The recent morpho-tectonic history of the Vaalputs radioactive waste repository and environs
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Date
2016-08-18
Authors
Brandt, Dion
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Abstract
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.
Description
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