Tephrostratigraphy, petrography, geochemistry, age and fossil record of the Ganigobis shale member and associated glaciomarine deposits of the Dywka group, late Carboniferous, southern Africa
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2000
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The Ganigobis Shale Member contains remains of paleoniscoid fishes (e. g. Namaichthys schroederi), bivalves (e. g. Nuculopsis), gastropods (e. g. Peruvispira), scyphozoa (e. g. Conularia), crinoid stalks, sponges and sponge spicules, radiolaria, coprolites and permineralised wood. These mostly marine body and trace fossils record the extent of the first of a series of marine incursions into the disintegrating Gondwanan interior as early as the Carboniferous
Within the Ganigobis Shale Member 21 bentonitic tuff beds were determined which in part can be traced laterally over tens of kilometres indicating an ashfall derivation. The thickness of the white to yellow and brownish tuff beds varies between 0. 1 and 2. 0 cm. Further bentonitic tuff beds of the Dwyka Group were detected in cut banks of the Orange River near Zwartbas in the Karasburg Basin (southern Namibia). The clay matrix of the 65 tuff beds is less frequently replaced by secondary gypsum minerals than in the tuff beds near Ganigobis. The tuff beds vary between 0. 1 and 4. 0 cm in thickness. Due to a similar fossil content and age of the background deposits, the tuff beds are thought to have originated from the same source area as those from the Aranos Basin
Tuff beds from the Western Cape Province of South Africa were sampled from SOEKOR cores and in roadcuts. The yellow tuff beds sampled from the cores in the uppermost parts of DS II and III (Dwyka Group) display a thickness between 0. 1 and 2. 0. Samples were especially used for the preparation of thin-sections and geochemistry. Apart from a tuffaceous bed in the uppermost part of DS III, tuff beds in roadcuts originated from the base of the Prince Albert Formation (Ecca Group). The numerous, bentonitic tuff beds are 0. 2-8 cm thick and light green to white. They do not show any internal bedding structures and could not be traced into the field due to the lack of exposure
Thin-sections reveal the derivation of the tuff beds as distal fallout ashes produced by explosive volcanic eruptions. The matrix consists of a micro- to cryptocrystalline quartz-clay mineral mixture. Most obvious are secondarily grown kaolinite booklets. Rare fragments of splinter, quartz, completely recrystallized ash-sized particles of former volcanic glass and few apatite and zircon grains are the only juvenile components which are determined with confidence
The tuff beds contain as non-opaque, heavy minerals mostly zircon, apatite, monazite and sphene but also biotite, garnet, hornblende and tourmaline. The 100 to 220 um large zircons are mostly euhedral and elongated prismatic to needlelike. Some of zircons show inclusions of apatite or glassy material. The apatites are clear and euhedral to subhedral. Few euhedral sphenes and monazites show distinct and unrounded crystal faces. Biotite flakes only occur in certain tuff beds in higher amounts whereas garnet, hornblende and tourmaline only turn up in tuff bed VIIIa
Geochemical analyses point to an original, intermediate to acid composition of the tuff samples. A12O3 and K2O are significantly enriched whereas all other major elements show lesser values than comparable igneous rocks, LREE enrichment and Eu-anomalies show that the parent magma of the tuff beds was a highly evolved calc-alkaline magma. La enrichment supports the involvement of a dominantly crustal source. Tectonomagmatic discrimination diagrams point to a volcanic arc setting. Bedding characteristics and the lack of any Carboniferous-Permian volcanic successions onshore Namibia makes an aeolian transport of the ash particles over larger distances likely. Siliceous, arc-related related magmatism of Carboniferous-Permian age is is known from South America (Patagonia). Siliceous ashes could thus have been transported by prevailing south-westerly winds to South Africa and Namibia. The thickness and frequency of the tuff beds is highest in South Africa and generally decreases towards the north (Namibia). A second, more local source area could have been located in an intracontinental rift zone along the western margin of southern Africa which is indicated by north-south directed ice-flow directions in the Late Carboniferous and which hosted a marine seaway during Permian times (Whitehill sea). Especially the large size of juvenile components within the ashes (e. g. zircons) contradicts an aeolian transport from South America
SHRIMP-based age determinations of juvenile magmatic zircons separated from the tuff beds allow a new time calibration of Dwyka Group deglaciation sequences II - IV and the Dwyka/Ecca boundary. The start of the deposition of the Dwyka Group will stay uncertain as basal deposits of DS I are not preserved. If sediment structures in the Waaipoort Formation (Witteberg Group) are of glacial origin, glaciation started at 342 Ma and DS I may have lasted for 36 Ma. Zircons of the Ganigobis Shale Member yield SHRIMP-ages of 302-300 Ma. This dates the uppermost part of the second deglaciation sequence in southern Namibia to the Late Carboniferous (Gzelian) and provides a minimum age for the onset of Karoo-equivalent marine deposition. The age of the uppermost argillaceous part of the third deglaciation sequence (297 Ma) was determined from zircons of a tuffaceous bed sampled in a roadcut in the Western Cape Province, South Africa. The deposits correlate with the Hardap Shale Member in the Aranos Basin of southern Namibia which are part of the much more widespread Eurydesma transgression. The age of the Dwyka/Ecca boundary was determined by SHRIMP-measurements of juvenile zircons from two tuff beds of the basal Prince Albert Formation sampled in the Western Cape Province (South Africa). The zircons revealed ages of 289 - 288 Ma which date the Dwyka/Ecca boundary at about 290 Ma. According to these ages, deglaciation sequences II-IV lasted for 5 Ma on average. Deposition of Dwyka Group-sediments in southern Namibia started by latest at about 306 Ma and ended at about 290 Ma before present
