Ground-penetrating radar and its use in sedimentology: principles, problems and progress

Luminescence dating : Laboratory procedures and protocols

Evidence increasingly suggests that sub-Saharan Africa is at the center of human evolution and understanding routes of dispersal “out of Africa” is thus becoming increasingly important. The Sahara Desert is considered by many to be an obstacle to these dispersals and a Nile corridor route has been proposed to cross it. Here we provide evidence that the Sahara was not an effective barrier and indicate how both animals and humans populated it during past humid phases. Analysis of the zoogeography of the Sahara shows that more animals crossed via this route than used the Nile corridor. Furthermore, many of these species are aquatic. This dispersal was possible because during the Holocene humid period the region contained a series of linked lakes, rivers, and inland deltas comprising a large interlinked waterway, channeling water and animals into and across the Sahara, thus facilitating these dispersals. This system was last active in the early Holocene when many species appear to have occupied the entire Sahara. However, species that require deep water did not reach northern regions because of weak hydrological connections. Human dispersals were influenced by this distribution; Nilo-Saharan speakers hunting aquatic fauna with barbed bone points occupied the southern Sahara, while people hunting Savannah fauna with the bow and arrow spread southward. The dating of lacustrine sediments show that the “green Sahara” also existed during the last interglacial (∼125 ka) and provided green corridors that could have formed dispersal routes at a likely time for the migration of modern humans out of Africa.

https://www.pnas.org/content/pnas/108/2/458.full.pdf

Ancient watercourses and biogeography of the Sahara explain the peopling of the desert

Comparison of GIS-based landslide susceptibility models using frequency ratio, logistic regression, and artificial neural network in a tertiary region of Ambon, Indonesia

Within sedimentological studies, ground penetrating radar (GPR) is being used with increasing frequency because it yields images of the shallow subsurface that cannot be achieved by any other non-destructive method. The purpose of this paper is to provide an introduction to the collection, processing and interpretation of GPR data so that future sedimentary studies can be improved. With GPR equipment now being lightweight, robust and portable, proper data collection and survey design methods need to be followed in order to acquire high resolution, subsurface digital data. Various factors are discussed including: Reflection profiling, velocity soundings, test surveys, topography, logistics, data quality and extreme environments. Basic data processing and visualization are then reviewed, followed by a discussion on GPR interpretation strategies including a background to radar stratigraphy. For the sedimentary geologist or geomorphologist, GPR offers unique data of the shallow subsurface including stratigraphy, geometry, architecture and structure.

GPR in sediments: advice on data collection, basic processing and interpretation, a good practice guide

Vegetation of the Tembe Elephant Park, Maputaland, South Africa

The luminescence chronology of dune development on the Maputaland coastal plain, southeast Africa

Gullies (locally termed dongas) are a characteristic feature of the landscape in central KwaZulu-Natal and are highly associated with widespread, stratified colluvial sediments and the buried palaeosols that have formed in this material. Research objectives were to find out which sediment property or properties contribute most to the vulnerability of the colluvium to gully erosion; to study the erosion susceptibility of the sequence of buried stratigraphic layers and palaeosols; and to improve understanding of the relationship between erosion susceptibility of the colluvial layers and palaeosols and their disposition in the landscape. Materials from several colluvial layers and palaeosols exposed in the donga sidewall of the Dabekazi donga, 13 km north of Nqutu in central KwaZulu-Natal, were subjected to chemical and mineralogical analyses, several dispersion and erodibility tests and were characterized physically. Exchangeable sodium percentages (ESP) ranged from 0% to 23% and the dispersivity and erodibility tests revealed a range of dispersion potential and erodibility across the sequence of colluvial layers and palaeosols. However, a poor correlation was found between the results of different dispersion tests, as well as between dispersion tests and dispersion-related properties. Dispersion tests showed significant correlation with silt content. An erodibility test using a flume showed very strong correlation with electrical conductivity (EC) and sodium adsorption ratio (SAR) of the saturated paste extract and ESP values. The plasticity index ranged from 0% to 0.23 and estimated hydraulic conductivities ranged from 33 to 0.7 m/day. High correlation with ESP and EC suggests that dispersion plays an important role in the erodibility of the materials under conditions of short duration, turbulent water flow. However, colluvium in which deep gullying occurs is not necessarily strongly sodic. Also, the degree of erodibility of the materials differs depending on the test used, illustrating the complexity of the interactions that govern dispersion and erosion. The erodibility of the colluvial layers and palaeosols from the Dabekazi donga, and properties with relevance to dispersion, differ considerably down the profile, indicating that geomorphic threshold conditions and position in the landscape with respect to surface runoff hydrology also codetermine donga erosion. Hydraulic conductivity estimates of the investigated materials are relatively high and do not differ sufficiently to enable lateral subsurface flow through, and piping of, these materials to be the main causes of donga formation.

Some physical and chemical properties of sediments / exposed in a gully donga in northern KwaZulu-Natal, South Africa and their relationship to the erodibility of the colluvial layers

Hillslope deposits in northern KwaZulu-Natal, South Africa are incised by gullies (dongas) which expose sequences of buried palaeosols developed within a succession of discontinuity-bounded colluvial sedimentary units. Infilled and buried palaeogully topography attests to at least four geomorphic cycles, comprising gully cut-and-fill and palaeosol formation, on hillslopes in the region during the past 135 ka. Past attempts at dating colluvial stratigraphic units were limited to radiocarbon dating of associated palaeosol organic matter and authigenic soil carbonates. This study presents a preliminary set of Infra Red Stimulated Luminescence (IRSL) age determinations pertaining to deposition of the succession of colluvial units infilling and burying palaeogullies at several sites of stratigraphic, geochronological and palaeoenvironmental importance in the region. Interpretation of the IRSL and 14 C dates in the context of cyclical hillslope processes suggests that past episodes of palaeogully erosion and infill were not synchronous and individual palaeosol landsurfaces were diachronous during the late Pleistocene.

G.A. Botha

Papers

Vegetation of the Tembe Elephant Park, Maputaland, South Africa

The luminescence chronology of dune development on the Maputaland coastal plain, southeast Africa

Some physical and chemical properties of sediments / exposed in a gully donga in northern KwaZulu-Natal, South Africa and their relationship to the erodibility of the colluvial layers

Episodic late quaternary palaeogully erosion in northern KwaZulu-Natal, South Africa

Soil chronosequence development in dunes on the southeast African coastal plain, Maputaland, South Africa