Page 140 - Sedimentology and Stratigraphy

Further Reading 127

terrigenous dust can all be distinguished by geochem- in periglacial regions. Wind blowing over the out-
ical analysis. wash plain can pick up sand and redeposit it locally,
Aeolian dust is dispersed worldwide, but most of it usually against topographic features such as the side
ends up in other marine and continental depositional of a valley. These patches of aeolian sand may there-
environments where it mixes with other sediment and fore occur intercalated with fluvio-glacial facies
its origin cannot easily be determined. In most places (7.4.3), but rarely form large deposits.
the proportion of aeolian dust is very low compared
with other sediment being deposited, but there are
some environments where terrigenous clastic deposi- 8.7 SUMMARY
tion is very low, and the main source of silt and clay
can be aeolian dust. Limestones formed in carbonate- Aeolian deposits occur mainly in arid environments
forming environments can usually be shown to con- where surface water is intermittent and there is little
tain a residue of dust if the calcium carbonate is dis- plant cover. Sands deposited in these desert areas are
solved, and dust settled on ice sheets and glaciers may characteristically both compositionally and mineralo-
be seen as layers within the ice. The parts of the deep gically mature with large-scale cross-bedding formed
oceans that are distant from any continental margin by the migration of dune bedforms. Oxidising condi-
receive very little sediment (16.5): airborne dust that tions in deserts preclude the preservation of much fossil
settles through the water column can therefore be an material, and sediments are typically red–yellow col-
important component of deep ocean deposits. ours. Associated facies in arid regions are mud and
evaporites deposited in ephemeral lakes and poorly
sorted fluvial and alluvial fan deposits. Aeolian depos-
8.6.2 Aeolian sands in other environments its are less common outside of desert environments,
occurring as local sandy facies associated with beaches
Beach dunes and glaciers, and as dust distributed over large dis-
Sand dunes built up by aeolian action can form adja- tances into many different environments, but, apart
cent to beaches in any climatic setting. In the inter- from Quaternary loess, rarely in significant quantities.
tidal zone of a foreshore loose sediment is subaerially Characteristics of aeolian deposits
exposed at low tide, and as it dries out it is available to . lithologies – sand and silt only
be picked up and redeposited by the wind. Beach dune . mineralogy – mainly quartz, with rare examples of
ridges form where the foreshore sediments are mainly carbonate or other grains
sandy, exposed at low tide and subject to removal by . texture – well- to very well-sorted silt to medium sand
onshore winds. The sand then accumulates at the . fossils – rare in desert dune deposits, occasional
head of the beach, either as a simple narrow ridge or vertebrate bones
sometimes extending for hundreds of metres inland. . bed geometry – sheets or lenses of sand
In humid climates the dunes become colonised by . sedimentary structures – large-scale dune cross-
grasses, shrubs and trees that stabilise the sand and bedding and parallel stratification in sands
allow the ridges to build up metres to tens of metres . palaeocurrents – dune orientations reconstructed
thickness. The roots of these plants and burrowing from cross-bedding indicate wind direction
animals disrupt any depositional stratification, so the . colour – yellow to red due to iron hydroxides and
cross-bedding characteristic of desert dunes may not oxides
be preserved in beach dune ridges. The association of . facies associations – occur with alluvial fans,
beach dune ridges with other coastal facies is dis- ephemeral river and lake facies in deserts, also with
cussed in 13.2.1. beach deposits or glacial outwash facies
Periglacial deposits
FURTHER READING
Glacial outwash areas (7.4.3) are places where loose
detritus that has been released from melting ice Glennie. K.W. (1987) Desert sedimentary environments,
remains exposed on the surface for long periods of past and present – a summary. Sedimentary Geology, 50,
time because plant growth and soil formation is slow 135–165.