The Ganigobis Shale Member contains remains of paleoniscoid fishes (e. g. Namaichthys schroederi), bivalves (e. g. Nuculopsis), gastropods (e. g. Peruvispira), scyphozoa (e. g. Conularia), crinoid stalks, sponges and sponge spicules, radiolaria, coprolites and permineralised wood. These mostly marine body and trace fossils record the extent of the first of a series of marine incursions into the disintegrating Gondwanan interior as early as the Carboniferous
Within the Ganigobis Shale Member 21 bentonitic tuff beds were determined which in part can be traced laterally over tens of kilometres indicating an ashfall derivation. The thickness of the white to yellow and brownish tuff beds varies between 0. 1 and 2. 0 cm. Further bentonitic tuff beds of the Dwyka Group were detected in cut banks of the Orange River near Zwartbas in the Karasburg Basin (southern Namibia). The clay matrix of the 65 tuff beds is less frequently replaced by secondary gypsum minerals than in the tuff beds near Ganigobis. The tuff beds vary between 0. 1 and 4. 0 cm in thickness. Due to a similar fossil content and age of the background deposits, the tuff beds are thought to have originated from the same source area as those from the Aranos Basin
Tuff beds from the Western Cape Province of South Africa were sampled from SOEKOR cores and in roadcuts. The yellow tuff beds sampled from the cores in the uppermost parts of DS II and III (Dwyka Group) display a thickness between 0. 1 and 2. 0. Samples were especially used for the preparation of thin-sections and geochemistry. Apart from a tuffaceous bed in the uppermost part of DS III, tuff beds in roadcuts originated from the base of the Prince Albert Formation (Ecca Group). The numerous, bentonitic tuff beds are 0. 2-8 cm thick and light green to white. They do not show any internal bedding structures and could not be traced into the field due to the lack of exposure
Thin-sections reveal the derivation of the tuff beds as distal fallout ashes produced by explosive volcanic eruptions. The matrix consists of a micro- to cryptocrystalline quartz-clay mineral mixture. Most obvious are secondarily grown kaolinite booklets. Rare fragments of splinter, quartz, completely recrystallized ash-sized particles of former volcanic glass and few apatite and zircon grains are the only juvenile components which are determined with confidence
The tuff beds contain as non-opaque, heavy minerals mostly zircon, apatite, monazite and sphene but also biotite, garnet, hornblende and tourmaline. The 100 to 220 um large zircons are mostly euhedral and elongated prismatic to needlelike. Some of zircons show inclusions of apatite or glassy material. The apatites are clear and euhedral to subhedral. Few euhedral sphenes and monazites show distinct and unrounded crystal faces. Biotite flakes only occur in certain tuff beds in higher amounts whereas garnet, hornblende and tourmaline only turn up in tuff bed VIIIa
Geochemical analyses point to an original, intermediate to acid composition of the tuff samples. A12O3 and K2O are significantly enriched whereas all other major elements show lesser values than comparable igneous rocks, LREE enrichment and Eu-anomalies show that the parent magma of the tuff beds was a highly evolved calc-alkaline magma. La enrichment supports the involvement of a dominantly crustal source. Tectonomagmatic discrimination diagrams point to a volcanic arc setting. Bedding characteristics and the lack of any Carboniferous-Permian volcanic successions onshore Namibia makes an aeolian transport of the ash particles over larger distances likely. Siliceous, arc-related related magmatism of Carboniferous-Permian age is is known from South America (Patagonia). Siliceous ashes could thus have been transported by prevailing south-westerly winds to South Africa and Namibia. The thickness and frequency of the tuff beds is highest in South Africa and generally decreases towards the north (Namibia). A second, more local source area could have been located in an intracontinental rift zone along the western margin of southern Africa which is indicated by north-south directed ice-flow directions in the Late Carboniferous and which hosted a marine seaway during Permian times (Whitehill sea). Especially the large size of juvenile components within the ashes (e. g. zircons) contradicts an aeolian transport from South America
SHRIMP-based age determinations of juvenile magmatic zircons separated from the tuff beds allow a new time calibration of Dwyka Group deglaciation sequences II - IV and the Dwyka/Ecca boundary. The start of the deposition of the Dwyka Group will stay uncertain as basal deposits of DS I are not preserved. If sediment structures in the Waaipoort Formation (Witteberg Group) are of glacial origin, glaciation started at 342 Ma and DS I may have lasted for 36 Ma. Zircons of the Ganigobis Shale Member yield SHRIMP-ages of 302-300 Ma. This dates the uppermost part of the second deglaciation sequence in southern Namibia to the Late Carboniferous (Gzelian) and provides a minimum age for the onset of Karoo-equivalent marine deposition. The age of the uppermost argillaceous part of the third deglaciation sequence (297 Ma) was determined from zircons of a tuffaceous bed sampled in a roadcut in the Western Cape Province, South Africa. The deposits correlate with the Hardap Shale Member in the Aranos Basin of southern Namibia which are part of the much more widespread Eurydesma transgression. The age of the Dwyka/Ecca boundary was determined by SHRIMP-measurements of juvenile zircons from two tuff beds of the basal Prince Albert Formation sampled in the Western Cape Province (South Africa). The zircons revealed ages of 289 - 288 Ma which date the Dwyka/Ecca boundary at about 290 Ma. According to these ages, deglaciation sequences II-IV lasted for 5 Ma on average. Deposition of Dwyka Group-sediments in southern Namibia started by latest at about 306 Ma and ended at about 290 Ma before present
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Includes bibliographical references
Keywords
Geology, Bentonite geology