The Ventersdorp contact reef; final phase of the Witwatersrand Basin, independent formation, or precursor to the Ventersdorp Supergroup?
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The Ventersdorp Contact Reef (VCR) is an Archaean conglomeratic gold placer, mined in the Carletonville, West Rand, and Klerksdorp goldfields of the Republic of South Africa. It belongs to a lithostratigraphic formation, the Venterspost Conglomerate Formation (VCF), which is considered to be stratigraphically independent of the underlying Witwatersrand Supergroup sediments and overlying Ventersdorp Supergroup volcanics. In this paper, the present formal stratigraphic placement of the Formation is compared with its alternative stratigraphic assignment to either of the underlying Witwatersrand or overlying Ventersdorp Supergroups. Evidence for a basin-wide, angular unconformity separating the VCF from Witwatersrand rocks, which were regionally lithified prior to deposition of the Formation, indicates that it is stratigraphically divorced from the Witwatersrand Supergroup. While similar angular, internal unconformities are present within the Witwatersrand Supergroup, a significant difference between these and the VCF unconformity is that the internal Witwatersrand unconformities are asserted to have basinward extents only to the tectonic hinge between continuous deposition and contemporaneous (Witwatersrand) foreland basin destruction. A genetic difference between the VCR and typical Witwatersrand placers is reflected by their differing mineralogical content and metamorphic grade. These differences, by implication, also reflect a substantial time gap separating their deposition. The fact that the conglomerates and sandstones of the VCF are locally interbedded with lava flows that are geochemically similar to the lava of the basal Westonaria Formation of the Ventersdorp Supergroup, indicates lithologic and genetic links to this Supergroup. These relationships are further supported by the presence of tuffaceous sandstone beds within the VCF. The occurrence of deformational load structures at the contact between the formation and the basal lava flows of the overlying Ventersdorp Supergroup indicates an unconsolidated, probably water-saturated state of the VCF sediments at the time of their envelopment by the lava, and implies a conformable relationship with the lower formations of the Supergroup. In conclusion, it is proposed that the VCF be stratigraphically assigned to the Ventersdrop Supergroup.Keywords:
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Systematic measured profiles on a regional scale are used to document the distribution of noritic and amphibolitic sills (787 in total) in the largely sedimentary latest Archaean–early Proterozoic succession of the Transvaal Supergroup in northeastern South Africa. An aggregate thickness of over 2.2 km of sills intruded this 12 km thick succession. The proportion of sill to sill plus sedimentary rock increases upward (average 16%), towards the overlying mafic phase of the Bushveld Complex. The average thickness of sills is 34 m, but increases in the same sense. The emplacement level of the Bushveld Complex transgresses across some 5 km of the stratigraphy of the upper formations of the Transvaal Supergroup. A node of maximum intensity of noritic sill intrusion is developed in these uppermost strata adjacent to the location where the transgressive relationship is most pronounced. The structural styles of the Transvaal Supergroup strata influence intensity of sill intrusion. Other factors being equal, maximum intrusion intensity is associated with areas of shallow homoclinal dip (<20°) and minimum intensity with areas of steeper dip and folding. A lithological control on sill emplacement is also documented, with quartzite- and shale-dominated formations being more heavily intruded than dolomite and basaltic lava. Supplementary material: A spreadsheet including all 440 formation traverses including grid references, formation thicknesses, and number and thicknesses of every sill therein is available at http://www.geolsoc.org.uk/SUP18807 .
