Page 187 - Sedimentology and Stratigraphy

174 The Marine Realm: Morphology and Processes

after it died. In some cases, the environmental condi-

tions might have actually caused the death of an
animal, such as a skeleton of a mammal enclosed in
volcanic ash. Most importantly, ichnofauna provide
precise information about the environment where
they were formed. For example, bird footprints are

either evidence of a land surface, or of very shallow
"

water where the bird may have been paddling, and a

complex of burrows in sea-floor sediment is evidence
of oxygenated conditions. Trace fossils are therefore a
very powerful tool in palaeoenvironmental analysis,
*
and we can use changes in trace fossil assemblages,
" !
known as ichnofacies, as evidence for changes in
( environment, such as rise and fall of sea level (23.8).

11.7.2 Trace fossil assemblages
The ecology of the sea floor and hence the ichnofauna
Fig. 11.11 Classification of trace fossils based on
found in the sediment is controlled by a number of
interpretation of the activity of the organism. (Adapted from
interrelated factors (Pemberton et al. 1992). These
Seilacher 2007.)
factors are:
1 substrata type, whether it is hard or soft, sandy or
type of trace fossil. There is also a lot of overlap muddy;
between categories, as an animal may have been 2 the strength of the currents that sweep the sea
walking and feeding at the same time. The most floor;
common trace fossils are some form of burrow made 3 the rate at which sediment is being deposited;
for dwelling or feeding or both. Escape burrows, 4 turbidity, which is the amount of fine suspended
formed by organisms moving up to the surface, are sediment in the water;
common in settings where there is rapid sedimenta- 5 oxygen levels in the water;
tion by storms or turbidity currents. 6 the salinity of the water;
7 the quality of the nutrient supply;
8 the quantity of nutrient supply.
11.7.1 Trace fossils in palaeoenvironmental These environmental variables can be simplified into
analysis a scheme based primarily on water depth (Fig. 11.12)
and the hardness of the substrate (Fig. 11.13)
Although we may not know the identity of many of (Pemberton et al. 1992; Pemberton & MacEachern
the animals that produced trace fossils, their presence 1995). Shallow marine environments tend to be
provides some very valuable information about higher energy and are richer in nutrients than deep
the behaviour of organisms and the nature of the water settings. There are, however, exceptions to this,
palaeoenvironments. From the perspective of the as some shallow water settings (shelf seas with
analysis of sedimentary rocks, ichnofossils will often restricted water circulation and lagoons) can be low
be more useful than fossil shells or bones because they energy and relatively poorly supplied with nutrient,
are conclusive evidence that an animal lived there. In so these ichnofacies are not necessarily definitive indi-
contrast, a body fossil is, of course, a dead animal, and cators of water depth.
it is not always certain whether it lived in the place The conditions of the substrate may vary from loose
where the fossil is found. A coral may be preserved as sand in a foreshore setting to hard rocks in another
part of the reef in which it lived, but a pelagic organ- beach environment: the ichnofacies that occur on hard
ism is not preserved where it lived, swimming in the or semiconsolidated shorelines (Trypanites and
open ocean, but on the ocean floor, where it ended up Glossifungites assemblages respectively) can also