Slopewash Deposits Research Articles

Weathering and geochronology of a Quaternary paleosol sequence in Lower Teleki Valley, Mount Kenya

A sequence of paleosols, including residual regolith (saprolite), till, aeolian, volcaniclastic, colluvial and slope-wash deposits, dating from the lower Brunhes Chron (<730 000 yr), was investigated to reconstruct the weathering effects and climatic intensities that occurred during three interglacial periods on Mount Kenya. Overall, weathering appears to have been least intense during the Lake Ellis-Naro Moru Interglaciation, and intensified progressively during the Naro Moru/Teleki and Teleki-Liki interglaciations. In comparison with weaker postglacial weathering of sediments emplaced close to the end of the Liki Glaciation (= Würm), each interglacial episode left a more intense weathering imprint, which probably resulted from wetter conditions for long periods of time. Scanning electron microscopy of the sand fractions in these paleosols reveals the full extent of weathering differences between interglacial and interstadial climates.

Studies on a Gelifluction Lobe, Jotunheimen, Norway: 14C Chronology, Stratigraphy, Sedimentology and Palæoenvironment

A turf-banked gelifluction lobe in south-central Norway is described together with detailed 14C dating of buried soil material, measurements of current activity, and sedimento-logical, geotechnical and micromorphological analyses. Three stratigraphic units are recognised; a silty sand upper unit (Unit 1) overlying heterogeneous sandy sediments containing organic bands up to 2 cm thick (Unit 2), which in turn overlies a buried soil of thickness 75 cm (Unit 3, the Main Buried Soil) developed on the underlying till. Analyses indicate that the gelifluction and frost creep responsible for the formation of this lobe affected not only till but also colluvial (slope wash) deposits. Particularly in Unit 2, well-sorted layers of fine sand and coarse silt alternating with organic bands, indicate episodic accumulation of colhivium separated by intervals of pedogenesis.Nineteen 14C dates were obtained from eight points along the length of the Main Buried Soil and from a vertical section through the lobe, inclu...

A consistency index and rainwash mechanisms on Belgian loamy soils

AbstractThe effects of soil crusting in Belgium are particularly conspicuous on the loamy plateaus and hill‐regions of southern Flanders, Brabant and Hesbaye. Field observations, made at the end of rainy winter seasons, permitted the making of a simple distinction between: (a) ‘stable’ soils showing crusting on less than 50 per cent of their total surface, without any evident slope wash, and (b) ‘unstable’ soils, crusting over more than 50 per cent of the total surface, with evident rainwash and basal colluviation.The Atterberg limits have been examined for both soil types. It was found that the upslope part of the liquid limit curves, in the range of low consistencies (high water contents), is steeper for stable than for unstable soils. So a distinctive consistency index C2–10 was deduced from the liquid limit curves, which may be used for prediction of the relative sensitivity of topsoils for crusting. Stable, well‐structured clods are more hydrophobic than unstable material. This was made clear by wettability tests under a rain simulator. Therefore more water must be added to remolded, stable material to lower its consistency, as is shown by the liquid limit curves.Crustability decreases and C5–10 values increase with increasing clay and organic matter contents, as one would expect. Stable soils are well aggregated. But rain simulator tests also showed that the detachment of this material by raindrop impact is high. Therefore, crumbs and clods of stable soils are relatively more sensitive to splash erosion. Their erosion results from the ejection of stable aggregates. Clods of unstable topsoils are compacted and flattened by raindrop impact, which causes internal liquefaction.Micromorphological analysis indicates that typical stable topsoils originate from well‐structured Ap‐horizons and from Bt‐horizons of grey‐brown podzolic soils (Alfisols). Laminated silt and mudcrust fragments characterize unstable topsoils which originate, at least partly, from slopewash deposits. The external stability of both stable and unstable clods is increased by compaction and by surficial concentration of gravel, coarse sands, root and other plant fragments. Thin sections of unstable material show the internal collapse of structures, due to liquefaction.A clear distinction must be made between the structural stability and the erodability of topsoils.

Stratigraphy and Petrography of North-East Otago Loess

Abstract An extensive loess mantle up to 36 ft thick, but generally less than 15 ft thick, covers much of north-east Otago, especially on the intermediate and high-level terraces south of the Waitaki River from Duntroon to the coast, and in the vicinity of Cape Wanbrow, near Oamaru. This loess is a pale yellow, angular, poorly sorted, positively skewed, medium silt. It is composed predommantly of quartz, feldspar (andesine?), accessory muscovite, and about 4% heavy minerals. The minerals are consistent with derivation from either a greywacke or schist terrain. Both slope-wash deposits and loess are discussed, because on steeper slopes slope-wash replaces loess deposition when the climate is severe. Hence, from Livingstone, a sequence of three loesses and two slope-wash deposits are described, and an attempt is made to correlate these units with loesses at Five Forks, and Maheno. The sequence of terrace gravel, loesses, and slope-washes, with soil development at one major and one minor horizon, is interpre...