Geology Reference
In-Depth Information
Fig. 4.5
Conceptual graph of
SDI distribution in embayed
depositional settings that are
influenced by small versus large
tidal prisms. Key factors in the
construction of this graph
include the greater landward
propagation of marine waters
with large tidal prisms,
increased residence time of fresh
water in settings with small tidal
prisms, and the inhospitable
nature of outer-bay settings in
locales where diurnal/semidiur-
nal tidal currents are exceed-
ingly strong
intrastratal food (e.g.,
Planolites, Teichichnus
and
Thalassinoides
) or resources that have settled on the
sediment-water interface (e.g.,
Skolithos, Arenicolites,
bivalve-generated
Siphonichnus
, and surficial stellate
probing marks) provide the dominant elements of the
preserved ichnocoenose (Gingras et al.
1999
; Hertweck
et al.
2007
). Stellate interface-feeding traces radiate
from a central location (Fig.
4.8a, b
), and are the sur-
face expression of vertical traces that include
Skolithos
(e.g., Fig.
4.3c
),
Arenicolites
(ibid.),
Siphonichnus
(Figs.
4.1
a and
4.7d
) and
Cylindrichnus
(e.g.,
Fig.
4.3m
). The stellate expression can only be seen on
uncommon bedding planes, however, and it is far more
common to observe abundant vertical traces that most
probably were associated with stellate surface/inter-
face feeding (Figs.
4.1
c and
4.2m
). An important dis-
tinction here is that although elements of the
Skolithos
Ichnofacies are abundant in tidal settings, many of the
vertically oriented ichnofossils represent an engage-
ment in deposit feeding rather than filter feeding
(Gingras et al.
1999
; Dashtgard et al.
2008
). This is,
again, an ethological strategy made possible by the
large amounts of resource that commonly accumulate
during slack-water periods.
In flaser bedded sediment, comparably systematic
deposit feeding may be used to harvest the localized
resource. Within large-scale flasers, this may be seen
as regular perforation of the flasers with
Planolites
or
even
Phycodes
-like trace fossils (Fig.
4.7e
). Small
and thin flasers are not typically exploited through
such selective efforts. These are more likely to be
targeted by large-scale convection of the sediment,
as conducted by “sand shrimp” (e.g.,
Neotrypea
,
Callianassa
) producing
Thalassinoides,
or “lug
worms” (e.g.,
Arenicola
) excavating large
Arenicolites
or
Polykladichnus
. Additionally, these animals are
also mining organics associated with the sand deposit
as well (Swinbanks and Luternauer
1987
).
4.4
Ichnologic Recognition of Tidally
Influenced Deposits
As with the primary physical sedimentary structures,
there are very few ichnological features that are pro-
duced solely in response to the influence of tides. As
such, the success of an ichnological interpretation of
tidal sedimentation depends upon accruing a prepon-
derance of supporting observations. In practice, with
our current level of understanding, it is unlikely that
ichnological data would be put forth as the primary
evidence for tidal reworking in the absence of more
diagnostic physical sedimentary structures. However,
in the interest of exploring the potential of trace fossils
as tidal indicators, this paper deliberately avoids such
dependence upon other corroborating datasets.
To establish an ichnological response to tidal pro-
cesses effectively, the trace-fossil dataset should be
