Page 130 - Sedimentology and Stratigraphy

Characteristics of Wind-blown Particles 117

become well-rounded, or even very well-rounded. tures that have been inherited from the earlier stage,
Grain roundness is therefore a characteristic that or cycle, of deposition (2.5.4).
can easily be seen in hand specimen using a hand
lens, or will be evident under the microscope if a
thin-section is cut of an aeolian sandstone. Inspection 8.3.2 Composition of aeolian deposits
using a hand lens reveals another feature, which is
more obvious if the grains are examined by scanning The abrasive effect of grain impacts during aeolian
electron microscopy (SEM) (2.4.4): the grain surfaces transport also has an effect on the grain types found
will have a dull, matt appearance that under high in wind-blown deposits. When a relatively hard
magnification is a frosting of the rounded surface. mineral, such as quartz, collides with a less robust
This is a further consequence of the impacts suffered mineral, for example mica, the latter will tend to
during transport and grain surface frosting is also a suffer more damage. Abrasion during transport by
characteristic of aeolian processes. Aeolian dust wind therefore selectively breaks down the more labile
shows similar grain characteristics but features on grains, that is, the ones more susceptible to change. A
these sizes of grains can be recognised only if viewed mixture of different grain types becomes reduced to a
under the high magnifications of an SEM. grain assemblage that consists of very resistant
A wind blowing at a relatively steady velocity can minerals such as quartz and similarly robust lithic
transport grains only up to a particular size threshold fragments such as chert. Other common minerals,
(Nickling 1994), and large, heavier grains are left for example feldspar, are likely to be less common in
behind. Grains close to the threshold for transport aeolian sandstones, and weak grains such as mica are
are carried as bedload and deposited as ripples and very rare. A deposit with this grain assemblage is
dunes (4.3.1 & 4.3.2), whereas finer grains remain in considered to be compositionally mature (2.5.3), and
suspension and are carried away. This effective and this is a common characteristic of aeolian sandstone.
selective separation of grains during transport means Most modern and ancient wind-deposited sands are
that aeolian deposits are typically well-sorted (2.5). quartz arenites.
Sands in dunes are normally fine to medium grained, In places where loose carbonate material is exposed
with no coarser grains present and most of the finer on beaches, the sand-sized and finer sediment can be
grains winnowed away by the wind. This winnowing transported and redeposited by the wind. If the wind
effect, the selective removal of finer grains from the direction is onshore, wind-blown carbonate sands can
sediment in a flow, also occurs in water flows, but is accumulate and build up dune bedforms. Dunes built
more effective in the lower density and viscosity med- up of carbonate detritus have many of the same char-
ium of air. acteristics as a quartz-sand dune, and are several
A clastic deposit that consists of only sand-sized metres high with slip faces dipping at around 308
material, which is well sorted and with well-rounded creating large-scale cross-bedding. The clasts may be
grains, is considered to be texturally mature (2.5.3). ooids, bioclasts or pellets, depending upon what is
Aeolian sandstones are, in fact, one of very few available on the beach, and are well-rounded and
instances where granulometric analysis (2.5.1) pro- well-sorted; if the clasts are bioclastic they will com-
vides useful information about the depositional envi- monly have a relatively low density, so wind-blown
ronment of the deposit. There is, however, a need for grains may be very coarse sand or granule size. Wind-
caution when using petrographic characteristics blown carbonates may accumulate in temperate as
alone as an indicator of environment of deposition. well as tropical settings: they are most commonly
Consider an area of bedrock made up of sandstone found near to coasts, but may also occur tens of kilo-
deposited in a desert tens or hundreds of millions of metres inland. Loose carbonate grains on land are
years ago. After deposition it was buried and lithified, subject to wetting by the rain and subsequent drying
then uplifted and eroded. The sand that is being in the sun; this leads to local dissolution and re-
weathered off this bedrock will have the characteris- precipitation of carbonate, which results in rapid
tics of the deposits of an aeolian environment, but is formation of cements and lithification of the sedi-
presently being transported and deposited by streams ment. Aeolian carbonate deposits are therefore more
and rivers in a very different climatic and depositional stable features than dunes made of quartz sand.
setting. In these circumstances the sands have fea- Lithified wind-blown carbonate deposits are termed