Page 189 - Sedimentology and Stratigraphy

176 The Marine Realm: Morphology and Processes

occur further out on the shelf if conditions result in a nutrient-rich sediment more random feeding struc-
hard or firm sea floor (11.7.4). It should be noted that tures are the norm.
the names of the assemblages are taken from
one particular ichnofossil which may be typical: the
Cruziana assemblage does not necessarily include the
11.7.3 Bioturbation
actual form Cruziana, and is in fact unlikely to unless
the deposits are Palaeozoic as they are thought to be The presence of evidence of organisms disturbing sedi-
formed by trilobites. Examples of trace fossils are ment is known as bioturbation, and is a very com-
shown in Fig. 11.14. mon feature in sedimentary rocks. In fact, the absence
Trace fossil assemblages that occur along shore- of bioturbation in shallow marine deposits may be
lines may be subdivided according to the degree taken as an indicator of something unusual about
of consolidation of the substrate. Along sandy shore- conditions, such as an anoxic sea floor. The intensity
lines Skolithos ichnofacies are characteristic. This of bioturbation in a body of sediment is an indication
facies is named after simple vertical tubes formed by of the number of animals living there and the length
organisms that lived in the high energy region of the of time over which they were active (Droser & Bottjer
foreshore. In this assemblage Ophiomorpha also occur, 1986). A scale of bioturbation intensity has been
a larger, mainly vertical burrow lined with faecal
devised to allow comparison between deposits in dif-
pellets, and Diplocraterion, a U-shaped burrow. The
ferent places.
animals that formed Skolithos, Ophiomorpha and Diplo-
Grade 1: a few discrete traces
craterion are thought to have moved up and down in
Grade 2: bioturbation affects less than 30% of the
the sediment with the changing water level of the
sediment, bedding is distinct
foreshore. Where the sediment is semiconsolidated
Grade 3: between 30% and 60% of the sediment
the Glossifungites ichnofacies assemblage occurs: the
affected, bedding is distinct
burrows are similar in form to those of the Skolithos
Grade 4: 60% to 90% of the sediment bioturbated,
assemblage but they tend to have sharp, well-defined
bedding indistinct
margins to the tubes and may extend into excavated
Grade 5: over 90% of sediment bioturbated, and bed-
dwelling cavities. Some organisms (such as bivalves,
ding is barely detectable
echinoids and some sponges) are able to bore into
Grade 6: sediment is totally reworked by bioturbation
rock to create dwelling traces: this assemblage is
It should be noted that when a body of sediment is
called Trypanites.
wholly bioturbated it can be difficult to recognise
In the shoreface zone of the shelf, the Cruziana
individual traces, and sometimes difficult to recognise
assemblage includes Cruziana itself, Rhizocorallium, that there is bioturbation present at all. The sediment
an inclined U-shaped burrow, Chondrites, a vertically will simply appear to be structureless, with the only
branching small burrow, Planolites, a horizontal evidence of trace fossils being that the sediment
branching burrow and Thalassanoides, larger appears to be slightly mottled or with patches of dif-
(>10 mm diameter) burrows in a complex three- ferent grain sizes.
dimensional network. In the deeper waters of the
outer bathyal zone the Zoophycos assemblage is the
characteristic ichnofacies. Zoophycos has a rather
variable, partly radial form that may be tens of 11.7.4 Trace fossils and rates
centimetres across. Few other trace fossils are of sedimentation
found in these depths. In the deeper bathyal to
abyssal depths the Nereites ichnofacies assemblage Ichnofacies can be used as indicators of the degree of
traces are characteristically feeding traces showing consolidation of the substrate (Fig. 11.13) and this
regular patterns. These include Helminthoidea, can be a useful tool in the analysis of a stratigraphic
which, like Nereites is a looping surface trace, and succession. Where rates of sedimentation are high,
the enigmatic Palaeodictyon which has a regular hex- the sea floor is covered by loose sandy or muddy
agonal pattern. The regular structure of the traces of material and a variety of ichnofacies occur according
this ichnofacies is attributed to the scarcity of nutri- to the water depth. Sediment exposed on the sea floor
ents and the need to move efficiently; in shallower, starts to consolidate if the rate of sedimentation is