Abstract
M.Sc.
The KaNyaka barrier island system consists of the vegetated KaNyaka and Portuguese Islands, and various partially exposed sandbars. It is situated in south-easternmost Mozambique where it forms a barrier between Maputo Bay in the west and the Indian Ocean in the east. It forms part of the northern most limit of the Maputaland Group, which consists of Pliocene to Holocene sediments. The island system is an example of a compound barrier island since it comprises a stacked succession of several ancient shoreline marine and aeolian sedimentary rock units and sediments, along with various active sedimentary environments. This thesis describe the general geology of the island system, the petrography of the various rock and sedimentary units and provide information on provenance of sediment based on detrital zircon age populations. It also provides several new 14C age dates of shell fossils and calcified rhizoliths.
KaNyaka Island consists of two high north-south trending dune cordons along its eastern and western shores, with a low-lying dune-covered area in between. The Western dune cordon consists of aeolian calcareous quartz arenite of the Ridjene Formation unconformably overlain by calcified aeolian dunes of the Alto Pocuane Formation in turn overlain by the unlithified red dunes of the Barreira Vermelha Formation. The core of the Eastern dune cordon consists of the calcareous quartz arenite of the Cabo Inhaca Formation comprised of stacked calcified aeolian dunes, the marine and tidal deposits of the Ponta Mazondue and Ponta Torres formations form the eastern and south-western flanks of the Eastern dune cordon. The formations are overlain by unlithified red to orange sands of the Changana Formation and partially lithified light grey to yellow sands of the Muamuluago Formation. The area between these two dune cordons is covered by low-lying aeolian dunes of the Alto Chumine Formation, ancient sandbar and beach deposits of the Chunhe Formation and both modern and paleo-intertidal flat deposits. Modern sedimentary environments include high-energy beaches, intertidal flats, sand spits, sandbars and aeolian dunes.
The presence of aeolian deposits extending below the present sea-level, and marine deposits up to 3m above it, indicates that the KaNyaka barrier island system records a long history of sea-level change. 14C dating of marine and freshwater shell fossils and a rhizolith collected from several of the formations on the KaNyaka barrier island system assist in defining the history of the island with reference to sea-level fluctuations. Results from two freshwater shell fossils in the aeolian Alto Pocuane and Cabo Inhaca Formations indicate that they were deposited and subsequently calcified 47 000 and 30 000 years ago respectively, at times when the sea-level was 40 - 60 m lower than at present. The red sand of the Barreira Vermelha and Changana formation overlying these calcified dunes are suggested to have formed when climate was drier and sea-level even lower, during the Last Glacial Maximum (LGM) around 17 000 years ago. 14C ages on formations overlying the red sand deposits all postdate the Last Glacial Maximum and record sediment deposition of the past 7 000 years when the sea-level was either rather similar or somewhat higher than the present.
Detrital zircon age populations, determined by laser ablation inductively coupled mass spectrometry (LA-ICP-MS), indicate that the majority of the sediment building the barrier island system was derived from the Grenvillian (~1100 Ma) and the Pan-African (~500 Ma) orogenic belts situated along the eastern side of Africa. Sediment was probably transported from exposed rock successions of these belts by large rivers like the Zambezi in the north and the Tugela in the south. The sediment was then dispersed along the coast by the south flowing Agulhas Current and northward directed long-shore current. Rivers feeding into Maputo Bay, like the Phongola, Umbeluzi and Incomati, draining the Kaapvaal Craton and Karoo Supergroup provided very little sediment to the barrier island system.