The Geocongress is pleased to offer seven field trips, six one-day trip on a vast range of topics and a two-day trip to the Vredefort Impact Structure. Registration is now open and will close on 30 June 2018.
|Field Trip Title||Date||Cost|
|Jo’burg Geoheritage||Saturday 21 July 2018||R600|
|Cradle of Humankind||Saturday 21 July 2018||R700|
|Mafic Volcanic Rocks of the Central Kaapvaal Craton||Sunday 22 July 2018||R600|
|Geology of the Vredefort Impact Structure||Saturday 21 & Sunday 22 July 2018||R1 600|
|The Johannesburg Dome, a window of Archaean crust in the centre of the Kaapvaal Craton||Saturday 21 July 2018||R500|
|Geology of Johannesburg||Sunday 22 July 2018||R450|
|A transect of Archean Witwatersrand-Paleoproterozoic Transvaal Depositional Systems and the Great Oxidation Event||Saturday 21 July 2018||R550|
Field leaders: Morris Viljoen, Richard Viljoen
Saturday 21 July 2018
Cost – R600-00 incl. VAT
- An introduction and overview of the geology, topography and outstanding landmarks of the Egoli region will be given from the rooftop of the Illovo Edge building, which provides an excellent 360″ view.
- Short tour of Northwards, mansion of Randlord John Dale Lace.
- The Orange Grove quartzite escarpment. An overview of Witwatersrand geology will be presented and cross-bedded, ripple-marked quartzite with an age of 2.9 billion years will be examined.
- Viewing of other historic houses and schools on Parktown Ridge and upper Houghton as well as in the suburbs off Yeoville, Berea, Hillbrow & Braamfontein. The history of these early suburbs will be discussed.
- Contorted Bed, folded Banded Iron Formation. The magnetic property of this rock led ultimately to the discovery of the West Wits or Carletonville goldfield. The importance of this discovery to the economy of South Africa will be discussed.
- Drive past and view historical buildings of early Johannesburg such as the Rand Club, JCI House, Consolidated building, Standard Bank building and others, in downtown Johannesburg. Story of early mining houses and Randlords.
- Ferrera Stope Museum. View original, near-vertical Ferreira Stope on the Main Reef Leader, dating from 1886. Story of Ferreira and early mining history and how the cyanidation process saved the Witwatersrand.
- The Workers Museum tells the story of migrant African mine-workers, essential to the success of the Witwatersrand.
- Footprint of Park Central conical sand dump on which the Top Star Drive-in cinema was situated. Story of reprocessing of dumps and environmental impacts of goldmining.
- Elsburg conglomerate exposed in Moffat Park and at locality at south end of Central St, with excellent views of the historic Victorian style house of Johannes Petrus Meyer (one of the Johannes’s after whom Johannesburg was named).
- Massive, amygdaloidal basaltic Ventersdorp lava outcrop.
- Exposure of talcose, chloritic, ultramafic (komatiitic), basal Ventersdorp lava.
Cradle of Humankind
Field leaders: Tebogo Vincent Makhubela, Stephanie Baker
Saturday 21 July 2018
Cost – R700-00 incl. VAT
The field trip will feature three well-known fossil sites of the Cradle of Humankind:
- Sterkfontein Cave – well-known for producing the largest number of Australopithecus fossils and for having the longest-running fossil excavations in the world.
- Drimolen fossil site – has yielded several specimens of Australopithecus robustus and early Homo dated between 2 and 1.5 Ma.
- Rising Star site – famous for the preservation of abundant Homo naledi fossils, an assemblage of the largest collection of fossils from a single primitive hominin species ever discovered in Africa.
The focus of the trip field will be the geology and geomorphology of the area and how they influence attempts to radiometrically date the fossil-bearing deposits in the caves. Discussions will also include geochronology issues commonly debated, such as the age of the Little foot skeleton. The visit to Sterkfontein Caves will include the tourist tour.
Mafic Volcanic Rocks of the Central Kaapvaal Craton
Field leaders: Grant Bybee, Fabien Humbert
Sunday 22 July 2018
Cost – R600-00 incl. VAT
The Kaapvaal Craton hosts several extrusive and intrusive Large Igneous Provinces as well as smaller volume volcanic outpourings. This 1-day field trip investigates two volcanic and volcanoclastic successions preserved at the base of the Ventersdorp Supergroup and in the uppermost portions of the Transvaal Supergroup (Hekpoort Formation). These Precambrian lava flows preserve a suite of remarkable volcanic and volcanoclastic textures, providing opportunity to observe and debate magmatic and eruptive processes operating during the outpouring of these lavas.
