Geology Reference
In-Depth Information
Fig. 4.7
Examples of tubular tidalites. In all photographs, the
white arrows
indicate the position of discernible sedimentary
couplets. (
a
)
?
Thalassinoides
with poorly developed couplets.
Jurassic, Upper Monteith Member, Nikanassin Formation,
British Columbia, Canada. (
b
) Undiagnosed domicile (similar to
Psilonichnus
) displaying several sedimentary couplets. Lens
cover is 6.2 cm diameter. Intertidal flat deposit, Pleistocene,
Willapa Bay, Washington, USA. (
c
) Undiagnosed domicile from
the same outcrop as (
b
) showing a side view through an analo-
gous domicile. This trace fossil also displays several sedimen-
tary couplets. Lens cover is 6.2 cm diameter. Intertidal
flat deposit, Pleistocene, Willapa Bay, Washington, USA.
(
d
) Vertical tube with well developed couplets. Miocene, Pebas
Formation, Peru. (
e
) Cropped x-ray image (x-ray negative:
white
is sand rich,
dark
is mud rich) from a modern intertidal-flat
deposit showing a recently abandoned crab burrow similar to the
Pleistocene examples shown in (
b
) and (
c
). Image is 4 cm wide.
Modern tidal flat, Willapa Bay, Washington, USA
of workers to establish as many of the following criteria
as possible:
1. The presence of thickly lined ichnofossils, with the
linings composed of sediment not otherwise pre-
served in associated beds. If this is so, it should be
established that the trace-fossil assemblage is also
of lower
diversity
than the baseline assemblage.
2. Rhythmically (but passively) infilled, larger-
diameter trace fossils such as
Thalassinoides
and
Psilonichnus
are present, corresponding to tubular
tidalites.
3. Regular waxing and waning of bioturbation inten-
sity is observable at the bed and/or bed-set scale.
4. Ichnofossil suites in highly bioturbated sediment
comprise both horizontally and vertically oriented
trace fossils (i.e., concurrent substratal- and inter-
face-deposit feeding can be established), and highly
bioturbated zones lie gradationally above domi-
nantly bedded strata.
5. In heterolithic bedding, zones dominated by
(presumed interface deposit-feeding) vertically
oriented trace fossils are present. In outcrop stud-
ies, surficial stellate feeding traces are commonly
observed as well.
6. The presence of systematic deposit feeding within
flaser-bedded intervals.
