Tephrostratigraphy, petrography, geochemistry, age and fossil record of the Ganigobis Shale Member and associated glaciomarine deposits of the Dwyka Group, Late Carboniferous, southern Africa. Dissertation zur Erlangung des naturwissenschaftlichen Doktorgrades der Bayerischen Julius-Maximilians-Universität Würzburg vorgelegt von Berthold Bangert aus Freiburg im Breisgau Würzburg 2000 PREFACE 1 Preface Thin, bentonitic tuff beds were initially recognised by Harald Stollhofen in the Dwyka Group of the Aranos Basin (southern Namibia) in 1994 and were described to greater detail by Hermann Grill in 1997. The following study concentrates on the volcanology, petrography, tephro-stratigraphy and tephrochronology as well as geochemistry of these ashfall tuff beds. The most detailed studied area was situated in the vicinity of Ganigobis within the Aranos Basin of southern Namibia where the tuff beds are preserved in a more than 40 m thick, marine silty mudstone-unit (Ganigobis Shale Member). Extensive field study of the Ganigobis Shale Member triggered the need for a comprehensive examination of all preserved features within the mudstone unit e.g. including the preserved fossils. Thicker and better preserved ashfall tuff beds of the Dwyka Group were sampled in cut banks of the Orange River near Zwartbas in the Karasburg Basin (southern Namibia). They were initially discovered by Volker Lorenz. The sedimentology, mineralogy and geochemistry of these ashfall tuff beds and the surrounding mudstones of the Dwyka Group were studied to greater detail by Markus Geiger. Own results but also parts of his results are incorporated in this study. A third study area is located in the Western Cape Province (South Africa) where ashfall tuff beds of the Dwyka Group and the overlying Prince Albert Formation (Ecca Group) were sampled in outcrops and cores of SOEKOR drill holes. Samples were used to study the petrology and geochemistry of the tuff beds while juvenile zircons of the beds were used for SHRIMP-based age determinations. Two tuffaceous beds, sampled in the Dwyka Group of KwaZulu-Natal (South Africa), were additionally included in this study. Würzburg, 29 July 2000 BANGERT, B. (2000): Tephrostratigraphy, petrography, geochemistry, age and fossil record of the Ganigobis Shale Member and associated glaciomarine deposits of the Dwyka Group, Late Carboniferous, southern Africa.- Dissertation (Dr.rer.nat.) Universität Würzburg, Germany; 260 pp., 154 Figures, 31 Tables, 8 Plates. 2 BERTHOLD BANGERT Abstract Thin pyroclastic marker beds are preserved in part can be traced laterally over tens of kilometres argillaceous units of the Dwyka Group in indicating an ashfall derivation. The thickness of southern Nambia and South Africa which are the the white to yellow and brownish tuff beds varies earliest witnesses of volcanism in Karoo- between 0.1 and 2.0 cm. Further bentonitic tuff equivalent strata of southern Africa. The aim of beds of the Dwyka Group were detected in cut this study is to present the field appearance of banks of the Orange River near Zwartbas in the these marker beds, to characterise their Karasburg Basin (southern Namibia). The clay mineralogy, geochemistry and heavy mineral matrix of the 65 tuff beds is less frequently contents and to present new radiometric age data replaced by secondary gypsum minerals than in from their juvenile zircons. the tuff beds near Ganigobis. The tuff beds vary Carboniferous-Permian Karoo deposits in the between 0.1 and 4.0 cm in thickness. Due to a Aranos Basin of southern Namibia include the similar fossil content and age of the background glacially dominated, Carboniferous Dwyka Group deposits, the tuff beds are thought to have and the shelf sediments of the overlying Permian originated from the same source area as those Ecca Group totaling approximately 400 m in from the Aranos Basin. sediment thickness. The Dwyka Group can be Tuff beds from the Western Cape Province of subdivided into four upward-fining deglaciation South Africa were sampled from SOEKOR cores sequences, each capped by relatively fine-grained and in roadcuts. The yellow tuff beds sampled glaciolacustrine or glaciomarine deposits. The from the cores in the uppermost parts of DS II uppermost part of the second deglaciation and III (Dwyka Group) display a thickness sequence comprises a thick fossiliferous between 0.1 and 2.0. Samples were especially mudstone unit, referred to as the ”Ganigobis used for the preparation of thin-sections and Shale Member”. An abundance of marine macro- geochemistry. Apart from a tuffaceous bed in the and ichnofossils as well as extrabasinally derived uppermost part of DS III, tuff beds in roadcuts ashfall tuff beds characterise the more than 40 m originated from the base of the Prince Albert thick mudstones and provide the basis for an Formation (Ecca Group). The numerous, integrated high-resolution biostratigraphic and bentonitic tuff beds are 0.2-8 cm thick and light tephrostratigraphic framework. The uppermost green to white. They do not show any internal part of the third deglaciation sequence of the bedding structures and could