Klipriviersberg Nature Reserve & the Basal Lava Successions of the Ventersdorp Supergroup:
- Overview of the local geology at the highest point in the Klipriviersberg Nature Reserve
- Viewing of successive ~2.7 Ga lava flows of the Klipriviersberg Group of the Ventersdorp Supergroup, including porphyritic and amygdaloidal units, flow top and bottom features, including preserved lava flow toes and possible lava tubes.
Travel to Meerdal Quarry to view and debate spheroidal textures preserved in the Klipriviersberg Group volcanics; possible origins of this texture include liquid immiscibility or magma mixing features.
‘Hekpoort variolites basin’ in and around Wedela (N12 direction Potchefstroom, then turn right direction Wedela/ Kusasalethu gold Mine):
- Excellent outcrop of Hekpoort variolite flows in cross section, flows are 40-80 cm thick and present succession of layers rich in cooling columns with layers rich in varioles (which are up to 10 cm thick).
- Short walk to the north (close to the mine dump of the Kusasalethu Gold Mine) to observe coalescent varioles including the ‘giant’ varioles layer whose diameter can reach 30cm wide.
Short drive to Wedela to observe thin layers of Hekpoort pyroclastic breccias and stratified lapilli tuff south of the town
- Last stop in the western part of the area where outcrop of the smallest Hekpoort varioles can be seen (constant diameter of 1cm) overlying massive tuff-breccia.
Geology of the Vredefort Impact Structure
Field leader: Roger Gibson
Saturday & Sunday 21 & 22 July 2018
Cost – R1 600-00 incl. VAT
Accommodation in shared rooms with communal showers (minimum two, max four people per room)
The Vredefort Dome represents the deeply eroded remnant of the central uplift of an originally 250-300 km diameter impact crater that formed 2.02 Ga ago.
The extreme uplift that formed the Dome rotated significant portion of the Kaapvaal craton crust on-edge, presenting an unprecedented window into almost 1.5 billion years of Mesoarchaean to Palaeoproterozoic Earth history and a 25 km-deep transect through the crust. In addition to providing an opportunity to examine rocks of the Dominion Group and Witwatersrand and Ventersdorp Supergroups, this field trip will also cover the Mesoarchaean TTG-greenstone rocks underlying the supracrustal succession and provide an opportunity to discuss their significance in debates about deep crustal structure and whether the Moho is exposed within the Dome. Impact features include pseudotachylitic and impact-melt breccias, shatter cones, impact-generated folds and unusual thermal metamorphic features. As much of the trip will be conducted in the Vredefort Dome World Heritage Site, aspects of the region’s archaeological and historical record will also be integrated.
The Johannesburg Dome, a window of Archaean crust in the centre of the Kaapvaal Craton
Field leaders: Carl Anhaeusser,Jeremie Lehmann, Axel Hofmann
Saturday 21 July 2018
Cost – R500-00 incl. VAT
The purpose of this fieldtrip on Saturday 21 July 2018 is to show a variety of rocks that make up the Johannesburg Dome. We will visit outcrops of granites located in the southern part of the dome, greenstone remnants, and a spectacular outcrop (Nooitgedacht Platform) summarising the stages of formation of the Archean crust in the northern part of the dome.
The area around Cosmo City is a granitoid-greenstone terrain within the Johannesburg Dome. A large greenstone remnant is surrounded and intruded by ca. 3.1 Ga porphyritic granodiorites. Referred to as the Zandspruit remnant, it consists of a shallow-dipping ultramafic complex of harzburgite and pyroxenite metamorphosed at greenschist facies grade. The Nooitgedacht Platform is an exceptional river exposure where crosscutting relationships between different granite-greenstone components allow inferring the following succession of events: (1) mafic and ultramafic volcanic rocks and their plutonic equivalents are intruded by (2) a suite of TTG dated at 3340 Ma. (3) Two mafic dykes cross-cut the trondhjemitic gneisses, and are in turn (4) cut by granitoids dated at 3121 Ma and pegmatites dated at ca. 3000 Ma. Greenstone remnants are found NE of Krugersdorp. These include the intrusive Muldersdrift and extrusive Roodekrans ultramafic complexes, the latter locally associated with pillow basalts.
