Lithofacies of the volcanic Machadodorp Member (Silverton Formation) South Africa

Lithofacies of the Machadodorp Volcanic Member (Silverton Formation), South Africa Nils Lenhardt, Patrick G. Eriksson Age (Ga) Introduction 28°E za Botswana Formations of the Pretoria Group Members Mo Houtenbek Namibia Burgersfort Johannesburg Steenkampsberg Nederhorst 30° Lakenvlei Republic of South Africa Km 100 200 15° Cape Town 20° 25° 30° Paleoproterozoic The Pretoria Group, an up to 7500 m-thick volcano-sedimentary succession (Eriksson et al., 1993) at the top of the Transvaal basin-fill and preceding the Bushveld Complex intrusion is interpreted as being deposited in an intracratonic sag basin (Allen and Allen, 1990), within which initial fault-controlled mechanical subsidence was followed by thermal subsidence (Eriksson and Reczko, 1995). Although the Pretoria Group succession is cratonic in character in terms of sedimentary rocks (Button, 1973) three main volcanic units occur within the succession, one being the Machadodorp Volcanic Member of the Silverton Formation (Crow and Condie, 1990; Reczko et al., 1995) of which no age data are available as yet. A large, shallow epicontinental sea was marked by the largely muddy sediments of the Silverton Formation (Eriksson et al., 2001) with the subaqueous Machadodorp Member volcanism mostly distributed in the eastern part of the basin (Fig. 1) (Button, 1973). The Silverton Formation consists of three different members, which include the Boven Shale Member, the Machadodorp Member and the Lydenburg Shale Member (Fig. 2) (Schreiber, 1990). So far, the volcanism of the Machadodorp Member has not been studied in detail. The Machadodorp volcanics are tholeiitic basalts and andesites and exhibit flat to LREE depleted patterns similar to MORB. A detailed facies analysis of the Machadodorp volcanism was initiated, allowing constraints on its volcanic evolution. mb iq u e Zimbabwe Era Department of Geology, University of Pretoria, South Africa Lydenburg (Mashishing) 35° Machadodorp Pretoria (Tshwane) Magaliesberg Silverton Lydenburg Shale Mem. Machadodorp Mem. Boven Shale Mem. Daspoort Strubenkop Dwaalheuwel 2.22 Carolina Johannesburg Pretoria Group 28°E Vermont Boshoek Machadodorp Member Hekpoort Fm. 2.32 Chuniespoort Group Timeball Hill Rooihoogte Fig. 1: Geological map showing the distribution of the Machadodorp Member within the eastern part of the Transvaal Basin. Massive lava flows Hekpoort Fig. 2: Stratigraphy of the Pretoria Group. Laminated tuff Earlier lava flows 10 cm 1m The massive lavas, reaching up to 250 m in thickness in drill cores, r e p r e s e n t nonchannelized sheet flows and are characteristic of higher effusion rates and temperatures compared to pillowed flows. Fluidal-clast breccia Cinder cones The well sorted, laminated tuffs are distributed over a wide area and are interpreted as the fine products of subaqueous eruptions that settled through the water column. Pyroclastic flow Bedded lapilli tuff Bedded lapilli-tuff Lava Fallout layers Fluidal-clast breccia 10 cm This facies is composed of clasts that are fluidally shaped to blocky and splintery, and moderately to highly vesicular. It is poorly sorted, internally massive and clast-supported, and reaches thicknesses of several 10s of metres. The clasts are interpreted to resemble bombs and fluidal lapilli formed by tearing apart of lava ribbons jetted upward from vents during Hawaiian-style fire-fountain eruptions. Fissures Fig. 3: The four main lithofacies types forming the Machadodorp Member of the Silverton Formation and their depositional environment. Results The Machadodorp Member comprises four main volcanic lithofacies types distinguished on the basis of rock type, sedimentary and volcanic structures or textures, and grain size. The vertical and lateral analysis of individual lithofacies types and their distribution within the study area allow constraints on the volcanic architecture and evolution of the Machadodorp Member (Fig. 3). The volcanic activity was heralded by a few isolated, thin tuff beds, intercalated with the mudrocks of the underlying Boven Shale Member. Interpretation of the lithofacies types suggests that the Machadodorp volcaniclastic rocks were generated by fountaining of low-viscosity magma in a relatively shallow submarine environment. The distribution of the rocks (200 km strike length) suggests that several submarine fire fountains, possibly located along a fault or fissure, were erupting simultaneously. Deposition was almost certainly proximal within 10s to 100s of metres of the source vents. In a later stage, sheet and subordinate pillow lavas were extruded, indicating reduced magma discharge rates at the end of the deposition of the Machadodorp Member. The lavas are unlikely to be fountain-fed lavas, given the non-welded character of the associated fluidal-clast breccia facies. Subaqueous fire-fountain deposits represent paleo-seafloor positions and indicate proximity to eruptive vents. Identification of such near-vent facies is important as submarine fire-fountain deposits have been identified in several massive sulfide districts (Allen et al., 1996). View publication stats Boven Shale Member sediments The planar bedded, fine- to mediumgrained deposits reach thicknesses up to several 10s of metres and show diffuse alignment of the clasts. The poor sorting and the limited extent of the tuffs are consistent with deposition or redeposition from a nearby subaqueous eruption. References Allen, P.A., Allen, J.R., 1990. Basin Analysis: Principles and Applications. Blackwell, Oxford, 451 pp. Allen, R.L., Hundstrom, I., Ripa, M., Simeonov, A., Christofferson, H., 1996. Facies analysis of a 1.9 Ga continental margin, back-arc, felsic caldera province with diverse Zn-PbAg-(Cu-Au) sulfide and Fe oxide deposits, Bergslagen Region, Sweden, Econ. Geol. 91, 979-1008. Button, A., 1973. A regional study of the stratigraphy and development of the Transvaal Basin in the eastern and northeastern Transvaal. (unpubl.) Ph.D. thesis, Univ. Wiwatersrand, Johannesburg. Crow, C., Condie, K.C., 1990. Geochemistry and origin of early Proterozoic volcanic rocks from the Transvaal and Soutpansberg successions, South Africa. Precambr. Res. 47, 17-26. Eriksson, P.G., Reczko, B.F.F., 1995. The sedimentary and tectonic setting of the Transvaal Supergroup floor rocks to the Bushveld Complex. J. Afr. Earth Sci. 21, 287-504. Eriksson, P.G., Schweitzer, J.K., Bosch, P.J.A., Schreiber, U.M., van Devender, J.L., Hatton, C.J., 1993. The Transvaal Sequence: an overview. J. Afr. Earth Sci. 16, 25-51. Eriksson, P.G., Altermann, W., Catuneanu, O., van der Merwe, R., Bumby, A.J., 2001. Major influences on the evolution of the 2.67-2.1 Ga Transvaal Basin, Kaapvaal Craton. Sed. Geol. 141, 205-231. Reczko, B.F.F., Oberholzer, J.D., Res, M., Eriksson, P.G., Schreiber, U.M., 1995. A reevaluation of the volcanism of the Palaeoproterozoic Pretoria Group (Kaapvaal craton) and a hypothesis on basin development. J. Afr. Earth Sci. 21, 505-519. Schreiber, U.M., 1990. A palaeoenvironmental study of the Pretoria Group in the eastern Transvaal. (unpubl.) Ph.D. thesis, Univ. of Pretoria, Pretoria. Contact: Dr. Nils Lenhardt Department of Geology University of Pretoria 0002 Pretoria Phone: 012 420 3310 E-mail: nils.lenhardt@up.ac.za