Geoscience Reference
In-Depth Information
some other cases, whether or not transportation has occurred
may be less clear. Coals (preserved peats), for example, can
either be allochthonous or autochthonous. Autochthonous coal
will have roots extending from its base and will lie directly on
a palaeosol, whereas allochthonous peats will not. In the case
of fossilized upright tree bases it is important to note whether
the roots penetrate into a palaeosol (fossilized soil) because tree
bases may well be transported in an upright position.
5
If any fossil is found broken, abraided or disarticulated then
some pre-burial transport is likely to have occurred. If the
fossil has attached or included sediment not found in the
entombing rock then re-working should be suspected. An
exception to this is breakage in situ during compaction of the
rocks, but this is easily identifi ed as all the parts will be in
close proximity.
Figure 5.6 Infaunal bivalve in
life position from the Jurassic, near
Scarborough, UK. (Angela L. Coe,
The Open University, UK.)
Also important is the fossil orientation. In fl owing fl uids
(usually air or water) allochthonous body fossils may become
oriented in relation to that fl ow post-mortem. This is most
commonly seen in elongate remains such as belemnites or tree
trunks. Often in the case of tree trunks branches become
snapped off during transport but roots are more resilient.
Stranding of the roots on the bed of the shallow parts of rivers
will cause the remaining trunk to swing round and point
downstream (Figure 5.5). If trees are blown down by wind, by a
volcanic pyroclastic fl ow or lateral explosion, then the tips of
the trunks will again point in the direction of fl ow away from
the source.
(a) Unstable: Settled from
suspension
Orientation is also an important indicator of life position. For
example many bivalves live in the sediment, so often get
preserved in that position. If the two shells (valves) of a bivalve
are attached and closed then there is a good chance that the
remains have not undergone any signifi cant post-mortem
transport. Careful inspection of the deposits may well indicate
other remains in the same orientation and, if so, they are likely
to be in life position (Figure 5.6).
current direction
If single valves of bivalves or brachiopods are found then
clearly they have undergone some disarticulation and
transport after death, even if the distances involved are
short. In this case it is useful to record which way up they
are lying. When populations of single valves are found
concave side up then it is likely that they were at some
time suspended in the water column (e.g. by storm action)
and then subsequently settled out of suspension to rest on
the sediment surface in that position. Alternatively, if the
valves are all oriented convex side upwards this would
suggest that prior to fi nal burial the shells were subject to
horizontally fl owing current that has fl ipped over the shells
(Figure 5.7).
(b) Stable: Deposited under
lateral flow
Figure 5.7 Sketch to show
single valves oriented (a) concave
up as a result of settling from
suspension and (b) convex up as a
result of lateral current fl ow.
 
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