Alluvial zones

Contrary to popular concepts, sands do not always dominate arid and semi-arid zones. Aridisols occur on a wide variety of landforms, lithological formations and are of different ages. They are most common on stable land surfaces of Late Pleistocene or greater age in tectonically active deserts where they comprise alluvial fan, alluvial flats or stream terraces. Arid zones also include mountainous terrain with steep slopes (Nettleton and Peterson, 1983). Many arid or semi-arid zones include fluvial and aeolian materials of Pleistocene age. 

Moreover, salts in arid soils increase concentrations of boron. In Israeli soils, total B content in saline alluvial soil is in the range of 150-170 mg/kg, four to six times higher than that in alluvial soils (25-40 mg/kg) (Aubert and Pinta, 1977). The soluble boron contents tend to increase with decreases in rainfall. Bioavailable Mo represents an average 2-20% of total Mo. The percentages of bioavailable Mo are higher in arid and semi-arid zones than in humid zones. Bioavailable Mo is 35.8% of the total Mo in soils of Israel and increases to 50-60% in saline alkali soils in India (Aubert and Pinta, 1977). Total Se concentrations in the western United States are higher... 

Large regional-scale dissolved contaminant plumes have been reported in many parts of the country. In California, many of the alluvial basin aquifers (i.e., major groundwater basins) were found to be impacted by chlorinated solvents during the period between 1979 to 1981. Beneath one valley a dissolved TCE plume incorporated the upper 50 ft of the saturated zone of a water table aquifer, was over 3 miles wide, and extended hydraulically downgradient over 14 miles. 

Relatively low permeability alluvial soils (silt, clay, and sandy clay), which corresponds with perching layers or aquitards. These soils were interpreted to be deposited in a low-energy, suspended load, fluvial-deltaic environment outside of channel zones. 

Surficial deposits at XY are dominated by tills and colluvium, which have been locally reworked by alluvial and glacio-fluvial processes (Figs. 1 2a). On the transect reported herein, the depth of overburden varies from negligible on mountain sides to at least 5 m thick in central parts of the till plain in central valley areas (unknown maximum thickness). In the section of the traverse the XY ore zone is c. 10 m thick, and occurs below c. 3 m of colluvial overburden (Fig. 2). Pedogenesis is generally restricted to minor to moderate horizonation that has produced predominantly brunisols (Canadian Soil Classification System, Agri-Canada 1998). 

A final example comes from the Nima basin located 450 km northwest of Lhasa at about 4500 m, in the southern part of the Bangong suture zone, which separates the Qiangtang and Lhasa terranes in central Tibet (Fig. lb). The southern Nima basin contains more than 4 km of Tertiary alluvial, lacustrine, and lacustrine fan-delta deposits that accumulated next... 

Deltas are generally divided into the following physiographic zones alluvial feeders delta plain delta front and prodelta/delta slope (figure 2.11 Coleman and Wright, 1975). The alluvial feeder is a valley within the drainage basin that supplies the water and sediment to the delta. Some of the alluvial feeders consist of a single channel or multiple channel (line-source) to the coast (Hart, 1995). The upper delta plain is an older section of... 

Virtually over its entire extension, the continental slope of the Black Sea is dissected by numerous faults and underwater canyons. These canyons, confined to tectonic dislocations (fracture zones or grabens), are later transformed by turbidity flows, which use them as channels for the transport of mineral particulate matter from the near-shore zone to the foot of the continental slope. At the places of discharge of turbidity flows, alluvial fans are formed, which may be cut by runoff channels [10]. 

The northwestern region is characterized by large provenance areas, from which alluvial matter is supplied via the deltas of the Danube, Dniester, and Dnieper rivers. The Danube River, which features a wide delta with numerous channels and branches provides the greatest solid runoff to the shelf zone. The delta is composed of terrigenous sediments and is advanced toward the sea by 7-10 km. 

Figure 3 Sheigra paleosol (bleached and reduced zone 1 m thick to right) under Torridonian (1,000 Ma) alluvial fan deposits, and Staca paleosol at same unfoncormity but on amphibolite (left-hand side) near the hamlet of Sheigra, northwest Scotland (photo courtesy of G. E. Williams geological age revised hy Williams and Schmidt, 1997).

Figure 2.1 Settings for calcrete development. In fluvial settings pedogenic calcretes can develop on floodplains and terraces, whereas groundwater calcretes may form in channel deposits or around the capillary fringe and upper part of the phreatic zone in more permeable parts of the floodplain. In alluvial fans paired calcretes may develop on the fans, with hydromorphic calcretes near discharge zones.

Figure 2.9 Characteristics of groundwater calcretes. (A) Generalised macroscopic features based on various sources. Some workers, such as Carlisle (1980), identify two zones in the massive phreatic unit an upper earthy zone with remnant soil and alluvium, and a lower, dense porcellaneous zone with abundant cracks and cavities. Phreatophytic plants may also produce features such as rhizocretions and laminar rhi-zolite crusts (Seminiukand Meagher, 1981). (B) Generalised model for the evolution of groundwaters and their precipitates in semi-arid to arid alluvial systems based on Arakel (1986).

About 25% of the State of Sao Paulo is covered by outcrops of the crystalline basement. The remaining area is covered by sedimentary rocks of the Parana Basin and the small Tertiary basins of Sao Paulo and Taubate, plus other restricted zones of coastal sediments and alluvial deposits. 

This is a zone of poorly drained alluvial-lacustrine plains, where features of continental salt accumulation are clearly expressed. Broad-Leafed Aspen Forest and Meadow Steppe ecosystems on Phaeozems and Chernozems with association with meadow-bog soils and Solods are characteristic of this region. 

Leeder, M.R. (1975) Pedogenic carbonates and flood sediment accretion rates a quantitative model for alluvial arid-zone lithofacies. Geol. Mag., 112(3), 257-270. 

Chlorite replaces kaolinite, clay pseudomatrix, infiltrated clays, micas and heavy minerals. It occurs as rims composed of platelets oriented perpendicularly to grain surfaces. The rims were formed by replacing infiltrated smectitic clay coatings, which were originally oriented tangentially to grain surfaces (Fig. 14A) (see Moraes De Ros, 1990). These infiltrated clays were presumably introduced into the vadose zone of alluvial continental sediments under semi-arid conditions by episodic floods (Walker et al., 1978 Moraes De Ros,... 

Field studies have ascribed the loss of Cr(VI) in alluvial groundwater systems to the reduction to Cr(III) on magnetite surfaces 26, 27), The latter study, which involved the injection of a Cr(VI) tracer into a well field at Cape Cod, Massachusetts demonstrated the strong attenuation of Cr(VI) in the suboxic zone. Breakthrough... 

In special circumstances such as footing for tank on soft alluvial ground, or deep basement with heavy loads over karstic limestone formation, raft and pile raft foundations, and barrette piles are used. To overcome the potential danger from slump zone at the contact between the pile toe and the limestone, compaction grouting is often carried out to infill the slump zone. 

---