Page 159 - Sedimentology and Stratigraphy

146 Rivers and Alluvial Fans

winnow the surface, removing fine-grained material plant or animal is covered by sediment soon after
from it. A desert varnish (8.2) may be seen on gravels death. Faunal remains are therefore relatively rare,
on fans formed in arid environments. occurring as scattered bones or teeth of vertebrates,
but plant fossils are more common and may be locally
abundant. Fossilised tree; stumps may be preserved
9.5.4 Controls on alluvial fan deposition in situ (in place) in overbank deposits as a result of
flood events that partially buried the tree; other plant
Although alluvial fan deposits are not the most sig- parts such as pieces of branches and leaves occur
nificant in a sedimentary basin in terms of volume, within beds of both channel and overbank sediments.
they are important because fan deposition is sensitive The most abundant plant fossil remains are those of
to tectonic and climatic controls. Alluvial fans develop pollen and seeds (palynomorphs) that are highly resis-
at the margins of sedimentary basins and these can be tant to breakdown and can survive long periods of
sites of tectonic activity, with faults along the basin transport before being deposited and preserved. This
margin creating uplift of the catchment area and makes them particularly useful for dating and corre-
subsidence in the basin (see Chapter 24). It is there- lation of terrestrial deposits (20.5.3).
fore possible to see evidence of tectonic activity within The footprints of animals in soft mud have a good
an alluvial fan succession, such as an influx of coarse preservation potential if the mud dries hard and is later
detritus onto the fan resulting from renewed tectonic covered with sand. These are examples of trace fossils
uplift (Heward 1978; Nichols 1987). Analysis of the (11.7) that in continental environments are largely
bed thicknesses and clast sizes within beds can there- restricted to floodplains and alluvial plains. Trace fos-
fore be used as a means of identifying periods of sils in these environments may range from the tracks
tectonic uplift in the high ground adjacent to the of animals such as dinosaurs to the burrows and nests
basin. A change in climate can also result in changes of insects such as beetles, bees and ants (Hasiotis
in the processes of deposition on a fan (Harvey et al. 2002). These traces provide information about the
2005): for example, with an increase in rainfall more palaeoenvironment, such as the level of the palaeo-
water is available and this may result in a predomi- water table: an ant or termite nest will be constructed
nance of sheetflood and stream-channel processes, only in dry sediment, so the presence of these and
with less debris-flow events occurring. The character other structures formed by insects is a reliable indica-
of the conglomerates deposited on the fan will reflect tor of how wet or dry the land surface was and hence
this climatic change, with more clast-supported and provides some information about the palaeoclimate.
fewer matrix-supported conglomerate beds. A further Trace fossils in continental environments have also
factor controlling fan deposition is the nature of the provided valuable information about the morphology
bedrock in the catchment area: lithologies that of extinct organisms. Footprints of dinosaurs, for
weather to form a lot of mud will tend to generate example, can provide an indication of the way that
muddy debris flows, whereas more resistant rocks will the animal walked in a way that the skeletons (which
break down to sand and gravel, which is transported are often incomplete) cannot.
and deposited by sheetflood and stream-channel pro-
cesses (Blair 2000a, Nichols & Thompson 2005).
9.7 SOILS AND PALAEOSOLS
9.6 FOSSILS IN FLUVIAL AND ALLUVIAL 9.7.1 Soils
ENVIRONMENTS
A soil is formed by physical, chemical and biological
In comparison to marine settings the terrestrial envi- processes that act on sediment, regolith or rock
ronment has a poor potential for the preservation of exposed at the land surface (Retallack 2001). Collec-
fossil plants or animals. An organism that dies on the tively these soil-forming processes are known as ped-
land surface is susceptible to scavenging by carrion or ogenesis. Within a layer of sediment the principal
the tissue will be broken down by oxidation. Preserva- physical processes are the movement of water down
tion occurs only if the organism has very resilient from or up to the surface and the formation of vertical
parts (e.g. teeth and bones of vertebrates) or if the cracks by the shrinkage of clays. Chemical processes