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Research Article| December 01, 2016 Early Permian diamond-bearing proximal eskers in the Lichtenburg/Ventersdorp area of the North West Province, South Africa Mike C.J. de Wit Mike C.J. de Wit University of Pretoria/Tsodilo Resources Ltd, P O Box 466, Kleinmond, 7195, South Africa e-mail: dewit@icon.co.za Search for other works by this author on: GSW Google Scholar Author and Article Information Mike C.J. de Wit University of Pretoria/Tsodilo Resources Ltd, P O Box 466, Kleinmond, 7195, South Africa e-mail: dewit@icon.co.za Publisher: Geological Society of South Africa First Online: 20 Nov 2017 Online Issn: 1996-8590 Print Issn: 1012-0750 © 2016 December Geological Society of South AfricaGeological Society of South Africa South African Journal of Geology (2016) 119 (4): 585–606. https://doi.org/10.2113/gssajg.119.4.585 Article history First Online: 20 Nov 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Mike C.J. de Wit; Early Permian diamond-bearing proximal eskers in the Lichtenburg/Ventersdorp area of the North West Province, South Africa. South African Journal of Geology 2016;; 119 (4): 585–606. doi: https://doi.org/10.2113/gssajg.119.4.585 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietySouth African Journal of Geology Search Advanced Search Abstract Diamond-bearing gravels of the Lichtenburg-Ventersdorp area of the North West Province are associated with north-south orientated sinuous ‘runs’ that occur almost entirely on a flat erosional surface of the Malmani dolomites (Transvaal Supergroup) at some 1,500 m elevation. East to west, this dolomite plain measures 150 km, and north-south it is on average 40 km wide. This unconformity, which first developed before the Pretoria Group sedimentation over a period of at least 80 Myr, is marked by siliceous breccias (palaeo-karst infill) and conglomerates (reworked breccias). It was exhumed in pre-Karoo and post-Gondwana times. Glacial pavements and remnants of thin Lower Karoo sediments are also found on this polyphase surface. The gravels that make up these ‘runs’ and sinkholes directly or indirectly linked to these runs, are coarse-grained, very poorly-sorted, and are best described as diamictites. The ‘runs’ are narrow, elongated, generally positive ridges that meander across the dolomite surface and are up to 30 km long and between 80 to 300 m wide. They have always been regarded as post-Cretaceous drainage features linked to southward-flowing river systems. Diamonds were discovered in these ‘runs’ and they have produced some 12 million carats. However, no Cainozoic fossils or artefacts have ever been found in almost 90 years of mining. From new field evidence, geomorphological studies, age dating from inclusions in diamond and zircon and clay analyses, it is proposed that these coarse-grained runs represent proximal palaeo-eskers of the last deglaciation of the Dwyka continental ice sheet, that are preserved on this ancient ‘palimpsest’ surface. The age of the deposit is constrained by two populations of agate within the diamictites that are linked to two separate volcanic units of the Pretoria Group. In addition, the youngest crustal zircon ages from the gravels are 1 Ba, but mantle zircons from Lichtenburg suggest that these have been derived from Cambrian age kimberlites. Analysis of inclusions in diamond support a Neoproterozoic to Cambrian source for the diamonds, so the absence of diamonds from Mesozoic kimberlites and Cainozoic fossils within the gravels support the conclusion that the runs are of Karoo age. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
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Boreholes drilled in the Tshabong area of southern Botswana have delineated two southwest-trending valleys infilled with Lower Karoo Supergroup strata. The valleys occur beneath a cover of Kalahari beds and are separated by positive topography related to pre-Karoo quartzites. The base of the Karoo strata comprises a thick glacial sequence (Dwyka Group) in which two lithostratigraphic formations can be recognized. These formations are tillites of a basal Malogong Formation overlain by the heterolithic Khuis Formation, with lithofacies indicative of a number of glacial environments, including glacial re-advances. Preserved palynomorphs in these rocks indicate a marine setting. The overlying Ecca Group strata are also divisible into two formations, again of regional extent. A thin sandstone sequence at the base of the lower Kobe Formation is interpreted as evidence for a basal Ecca marine transgression. Overlying rocks contain freshwater palynomorphs. The upper Otshe Formation is made up of deltaic sediments. Locally, the Ecca Group is overlain by a thin sequence of Beaufort Group argillites. Palynology indicates that the Dwyka Glaciation in the southeastern part of the Gemsbok Sub-basin lasted from about 303 Ma to about 280 Ma, that is from the Late Carboniferous to the Early Permian. The overlying Ecca Group ranges in age from the Early to Late Permian. The palynology has confirmed the presence of Beaufort Group strata in southern Botswana.
