Geoscience Reference
In-Depth Information
respect of sediment type(s) compared with the sediment
burrowed, and any geometries associated with the infi ll
(simple collapse versus structures that indicate deliberate
backfi lling by the organism);
•
look for tiering, i.e. the pattern of trace fossil types
vertically in the sediment (Appendix A5, Figure A5.4), in
relation to a given horizon marking a change in
sedimentation - some burrows are typically formed in
shallow sediment depths while others penetrate deeper,
and loss of shallow tiers can indicate erosion;
•
cross-cutting relationships indicating successive
colonizations of the sediment and changing conditions
through time.
When looking for trace
fossils on bedding planes do
not forget to look up at the
underside of overhangs.
These are often cleaner and
less abraded than bedding
planes that may have been
walked over and so the
fossils are easier to see and
better preserved.
5
While trace fossils carry information on how ancient organisms
move, and thus their biology, information also can be gleaned
about sediment deposition and erosion (e.g. Figure 5.4).
Burrows may be fi lled with sediment quite unlike that into
which the burrow is made, or that overly it, suggesting
sediment was deposited and then eroded. Often this may be the
only evidence of an unconformity. Trace fossils with defi ned
geometries relative to the ancient sediment surface (e.g. vertical
U-shaped burrows) can also be used as 'way up' structures.
Any displacement from the normal orientation will indicate
rock movement and/or deformation.
deposition
original sediment
surface
erosion
3 to
15cm
(a) original tube
(b) spreite beneath
U-tube
(c) spreite between
limbs of U-tube
Figure 5.4
Sketches to show a variety of spreiten patterns in the trace
fossil
Diplocraterion
formed in response to deposition and erosion.
(a) Stable situation where no sediment deposition or erosion is taking
place. In (b) sediment has been deposited causing the organism to
migrate upwards leaving traces of the original burrow as disturbed
sediment below. In (c) erosion has caused the organism to burrow
deeper leaving disturbed sediment (spreiten) between the vertical
burrows. Figures 6.3e and 6.3f (p. 113) show fossilized
Diplocraterion
.
