Aspects of the structure, tectonic evolution and sedimentation of the Tygerberg terrane, southwestern Cape Province

Bibliography: pages 88-93.A structural, deformational and sedimentalogical analysis of the Sea Point, Signal Hill and Bloubergstrand exposures of the Tygerberg Formation, Malmesbury Group, has been undertaken, through the application of developed geomathematical, digital and graphical computer-based techniques, encompassing the fields of tectonic strain determination, fold shape classification, cross-sectional profile preparation and sedimentary data representation. Emplacement of the Cape Peninsula granite pluton led to significant tectonic shortening of the sediments, tightening of the pre-existing synclinal fold at Sea Point, and overprinting of the structure by a regional foliation. Strain determinations from deformed metamorphic spotting in the sediments yielded a mean, undirected λ₁ : λ₂: λ₃ value of 1.57:1.24:0.52. This strain increment increases slightly towards the contact and it is proposed that it is non-coaxial to, and late relative to the ca 600Ma orogenic episode. The oblate nature of the strain ellipsoid lends support to the mechanism of radial distension as a means of pluton emplacement. An insight into the effect of the deformation on the structural configuration of the fold can be gained through a reconstruction of the 'pre-granite emplacement' down-plunge projection. In a reconstructed section, the matching of lithological units along strike was poor. Through detailed sedimentary logging of both limbs of the fold distinct lithological zones and facies, commonly displaying turbiditic characteristics, were recognised. Significant down-current changes in sedimentary structures, identified from a matching of the zones in the two limbs, are indicative of a small-sized depositional basin. The stratigraphic sequence thickens and coarsens upward and is consequently assigned to the lower and mid fan parts of a prograding submarine fan model