Geology of Johannesburg
Field leaders: Terence McCarthy and Jeremie Lehmann
Sunday 22 July 2018
Cost – R450-00 incl. VAT
Stop 1: Orange Grove Formation (Hospital Hill Subgroup)
- Basal non-conformity of the Witwatersrand Supergroup on altered and sheared basement granodiorite (either a palaeosol or hydrothermally altered granite – HAGS)
- Shale/siltstone member of the Orange Grove Formation, showing moderate cleavage.
- Shale dyke intruding a quartzite member of the Orange Grove Formation
- Herring-bone cross bedding in the Orange Grove quartzite indicating deposition under marine conditions (Kallenbach Drive).
Stop 2: Water Tower Slate marker. This iron formation is the lowermost of up to eight iron formations developed in the West Rand Group. Its lateral persistence is remarkable and it has been traced from KwaZulu-Natal (in the Mozaan Group) to Koster, south of Rustenburg (Koster borehole) southwards to the Free State Goldfield. The iron formations of the West Rand Group played an important role in the discovery of the Carletonville, Welkom and Evander goldfields. (cnr. Aida and Elsa Streets)
Stop 3: Ripple-marked Quartzite marker and basal unconformity of the Ventersdorp Supergroup on West Rand Group. The Ripple-marked Quartzite is another persistent marker bed in the shales of the Parktown Formation (Hospital Hill Subgroup). At this locality the marker bed, which dips at about 30°S, is unconformably overlain by a sequence of sedimentary rocks including diamictites, boulder beds and greywackes which form the base of the Ventersdorp Supergroup (Venterspost Formation or VCR). (St. Georges Street).
Stop 4: Alberton Formation of the Klipriviersberg Group (Ventersdorp Supergroup), a meta-basalt characterized by the presence of numerous feldspar phenocrysts. (Van Derel Street)
Stop 5: Highly deformed boulder beds and diamictites of the Platberg Group (Kameeldoorns Formation) overlying Klipriviersberg Group volcanic rocks. These rocks underlie Bezuidenhout Valley and occupy the core of a large graben structure.(Turnstone Street)
Stop 6: Government Subgroup and Mondeor Formation. These rocks are exposed in a narrow, fault-bounded sliver (Langermanskop) which is flanked by Platberg Group rocks to the north and Hospital Hill Subgroup rocks to the south.
- Southerly dipping quartzites of the Government Subgroup form the crest of Langermanskop.
- Mondeor Formation conglomerate containing locally derived, angular boulders of quartzite, most likely derived from the Government Subgroup.
- Unconformity between the Mondeor Formation and the Government Subgroup. The conglomerate beds of the Mondeor Formation on the northern face of Langermanskop dip northwards at about 40° whereas the quartzite of the Government subgroup dips southwards at about 30° indicating a pronounced angular unconformity between the two groups of rocks. Exposures (a) to (c) suggest that by Mondeor (Elsburg) Formation times, the underlying units of the Witwatersrand Supergroup had been severely deformed and were being reworked into the shrinking Witwatersrand basin. This locality marks the margin of the Witwatersrand Basin in Mondeor Formations times.
- On the western side of Langermanskop, Government Subgroup strata are overthrust on Mondeor Formation. Pseudotachylite fills the thrust plane and has been injected into the pebbly quartzites of the Mondeor Formation. The exposure reflects the continued effects of regional compression and on-going deformation during Mondeor Formation (Elsburg) times. All of the rocks in this area show a pronounced southerly dipping cleavage which post-dates pseoudotachylite formation. (Langermanskop)
Stop 7: Brixton Formation. The Brixton quartzite at this locality has been overturned and dips northwards at about 70°. The younging direction, as revealed by cross bedding, indicates that this unit has been overfolded towards the south and forms part of a large, east-west striking monocline which extends along almost the entire length of the Witwatersrand range of hills. The monocline is flanked to the north by the Rietfontein Fault. This fault appears to have acted as a thrust fault (probably with a left lateral component of movement as well) during Central Rand Group times, causing overfolding of the strata in the footwall of the fault plane. In middle-Ventersdorp times, however, the fault was reactivated as an extensional fault resulting in the development of a graben along its northern flank which became filled by sedimentary rocks of the Platberg Group (Stop 5). Palaeocurrents indicate a marine origin. (Scottish War Memorial, Highland Road)
Stop 8: Coronation diamictite, the Earth’s oldest known glacial deposit (although an older diamictite has recently been proposed). (Highland Road)
Stop 9: Overturned shale and quartzite of the Government Subgroup. (Katoomba Street)
Stop 10: Bird Reefs, The road cutting exposes a sequence of small-pebble conglomerates interbedded with quartzites dipping at about 20°S. The rocks are relatively friable due to the high phyllosilicate content. Cross bedding is well developed, indicating north to south palaeo-flow. (Vickers Road).
