Geology Reference
In-Depth Information
4.3.4
Characteristic Feeding Behaviors
in Tidal Settings
Food distribution is temporally and spatially heteroge-
neous in tidal settings. Due to attenuated tidal-current
energies and the settling of water-borne organics with
falling tide and during slack water, intertidal-flat deposits
are generally food-rich compared to subtidal settings.
Subtidal locales tend to suffer higher energy tidal
currents, which inhibit the deposition of food; this is
exacerbated with increasing tidal-current strength. In
subtidal settings, food is deposited most commonly
with fine-sediment IHS or with slack-water mud drapes
that characterize wavy through to flaser bedded com-
posite bedsets. This localized resource is either
exploited at the sediment-water interface in the form of
stellate feeding traces (Figs.
4.2
m and
4.7a
), or is tar-
geted in the substratum using simple (e.g.,
Planolites
;
Fig.
4.3e, g, m
) through to more complex (e.g.,
Teichichnus
Fig.
4.3b
; “
Phycodes
” Fig.
4.7e
; possibly
Gyrolithes
Fig.
4.3i
) deposit-feeding strategies.
With more thickly interbedded media (IHS and
wavy bedding), both surface feeding and substratal
deposit feeding are commonly employed (Fig.
4.8
).
Surface feeding is observed as abundant
Cylindrichnus
or
Skithos
generated by organisms that colonized either
sand or mud beds and gathered tidally delivered food
from the surface (Figs.
4.1
c and
4.2i, m
). Intrastratal
deposit-feeding ethologies are most commonly
observed as
Planolites
and
Teichichnus
, and reflect
organisms that bioturbated the deposit with varying
degrees of thoroughness (Figs.
4.3b, e, g
, m and
4.7c
).
These are behaviors that are well suited to the relatively
rapid exploitation of intastratal food. Unlike the inter-
face-feeding behaviors, it is likely that the intrastratal
activities (especially within IHS) are aimed at exploit-
ing seasonally deposited beds as opposed to directly
benefitting from tidally delivered resources (Gingras
et al.
2002b
; Pearson and Gingras
2006
). As such, the
presence of vertically oriented trace fossils in seasonally
generated IHS is likely a better indicator of the presence
of tidal currents. Inclined heterolithic stratification that
consist predominantly of tidally deposited strata with
little or no seasonal cycle will likely be unburrowed,
due to high sedimentation rates.
In contrast, the abundance of resources in tidal-flat
deposits leads to a preponderance of characteristic ich-
nogenera. Although not limited to these settings, feed-
ing behaviors that facilitate the rapid exploitation of
Fig. 4.4
Size-diversity index (SDI = maximum burrow diameter
X diversity of trace types) based on field studies in two modern
locales (
a
) Ogeechee River, Georgia, USA and (
b
) Kouchibouguac
River, New Brunswick, Canada (Data after Gunn et al.
2008
and
Hauck et al.
2009
)
fossils
Skolithos
,
Thalassinoides
,
Psilonichnus
and
Arenicolites
are most commonly associated with tubu-
lar tidalites.
Although tubular tidalites are distinctive, their
appearance may bear some similarities to meniscate
backfill (
Scolicia
,
Taenidium
) or to spreite (
Teichichnus
,
Zoophycos
). Key differentiating features from menis-
cae and spreite include: burrow-infill laminae that are
consistently oriented with a dip of less than 20° from
(depositional) horizontal; the presence of laminae
that abut the burrow wall (i.e. are non-tangential to the
burrow margin); and the presence of sedimentary
couplets.
