Abstract

Thyspunt is located along the southern coastal margin of the Eastern Cape Province, South Africa, between Oyster Bay and Cape St. Francis (~88 km west of Port Elizabeth). Field observations, thin section analysis, interpretation of geophysics, a review of more than 247 boreholes and the creation of a 212 D model are used to investigate the bedrock lithology and structure, stratigraphic contacts, palaeotopography of the Ordovician to early Devonian (485 to 419 Ma) Table Mountain Group (TMG) and thickness of the Miocene to Holocene (<23 Ma) overburden of the Algoa Group.

The northwest-southeast striking bedrock forms part of the regional southeast plunging, north verging Cape St. Francis anticline. Locally, folds plunge gently southeastward at shallow angles, with axial planes dipping steeply southwest or northeast. Lineaments previously interpreted as inferred faults, are here re-interpreted as zones of closely spaced jointing (shatter zones), that show little to no recognisable displacement. The 40 km long, northwest-southeast trending Cape St. Francis fault occurring offshore within 17.5 km of Thyspunt does not continue onshore within the study area. Jointing is pervasive and five joint systems are identified. Faults and joints do not extend into the younger cover deposits of the Algoa Group and are therefore older than ~23 Myrs.

Interpolation of bedrock elevation points and sections across bedrock reveal four previously unknown northwest-southeast trending palaeovalleys at Thyspunt, Tony’s Bay, Cape St. Francis and St. Francis, where the bedrock relief beneath overburden is below present day sea-level. Roughly 450 m northwest of Thys Bay, a 1050 m2 (area below sea-level) palaeovalley, gently sloping southeast to a depth of 15.5 m below msl, is cut into bedrock of the Goudini Formation. Bedrock is covered by aeolian and marine sequences of the Algoa Group reaching 61 m in thickness. Understanding of the bedrock and overburden characteristics allow greater understanding of where additional design requirements will potentially be required at Thyspunt.

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