SECTION 4: TECHNICAL ASPECTS

Semi-Regional and Site Geology

• 
  General
  

The reason for the geological, structural geological and seismo-tectonic studies is that they are a requirement of nuclear siting practices as prescribed by the Code of Federal Regulations, 10 CFR 100. This regulatory guide requires that investigations into surface faulting should include the following:-

• 
  Determination of the lithologic, stratigraphic, hydrologic (not covered by this study) and structural geological conditions at the site and in the area surrounding the site, including its geological history;
  
• 
  Evaluation of the tectonic structures underlying the site, whether buried or expressed at the surface with regard to their potential for causing surface displacement at or near the site (structural section);
  
• 
  For faults greater than 1000 feet (300m) long, any part of which is within 5 miles (8km) of the site, determination of whether these faults are considered as capable faults (structural section);
  
  
  
• 
  Listing of all historically reported earthquakes that can reasonably be associated with capable faults greater that 1000 feet (300m) long, any part of which is within five miles (8km) of the site (this is part of the Seismic Hazard Assessment).
  

(a) “It shows evidence of past movements of a recurring nature within such a period that it is reasonable to infer further movement at or near the surface can occur. In highly active areas where both historical and geological data consistently reveal short earthquake recurrence intervals, periods of the order of tens of thousands of years may be appropriate for the assessment of capability (upper Pleistocene – Holocene). In less active areas it is likely that much longer periods may be required.

It has a demonstrated structural relationship to a known capable fault such that movement of the one may cause movement on the other at or near the surface.

(c) The maximum potential earthquake associated with the seismogenic structure, to which the fault belongs, is sufficiently large and at such a depth that it is reasonable to infer that surface faulting can occur”.

• 
  Basement and Cenozoic Geology
  

The Malmesbury Group consists predominantly of a marine sedimentary assemblage with a large lithological variation which have been deformed by two tectonic events.

The intrusion of the Cape Granites took place in two phases along northwest to southeast trending lines of weakness. The younger granites have been dated at 500 15 million years and the Malmesbury sediments have a minimum isotopic age of 600 million years. .

The late Precambrian Malmesbury orogeny was followed by a period of erosion and planation, preceding the deposition of the Klipheuwel Group. These sediments are mainly arenaceous in character are unmetamorphosed and show little deformation.

There is a large depositional gap in the geological history over most of the south-western Cape Province due to the absence of the Cape Supergroup of sediments. It is possible that this Supergroup once overlay the area but has since been removed by tectonic uplift and subsequent planation.

Following the Cretaceous break-up of Gondwanaland (Dingle and Scrutton, 1974), the Late Precambrian rocks were exposed by erosion and subjected to tropical and sub-tropical weathering (Glass, 1977), probably in the Early Tertiary, which resulted in deeply weathered and highly leached bedrock, particularly along the coastal area (Rogers, 1980). On the published geological maps, the sandy surface material mapped in the Western Cape is described as ‘Tertiary to Recent’. Over the last couple of years, the stratigraphy of the Cenozoic sediments in the Western Cape is slowly being formalized. The South African Committee for Stratigraphy (SACS, 1980) has ratified some of the old names and proposed that the entire Western Cape Cenozoic succession be termed the Sandveld Group. The formations encountered between Cape Town and Eland’s Bay are the Miocene, fluviatile Elandsfontyn Formation, the Miocene, littoral Saldanha Formation, the Mio-Pliocene, phosphatic littoral and shallow-marine Varswater Formation, the Early Pleistocene, aeolian Springfontyn Formation, the Late Pleistocene, littoral Velddrif Formation, the Late Pleistocene, aeolian Langebaan Formation and the Holocene, aeolian Witzand Formation. At present SACS, (1980) has accepted but not yet approved all the formational names.

Rogers (1980), examined the sedimentary successions in three excavations made for the Koeberg Nuclear Power Station at Duynefontein No 34 and recognized the Varswater- (oldest), Springfontyn-, Milnerton- and Witzand (youngest) Members. These have since been upgraded to Formations and the Milnerton Member has been renamed the Velddrif Formation. The Geological formations are shown in Table 43.

Table 43: Geological Formations

Figure 10: Main Structural Map of Koeberg