The recent morpho-tectonic history of the Vaalputs radioactive waste repository and environs

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2016-08-18

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Brandt, Dion

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This study deals with a region in the Northern Province, centred around Vaalputs, the National Radioactive Waste Disposal site. The study area constitutes two distinct geomorphological terrains: the western section consists of rugged, mountainous, granitic terrane, with steep cliffs that include weathered and silicified basement; the eastern section consists of the generally reatureless, gently undulating Bushmanland Plateau. The latter is characterized by low-amplitude palaeo-dunes which have a north-northeasterly trend. Precambrian crystalline rocks belonging to the Namaqualand Metamorphic Complex and late Palaeozoic (Karoo) rocks form the basement to Cretaceous and Cenozoic deposits in the region. Seismic studies in the Vaalputs area have indicated that there is appreciable seismic activity over a broad region of the northern Cape. In addition, satellite images and aerial photographs have recorded the presence of a network of lineaments in recent cover. The objectives of this study were to obtain a better understanding of the Late-Mesozoic and Cenozoic tectonic history of the region. Geomorphological, sedimentological and geological episodes were identified, described and correlated to the major tectonic events in the area. During the Late-Cretaceous the Dasdap Formation was deposited as a result of uplift along the marginal escarpment (west coast), in an alluvial fan setting. Crater sediments overlying olivine melilitites and kimberlites were also deposited contemporaneously with the Dasdap Formation. The late Cretaceous saw tropical conditions which caused extensive deep weathering and silicification of the Namaqualand Metamorphic Complex and the Dasdap Formation. This wet period was terminated approximately at the end of the Cretaceous with the onset of aridification. Tectonism largely in the form of reactivated older structures caused differences in the elevation of the palaeo-weathered and silicified surfaces and the initial deepening of a north-northwest oriented graben. Continued deepening of the graben during the Tertiary allowed for the deposition of the fluvial (floodouts) and aeolian Vaalputs sediments. Calcrete horizons preserved in the Vaalputs sequence of sediments indicate drier interludes and define previous land surfaces during tectonically less active periods or during periods of lower sediment supply. Postdepositional alteration and bioturbation resulted in the generally structureless and massive sediments of the Vaalputs Formation Overall desiccation throughout the Cenozoic finally gave rise to the aeolian dominated Gordonia Formation and present day microenvironments, consisting predominantly of aeolian deposits, deflation pans, and lag deposits. The source of the Gordonia sediments is primarily the underlying Vaalputs Formation which is in a state of degradation. Surface processes (primarily aeolian) have reworked and modified the surface into longitudinal dunes with a transverse component. Since the Cretaceous two primary stress fields have given rise to the numerous tectonic products observed in the study area. The study area is affected by two important uplift axes: the Griqualand-Transvaal and the Kamiesberge axes, which were active between the Miocene and the Plio-Pleistocene. On a regional scale the larger and older Pan African age (700 Ma.) faults are predominantly north-northwesterly trending and have been reactivated. At about 60 Ma ago, the minimum principal stress was most probably oriented to the NW causing extensive dip-slip faulting with a northeasterly strike. The stress field, active during most of the Cenozoic, had an ENE oriented minimum principal horizontal stress, and accompanied the development of a fault-bound sedimentary basin, which was filled by the sediments of the Vaalputs Formation. This stress field is probably still active as evidenced by numerous recent structural features.

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Thesis submitted to the Faculty of Science, University of the Witwatersrand, in fulfilment of the requirements for the degree of DOCTOR OF PHILOSOPHY Johannesburg, 1998

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