THE SEDIMENTOLOGY, PALAEONTOLOGY AND STRATIGRAPHY OF COASTAL-PLAIN DEPOSITS AT HONDEKLIP BAY, NAMAQUALAND, SOUTH AFRICA JOHN PETHER University of Cape Town The copyright of this thesis vests in the author. No quotation from it or information derived from it is to be published without full acknowledgement of the source. The thesis is to be used for private study or non- commercial research purposes only. Published by the University of Cape Town (UCT) in terms of the non-exclusive license granted to UCT by the author. University of Cape Town Thesis Presented for the Degree of MASTER OF SCIENCE in the Department of Geology UNIVERSITY OF CAPE TOWN February 1994 John Pether Supervisor Dr John Rogers ABSTRACT The exposures in diamond mines on the Namaqualand west coast of South Africa provide a rare opportunity to examine a record that is normally inaccessible beneath a thick cover of aeolian sands. This study presents the main results of fieldwork in mine excavations on the farms Hondeklip and Avontuur-A, near Hondeklip Bay. Sections in the deposits were described in detail and the vertebrate and invertebrate faunas were sampled. The buried topography of the gneiss bedrock, obtained by prospecting, is complex, with the main feature consisting of a coast-parallel ridge flanking a wide palaeochannel on its landward side. Advanced kaolinitic weathering affected both the bedrock and a diamondiferous, basal kaolinitic sediment patchily preserved in the channel. The incision of the channel is related to the Oligocene regression and the basal kaolinitic sediment is interpreted as a fluvial arkose deposited in the channel. Both the bedrock and the deposit in the channel were then kaolinized during humid climatic conditions in the late Oligocene and early Miocene. Weathering­ profile silcrete also developed in the basal kaolinitic sediment. It is tentatively proposed that this weathering period may be represented in the Namib Desert by the thick laterite capping Eocene sediments at Kakaoberg. Subsequently, the palaeochannel was exhumed and was ultimately filled by late Tertiary marine deposits. The marine deposits were laid down in shallowing-upwards sequences of the shoreface environment. Two regressive, progradational packages (alloformations) are recognized. The older extends seawards from at least -50 m asl. and is the "45-50 m Complex" of Carrington and Kensley (1969), now called the 50 m Package. East of the channel, on the exposed coast, high-wave-energy storm­ deposition in the lower shoreface dominates the preserved record. With lowering of sea-level, the bedrock ridge emerged to the seaward of the prograding palaeoshoreline, reduced the level of incident wave energy and profoundly influenced the development of sub-environments within the progradational regime. Ultimately, low-energy bay deposits filled the palaeochannel in the bedrock. On the basis of vertebrate evidence and correlation with global sea-level trends, the age of the 50 m Package is middle Pliocene. The upper facies of the 50 m Package (foreshore and upper shoreface) have been extensively removed by later subaerial erosion. The subsequent transgression truncated the seaward extent of the 50 m Package, reached -30 m asl. and prograded seaward from that elevation. It is called the 30 m Package and combines the "29-34 m Beach" and "17-21 m Complex" of Carrington and Kensley (1969). A late Pliocene age is envisaged. The upper-shoreface facies of the 30 m Package is usually preserved, but may be disguised by pedogenesis. The diamondiferous marine gravels mined in the area are mainly lower-shoreface storm deposits and pre-existing transgressive lags and shelf deposits have generaly been reworked during regression. Enigmatic, muddy and/or phosphatic units, previously called "E-stage," are patchily preserved in the base of the 50 m Package and are revealed to be distal storm deposits laid down in the transitional shoreface to offshore environment. They are part of the overlying regressive sequence, but may include a fragmentary , petrified, mixed, vertebrate remanie. Nevertheless, eroded remnants of older deposits must also occur in places. ii ACKNOWLEDGEMENTS I am indebted to Mr F. Hoffman of Trans Hex Group Ltd., for unlimited access to the Hondeklip mine exposures. To Mr M. Mittelmeyer and Mr H. Bruwer, mine geologists, for their assistance and hospitality at Hondeklip. To the people of the Hondeklip mine, present and past, from mine managers to workers, for their friendliness and willlingness to help. To Mr R.G. Molyneux, of De Beers Namaqualand Mines, for his assistance and hospitality during field visits to neighbouring areas. To my field assistants at various times, M. Pether, S. Cloete and V. Bartnick, for their patience. To Dr John Rogers, for his guidance as my supervisor. To Ors Q.B. Hendey R.V. Dingle and B. Kensley, for their encouragement during various stages of this project. To Dr I.K. Macmillan, for analysis of the foraminif era. To Autodesk, via the AfraCAD dealership, for the provision of AutoCAD at a significant discount. To M. Joubert for kindly volunteering to photograph the fossil shells. To C. Booth for his photographic assistance. To J. Boltman, for last-minute binding. To my mother-in-law, without whose toddler-sitting help, I would not have had the assistance of my wife. To my wife, Marilyn, for still being my wife and for her invaluable assistance in this project. To the Director and Trustees of the South African Museum. To those many people here unmentioned who have contributed. iii TABLE OF CONTENTS CHAPTER 1 ................................................................................................................................. 1 INTRODUCTION .............................................................................................................. 1 1.1 OBJECTIVES ................................................................................................ 1 1.2 LOCATION OF THE STUDY AREA .............................................................. 1 1.3 SCOPE OF THE STUDY ............................................................................... 1 1.4 DATA AND METHODS ................................................................................. 4 Fieldwork ................................................................................................. 4 Survey and mapping .................................................................... 4 Section descriptions ..................................................................... 4 Sediment sampling ...................................................................... 6 Palaeontological description and sampling ................................... 6 Laboratory work ....................................................................................... 7 Grain size analysis ....................................................................... 7 Sediment compositions ................................................................ 7 Petrography ................................................................................. 8 Scanning electron microscopy ..................................................... 8 X-ray diffraction and X-ray fluorescence analyses ........................ 8 CHAPTER 2 ................................................................................................................................. 10 REGIONAL SETTING ...................................................................................................... 10 2.1 CLIMATE ...................................................................................................... 10 2.2 OCEANOGRAPHY ........................................................................................ 12 2.3 TIDAL AND WAVE REGIME ......................................................................... 15 2.4 THE CONTINENTAL SHELF ......................................................................... 16 CHAPTER 3 ................................................................................................................................. 19 PREVIOUS RESEARCH .................................................................................................. 19 3.1 INTRODUCTION ........................................................................................... 19 3.2 THE EARLIER WORK ................................................................................... 19 3.3 THE LATER WORK ...................................................................................... 21 iv The State Alluvial Diggings ....................................................................... 21 Kleinzee ................................................................................................... 22 The Olifants River Area ............................................................................ 22 The Hondeklip Bay Area .......................................................................... 23 3.4 REGIONAL SYNTHESES ............................................................................. 25 CHAPTER 4 ................................................................................................................................. 26 GENERAL GEOLOGY OF THE STUDY AREA ................................................................ 26 4.1 SURFACE GEOLOGY AND TOPOGRAPHY ................................................ 26 Bedrock Geology ..................................................................................... 26 Surficial Sediments .................................................................................