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Accueil du site → Doctorat → Royaume-Uni → 1989 → Pediments of the Al Aqiq and Al Jobub areas, South-West Saudi Arabia

University of Lancaster (1989)

Pediments of the Al Aqiq and Al Jobub areas, South-West Saudi Arabia

Sadah, A.S

Titre : Pediments of the Al Aqiq and Al Jobub areas, South-West Saudi Arabia

Auteur : Sadah, A.S

Université de soutenance : University of Lancaster

Grade : Doctor of Philosophy (PhD) 1989

Résumé
This thesis presents a detailed account of the gcomorphology of five pediments located in the Al Aqiq and Al Jobub areas of south-western Saudi Arabia. The thesis comprises three introductory chapters, five core chapters and a conclusion. The five core chapters provide a detailed analysis of pediment profile form, rcgolith clast shape and fabric studies, rcgolith particle size analysis and a chemical and mineralogical study of pediment saprock.Pediment form was measured by detailed theodolite survey and the results analysed by profile curvature studies and regression methods. Most pediments were gently concave. Pediment length and pediment slope were found to have a significant negative correlation.Most clasts in transit over the pediment surfaces were found to be angular to sub-angular blades. In spite of the relatively short distances, and low angles, involved it was found that pediment clasts became rounder down pediment.Two-and three-dimensional fabric studies of regolith clasts indicated that the regolith is well organised. In all cases two-dimensional fabrics indicated a strong down-pediment resultant for the c-axcs. Three-dimensional fabrics showed a strong, high-angle up-pcdinienl dip. This is inconsistent with mudflow fabrics but probably consistent with those generated in sheetflood environments.Particle size studies showed a clear down-pediment decrease in the regolith particle size. The rate of decrease was similar to the rate of pediment angle decline and it is concluded that the pediments arc acting as transportation slopes. No significant differences between pediments was detected using ANCOVA.Abrasion pH measurements of saprock samples generally declined down-pediment suggesting that the pediment surface was more weathered away from the piedmont angle. Conductivity levels of saprock pastes were low and now spatial variation was detected. Thin-sections of saprock showed that the biotites were particularly weathered. This is probably the reason for the production of grus which litters most pediment surfaces. XRD analyses revealed that kaolinite and montmorillonite were the dominant clay minerals in the saprock. Kaolinite appears to become more common away from the piedmont angle.It is concluded that the pediments are the modified margins of ctchplains. In the present-day arid environment occasional shectfloods strip and level the pediment surfaces and help extend the pediment toward the mountain front.

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