Stop 11: Mondeor Formation conglomerate. At this more distal locality, the clasts in the Mondeor Formation conglomerates are considerably smaller than seen at Stop 6. Quartzite and other clasts are similar in size and the former are fairly well rounded. The rocks dip at about 15°S. From this locality, which is underlain by the complete Witwatersrand succession, the hills marking the basin margin are clearly visible, providing a sense of the scale of the braid plain systems that developed towards the end of Witwatersrand sedimentation. (Moffat Park, South Rand Road)
Stop 12: Klipriviersberg Hills, type area for the lavas forming the Klipriviersberg Group lavas. (Comaro Road)
Stop 13: Mondeor Formation and Klipriviersberg. At this location the uppermost quartzites of the Witwatersrand Supergroup are exposed. The hills to the south are formed by the volcanic rocks of the Klipriviersberg Group (Ventersdorp Supergroup), the crest of which is underlain by the Alberton Formation seen at Stop 4. The soft, less resistant ultramafic rocks of the Westonaria Formation have been preferentially eroded, resulting in poor exposure of the base of the Klipriviersberg Group. (Frandaph Drive)
Stop 14: (time permitting) Contorted Bed banded iron formation. Axial planes of the folds are inclined out of the basin, suggesting that the folds are not the result of soft-sediment slumping but are more likely tectonic in origin, possibly related to the regional cleavage. (Melle Street)
A transect of Archean Witwatersrand-Paleoproterozoic Transvaal Depositional Systems and the Great Oxidation Event
Field leader: Nic Beukes
Saturday 21 July 2018
Cost – R550-00 incl. VAT
The focus of this one-day excursion would be to present participants with an overview of some of the major secular variations in depositional systems and environments that existed on the Kaapvaal Craton in Archean to Paleoproterozoic times and what they could imply for the redox conditions in the early atmosphere and oceans before and after the ~ 2,32 Ga Great Oxidation Event (GOE)
The latter is defined by the permanent disappearance of sulfur mass-independent isotopic signatures (S-MIF) in early diagenetic pyrite and/or sedimentary carbonates. We shall make only four major stops each with sufficient time available to observe in detail sedimentary structures and sequences and to discuss what they may imply about the presence or absence of free oxygen in the water column and ambient atmosphere.
These stops are as follows:
- The Mesoarchean basal Orange Grove to Parktown succession of the Witwatersrand Supergroup at the Walter Sisulu Botanical Garden. It represents an excellently exposed transgressive depositional systems tract from inshore shallow marine Orange Grove quartzites to outer shelf lower Parktown mudstones and the distal starved shelf Water Tower iron formation bed. A very clear example of a forced regression in the form of the Bulskop quartzite bed is also developed immediately above the iron formation bed. The short hike up the ridge also provides excellent views of the Johannesburg dome and the continental divide separating water flowing to the Indian and Atlantic oceans. There is also a Geological Garden with displays of megascale rock samples from the Archean basement up to coal of the Karoo Supergroup.
- Two closely spaced stops on the late Neoarchean Malamani carbonate succession north of Lanseria illustrating differences in the nature and composition of shallow subtidal and intertidal to supratidal stromatolitic dolomites.
- A stop on the oldest known pisolitic ironstone in the world namely that of the Paleoproterozoic Timeball Hill Formation at Klapperkop in Pretoria deposited immediately after the Great Oxidation Event.
- A stop on the Rietfonteindam glacial diamictite in the upper part of the Timeball Hill Formation either near Magaliesburg or Pretoria. This is one of the Paleoproterozoic glacial deposits commonly considered broadly correlative one of the so-called Huroniam glacial events in Earth’s History.
We shall also travel across the Great Oxidation Event located in the Rooihoogte Formation at the base of the Pretoria Group but due to poor outcrop, we would only make a stop if time permits. Rather a drill core intersection would be made available that we can look at during the field trip.