Page 114 - Sedimentology and Stratigraphy

Further Reading 101

Analysis are therefore thermochronological techni- 6.9 DENUDATION AND SEDIMENT
ques which make it possible to determine at what date SUPPLY: SUMMARY
in the past a crystal was at a certain temperature.
Converting thermochronology data into rates of The flux of material as bedload, suspended load and
erosion requires knowledge of the geothermal gra- ions in solution to depositional environments is a
dient, that is, the change in temperature with depth primary control on the character of the sediments
in the crust. In many parts of the world, the tempera- and facies that ultimately form. Thick successions of
ture increases by about 258 for every thousand metres evaporite minerals cannot precipitate in lacustrine
with depth, a geothermal gradient of 258 per kilo- environments (Chapter 10) without an abundant
metre, but is higher in places where there is volcanic supply of the relevant anions and cations from rivers
activity. A rock that is at 4 km depth will be at about draining nearby uplands. The characteristics of del-
1208C (assuming a surface temperature of 208C and taic facies are fundamentally controlled by the grain
an increase of 258C with every kilometre), and there- size of the sediment supplied (12.4), and, in fact, a
fore all the fission tracks in apatite crystals will be delta can only form if there is sufficient sediment
annealed. Tectonic movements may cause the body of supply in the first place. Carbonate-forming environ-
rock to be uplifted, and then, as the rock above the ments on shallow marine shelves can exist only in
sample is removed by erosion, it will start to cool as it places where there is a reduced flux of terrigenous
comes closer to the land surface. Fission tracks can clastic material (Chapter 15). The starting point in
then start to form in the apatite crystals in the sample, any holistic view of depositional systems is therefore
and continue to form until all the rock above has been the source of the sediment and the linked tectonic and
eroded away and the sample is at the surface, avail- climatic processes that ultimately control the denuda-
able for collection and analysis. Measurement of the tion of continental landmasses.
fission tracks can therefore tell us when the sample
was at a certain depth, and hence how long it has
taken to erode the rocks above: this provides us with FURTHER READING
an indication of the rate of erosion.
Thermochronological techniques also include the Einsele, G. (2000) Sedimentary Basins, Evolution, Facies and
Sediment Budget (2nd edition). Springer-Verlag, Berlin.
use of Ar–Ar dating (21.2.2) and there are relatively
Molnar, P. & England, P. (1990) Late Cenozoic uplift of
new techniques such as U/Th–He. Using combina-
mountains ranges and global climate change: chicken or
tions of approaches makes it possible to determine
egg? Nature, 346, 29–34.
the dates when the rocks were at different tempera- Ollier, C.D. (1984) Weathering. Longman, London.
tures and hence different depths. Statistical modelling Selby, M.J. (1994) Hillslope sediment transport and deposi-
of fission track data can be used to create a geother- tion. In: Sediment Transport and Depositional Processes
mal history of rock samples and hence a history of (Ed. Pye, K.). Blackwell Scientific Publications, Oxford;
erosion in an area. 61–88.