aged deposits of Amazonia; Gingras et al.
2002b
). Tubular tidalites are dominantly observed in
large diameter (>1 cm), vertical to inclined shafts, and
in J- through U-shaped burrows. As such, the trace
toward the top of the photo. The unit shows BI 3-5, with diminu-
tive
Planolites
(P),
Teichichnus
(Te), and
Cylindrichnus
(Cy).
Scale is 3 cm. Lower Cretaceous Glauconite Formation, Alberta,
Canada. (
h
) Sandy estuarine-bay deposit showing BI 4-5, and a
low-diversity suite dominated by re-equilibration structures and
vertical dwelling/deposit-feeding structures. Dominant ichno-
genera are
Lingulichnus
(Li) and
Rosselia
(Ro), with subordi-
nate
Siphonichnus
(Si), and
Planolites
(P). Scale is 5 cm.
Permian Pebbley Beach Formation, south Sydney Basin,
Australia. (
i
) Thoroughly bioturbated (BI 4) IHS, with a mono-
specific suite of
Gyrolithes
(Gy) in the sand layers, and
Planolites
(P) in the mud beds. An isolated
Cylindrichnus
(Cy) occurs near
the base of the photo. Down-slope creep of the sediment has
resulted in deformation of the burrows. Lower Cretaceous
McMurray Fm, Alberta, Canada. (
j
) Laminated sand and mud
near the top of a tidal-creek point bar, showing Nereid-generated
horizontal dwelling burrows that have been shifted upward dur-
ing sedimentation. This structure is attributable to the ichnoge-
nus
Teichichnus
(Te). Scale is 3 cm. Modern, Bay of Fundy, New
Brunswick, Canada. (
k
) Sandy heterolithic estuarine-bay inter-
val, containing bivalve-generated equilibrium adjustment struc-
tures (ea) and
Siphonichnus
(Si). Sediment-swimming structure
or navichnia (na) is associated with water-rich muds.
Planolites
(P) are common to the mud layers. Scale is 15 cm. Pleistocene,
Willapa Bay, Washington, USA. (
l
) Muddy bay deposits with
heterolithic lenticular-bedding containing remnant oscillation
ripples, wavy parallel lamination, dark unburrowed fissile mud-
stone drapes, and abundant synaeresis cracks (sy). The facies
shows BI 2-3, with a low-diversity suite of diminutive
Planolites
(P),
Teichichnus
(Te),
Cylindrichnus
(Cy). Scale is 3 cm. Lower
Cretaceous McMurray Formation, Alberta, Canada. (
m
) Mud
bed capping a sand layer in tidally influenced IHS. The mud
layer shows BI 4, with abundant
Cylindrichnus
(Cy). Rare
Planolites
(P) occur locally. Scale is 5 cm. Lower Cretaceous
McMurray Formation, Alberta, Canada
Fig. 4.3
Examples of trace fossil suites characteristic of brack-
ish-water settings. (
a
) Current-rippled sandstones showing BI 2.
Ichnological suite includes diminutive and sporadically distrib-
uted
Cylindrichnus
(Cy),
Planolites
(P),
Skolithos
(Sk), and
fugichnia
(fu). Scale is 3 cm. Lower Cretaceous McMurray Fm,
Alberta, Canada. (
b
) Bioturbated muddy sandstone of a sandy
bay, showing BI 5. Suite is very low diversity, and dominated by
Teichichnus
(Te),
Planolites
(P), and
Thalassinoides
(Th). Scale
is 3 cm. Lower Cretaceous Basal Colorado Sandstone, Alberta,
Canada. (
c
) Sandy bay deposit showing BI 1-2, with alternating
horizontal structures such as
Planolites
(P), and vertical struc-
tures such as
Arenicolites
(Ar), and
Skolithos
(Sk). Oscillation
and combined-flow ripples show
fugichnia
(fu). Diminutive
syneresis cracks (sy) might suggest salinity fluctuations. Scale is
3 cm. Lower Cretaceous Grand Rapids Fm, Alberta, Canada.
(
d
) Estuary-mouth deposit in an incised valley, consisting of low-
angle undulatory parallel-laminated sandstone and oscillation
ripples with dark, carbonaceous mudstone interlaminae. Unit
shows sporadically distributed bioturbation (BI 0-3). Stacked
event beds contain
fugichnia
(fu), and display recolonization
suites of
Diplocraterion
(D), and
Planolites
(P). Scale is 3 cm.
Lower Cretaceous Viking Formation, Alberta, Canada. (
e
) Muddy
IHS showing sporadically distributed bioturbation (BI 1-3), with
a low-diversity suite consisting of diminutive, facies-crossing
ichnogenera such as
Cylindrichnus
(Cy),
Skolithos
(Sk), and
Planolites
(P). Lens cap is ~8 cm in diameter. Pleistocene,
Willapa Bay, Washington, USA. (
f
) Heterolithic current-rippled
sandstone with mudstone drapes forms wavy-bedded composite
bedsets, interpreted as a tidal-estuarine bay deposit. Bioturbation
intensity is low (BI 0-2), with a low-diversity suite of diminutive
Planolites
(P),
Teichichnus
(Te), and uncommon
Rosselia
(Ro).
Permian Pebbley Beach Formation, south Sydney Basin,
Australia. (
g
) Sandy central-basin deposits of a riverine (tidally
influenced) estuary. Facies consists of wavy-bedded sandstone
and sandy mudstone containing remnant, wavy parallel lamination