not be traced into Dwyka Group consists of approximately 75 m the field due to the lack of exposure. thick siltstones and mudstones referred to as the Thin-sections reveal the derivation of the tuff ”Hardap Shale Member” which contain fossils beds as distal fallout ashes produced by explosive such as the bivalve Eurydesma mytiloides. The volcanic eruptions. The matrix consists of a Hardap Shale Member is apparently devoid of micro- to cryptocrystalline quartz-clay mineral tuff beds. mixture. Most obvious are secondarily grown The Ganigobis Shale Member contains remains kaolinite booklets. Rare fragments of splinter of paleoniscoid fishes (e.g. Namaichthys quartz, completely recrystallized ash-sized schroederi), bivalves (e.g. Nuculopsis), particles of former volcanic glass and few apatite gastropods (e.g. Peruvispira), scyphozoa (e.g. and zircon grains are the only juvenile Conularia), crinoid stalks, sponges and sponge components which are determined with spicules, radiolaria, coprolites and permineralised confidence. wood. These mostly marine body and trace fossils The tuff beds contain as non-opaque, juvenile record the extent of the first of a series of marine heavy minerals mostly zircon, apatite, monazite incursions into the disintegrating Gondwanan and sphene but also biotite, garnet, hornblende interior as early as the Carboniferous. and tourmaline. The 100 to 220 µm large zircons Within the Ganigobis Shale Member 21 are mostly euhedral and elongated prismatic to bentonitic tuff beds were determined which in needlelike. Some of zircons show inclusions of ABSTRACT 3 apatite or glassy material. The apatites are clear South America. and euhedral to subhedral. Few euhedral sphenes SHRIMP-based age determinations of juvenile and monazites show distinct and unrounded magmatic zircons separated from the tuff beds crystal faces. Biotite flakes only occur in certain allow a new time calibration of Dwyka Group tuff beds in higher amounts whereas garnet, deglaciation sequences II - IV and the hornblende and tourmaline only turn up in tuff Dwyka/Ecca boundary. The start of the bed VIIIa. deposition of the Dwyka Group will stay Geochemical analyses point to an original, uncertain as basal deposits of DS I are not intermediate to acid composition of the tuff preserved. If sediment structures in the Waaipoort samples. Al2O3 and K2O are significantly Formation (Witteberg Group) are of glacial enriched whereas all other major elements show origin, glaciation started at 342 Ma and DS I may lesser values than comparable igneous rocks. have lasted for 36 Ma. Zircons of the Ganigobis LREE enrichment and Eu-anomalies show that Shale Member yield SHRIMP-ages of 302-300 the parent magma of the tuff beds was a highly Ma. This dates the uppermost part of the second evolved calc-alkaline magma. La enrichment deglaciation sequence in southern Namibia to the supports the involvement of a dominantly crustal Late Carboniferous (Gzelian) and provides a source. Tectonomagmatic discrimination minimum age for the onset of Karoo-equivalent diagrams point to a volcanic arc setting. Bedding marine deposition. The age of the uppermost characteristics and the lack of any Carboniferous- argillaceous part of the third deglaciation Permian volcanic successions onshore Namibia sequence (297 Ma) was determined from zircons makes an aeolian transport of the ash particles of a tuffaceous bed sampled in a roadcut in the over larger distances likely. Siliceous, arc-related Western Cape Province, South Africa. The magmatism of Carboniferous-Permian age is deposits correlate with the Hardap Shale Member known from South America (Patagonia). in the Aranos Basin of southern Namibia which Siliceous ashes could thus have been transported are part of the much more widespread Eurydesma by prevailing south-westerly winds to South transgression. The age of the Dwyka/Ecca Africa and Namibia. The thickness and frequency boundary was determined by SHRIMP- of the tuff beds is highest in South Africa and measurements of juvenile zircons from two tuff generally decreases towards the north (Namibia). beds of the basal Prince Albert Formation A second, more local source area could have been sampled in the Western Cape Province (South located in an intracontinental rift zone along the Africa). The zircons revealed ages of 289 - 288 western margin of southern Africa which is Ma which date the Dwyka/Ecca boundary at indicated by north-south directed ice-flow about 290 Ma. According to these ages, directions in the Late Carboniferous and which deglaciation sequences II-IV lasted for 5 Ma on hosted a marine seaway during Permian times average. Deposition of Dwyka Group-sediments (Whitehill sea). Especially the large size of in southern Namibia started by latest at about 306 juvenile components within the ashes (e.g. Ma and ended at about 290 Ma before present. zircons) contradicts an aeolian transport from Zusammenfassung Geringmächtige, bentonitische Tuffe treten in Petrographie, Mineralogie und Geochemie Tonsteinabschnitten der karbonen Dwyka Gruppe zusammen. Im Rahmen einer im südlichen Namibia und Südafrika auf.