Geoscience Reference
In-Depth Information
Figure 12.11
Examples of the pedogenic features and landforms associated with chemical sedimentation along dryland rivers and
on floodplains. (a) Schematic cross-section across a typical ephemeral river in the Negev Desert, Israel, showing the location and
relationship of the 'fluvial active unit' (FAU) and 'fluvial pedogenic unit' (FPU) in the channel and the palaeo-FAU and palaeo-FPU
in late Pleistocene and Holocene alluvial terraces. The presence of the palaeo-FPU indicates that the hydrologic regime of past
floods was similar to the present, with arid conditions prevailing during their formation. On the alluvial terrace, a gypsic-saline
reg soil has developed on the calcium carbonate-coated clasts that represent the palaeo-FPU (from Amit
et al.
, 2007). (b) Oblique
aerial view illustrating the numerous elevated, near-circular islands with woody fringes and sparsely vegetated or barren interiors
on a broad unchannelled floodplain (floodout) in the Nyl River valley, northern South Africa. During extensive flooding, sheetflow
occurs between the islands with the flow direction towards the camera. (c) Diagrams illustrating the location of drill holes and
distribution of ions beneath two islands on the unchannelled floodplain in the Nyl River valley: sodium (Na
2
O, wt %), chloride (Cl,
ppm), calcium (CaO, wt %). Elevated levels of sodium and chloride are found beneath the islands owing to transpirative losses
induced by the woody vegetation, but there is no difference in the abundance of calcium (from Tooth
et al.
, 2002b). Reproduced
by permission Taylor & Francis Ltd., http://www.informaworld.com.
and the limited moisture left in the sediment is sufficient
for calcium carbonate deposition (Lekach
et al.
, 1998;
Amit
et al.
, 2007). Extreme cases of alluvial induration
can occur where dryland rivers experience more seasonal
flow regimes (Nanson, Tooth and Knighton, 2002), as
sufficient warmth and moisture can lead to pronounced
chemical alteration of bedrock and alluvium, particularly
reactive lithologies, such as igneous rocks or carbonates.
Chemical weathering releases certain elements in solu-
tion that are transported along moisture gradients and
may be reprecipitated locally to form indurated or ce-
mented horizons of ferricrete, silcrete, calcrete, gypcrete
or halite, depending on local conditions of aridity and pH.
Such induration, and in extreme cases lithification, of al-