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Two different localities within the fluviatile Neoproterozoic Malagarasi Supergroup of north-western Tanzania show striking evidence for tsunami related clastic sedimentary deposits. In each locality, the outcrops are characterised by the association of fine grained, thinly laminated shales at the bottom that are overlain by thick deposits of sandstones and conglomerates whose clasts and pebbles vary in size and angularity/roundness. In each case, the two rock units are separated by an erosional surface. The basal shale layers are consistent with deposition in deep shelf environments which are in stark contrast to the immediately overlying conglomerates/sandstones that suggest reworking under high energy conditions. The consistent association of erosional surfaces coupled with the deposition of adjacent low and high energy facies are interpreted as a result of an ancient earthquake triggered tsunami or storm that abruptly changed the depositional energy at the two localities. We propose that a backwash wave transported pebbles and sediments from the shore setting towards the basin interior depositing them on the shale units. Given the limited preservation of such unusual sedimentological deposits in ancient terranes, these two localities in the Neoproterozoic Malagarasi basin provide information on the effects of tsunami or storm impacts in Precambrian basins of Tanzania.
Keywords: Malagarasi Supergroup; Tsunami deposits; Shales; Sandstones; Conglomerates
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Research Article| September 01, 2015 LITHOSTRATIGRAPHY OF THE EMAKWEZINI FORMATION (KAROO SUPERGROUP), SOUTH AFRICA E.M. BORDY; E.M. BORDY Department of Geological Sciences, University of Cape Town, Private Bag X3, Rondebosch, 7701, South Africa, e-mail: emese.bordy@uct.ac.za Search for other works by this author on: GSW Google Scholar R. PREVEC R. PREVEC Albany Museum and Rhodes University, Somerset Street, Grahamstown, 6139, South Africa, e-mail: r.prevec@am.org.za Search for other works by this author on: GSW Google Scholar Author and Article Information E.M. BORDY Department of Geological Sciences, University of Cape Town, Private Bag X3, Rondebosch, 7701, South Africa, e-mail: emese.bordy@uct.ac.za R. PREVEC Albany Museum and Rhodes University, Somerset Street, Grahamstown, 6139, South Africa, e-mail: r.prevec@am.org.za Publisher: Geological Society of South Africa First Online: 09 Mar 2017 Online ISSN: 1996-8590 Print ISSN: 1012-0750 © 2015 Geological Society of South Africa South African Journal of Geology (2015) 118 (3): 307–310. https://doi.org/10.2113/gssajg.118.3.307 Article history First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation E.M. BORDY, R. PREVEC; LITHOSTRATIGRAPHY OF THE EMAKWEZINI FORMATION (KAROO SUPERGROUP), SOUTH AFRICA. South African Journal of Geology 2015;; 118 (3): 307–310. doi: https://doi.org/10.2113/gssajg.118.3.307 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietySouth African Journal of Geology Search Advanced Search Abstract The middle (?) to upper Permian Emakwezini Formation is part of the lower Beaufort Group (Karoo Supergroup) in the Lebombo Basin in the eastern part of South Africa. The unit is highly significant both palaeobotanically and economically due to its plant-rich carbonaceous mudstones and multiple semi-anthracite to anthracite seams that range in thickness from <1 m to up to 15 m, respectively. Owing to rare high quality outcrops as well as post-depositional faulting and intrusions, some of its lithostratigraphic attributes (e.g., thickness, regional distribution) are undefined; however its abundant and diagnostic Late Permian plant fossils are helpful in the regional mapping of the unit. The Formation is considered a stratigraphic equivalent of the Normandien Formation (and informal Estcourt Formation) of the eastern and north-eastern, and the Balfour Formation of the southern main Karoo Basin. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
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