Geology Reference
In-Depth Information
Table 4.1
Ichnological criteria for the differentiation of subtidal, intertidal and supratidal settings
Other Key Sedimentological
Observations
Trace-fossil Content
Bioturbation Index
Distribution of Trace Fossils
Character of Lower Contact
(1) Degree of animal-
generated bioturbation
is generally obscured
by rhizoturbation and
eluvination, lending the
sediment a massive
appearance
(1) Notwithstanding the
rooted fabric, trace fossils
are sporadically and
heterogeneously
distributed
(1) Generally abrupt transition
from burrowed intertidal to
rhizoturbated, massive
appearing supratidal media
(2) Roots and eluviation
commonly descend into
intertidal unit
(1) Common coalified plant
fragments
(2) Locally common
Fe-minealization
(3) Deposits of steep-banked,
small scale run-off creeks
may be present
Supratidal
Deposits
(1) Commonly contain
rhizoliths and
otherwise generally
devoid of burrows
(2) Where present, trace
fossils may include
care
Psilonichnus
and
upwards, rare
Scoyeria
and / or
Noktodermois
Intertidal
Deposits
(1) A mixed assemblage
of co-occurring
vertical and horizontal
trace fossils, most
commonly dominated
by
Planolives,
Teichichrus,
Thalassinoides
.
Siphonichnus,
Skolithos
, and
Arenicolies
(2) Subordinate numbers
of
Cylindrichnus,
Polyklodichnus,
Lockeia
, and <$$$>
tellate interface trace
fossils (the latter two
observed on bedding
planes) may be
preserved
(1) Mud-dominated flats are
commonly highly
bioturbated (B1 4-5)
with bioturbation
increasing in intensity
upwards. Primary
bedding is locally
preserved in association
with tidal run-off creeks
(B1 0-2) 0-2
(2) Sand-dominated flats
commonly display more
primary sedimentary
structures due to
wave-reworking,
tidal-dune migration or
the presence of variably
scaled tidal run-off
creeks. In these cases all
ranges between 0 to 3,
potentially with rare beds
displaying B1 6
(1) Lower energy tidal flats
characterized by slow
sedimentation rates
display homogeneous
bioturbation at the core /
small outcrop scale
(2) Increased sediment
shifting due to wave
exposure and increased
tidal run-off leads to
sporadic heterogeneous
distributions
(3) More rarely, seasonally
influenced tidal flats may
display nearly horizontal,
planiform regular
heterogeneous
distributions
(1) Gradational with underlying
subtidal deposits, but is
commonly discenible by an
abrupt increase in bioturba-
tion intensity and a switch
to comparably homoge-
neous burrow distributions.
In modern settings, this
switch in burrowing
intensity and distribution
approximately corresponds
to the outer part of the
middle intertidal flat
(2) With increasing tidal range,
intertidal flat deposits and
the transition zone become
thicker, leading to an
increasingly gradational
transition
(1) Most commonly, sand
and mud are admixed by
bioturbation
(2) Where present, preserved
sedimentary features
include a range of
primary structures, such
as lenticular through
wavy to flaser bedding
(3) Local scour and fill
associated with tidal run
off and the presence of
larger scale tidal run-off
creeks creeles
