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Pre 16 Geology of the Barberton Greenstone Belt: Processes of the early Earth

Field Trip Pre-16 took place August 22 (arrival) to August 28 (departure) and focused on the geology of the Barberton Mountain Land. The field trip was led by Profs. Don Lowe (Stanford University, USA), Christoph Heubeck (Jena University, Germany), and Gary Byerly (Louisiana State University, USA). Eighteen participants from China, Japan, Italy, Brazil, Belgium, the UK, and the US learned about early life, the growth of continents, ultramafic magmatism, historical and active gold mining and the many unusual factors that shaped the surface conditions of the early Earth.

Day-to-day organization of the trip lay in the capable hands of Astrid Christianson of Barberton Community Tourism who had assembled seven bakkies and staffed them by experienced local drivers, all of them active members of the community and involved either in the tourism industry or in mining / geology. This proved ideal for both sides: The international guests, all of them newcomers to greenstone belt geology, not only learned about geology from the three field trip leaders but also about local history, botany, wildlife and culture from their drivers; the drivers, in turn, experienced first-hand what attracts worldwide scientific attention to their region and also improved their own understanding of their home mountain range.

On the first day, Gary Byerly introduced the participants to the world of ultramafic magmatism by presenting komatiites, pyroxenites, and peridotites from the Sawmill and Pioneer Ultramafic Complexes west of Barberton. Many fine-grained interbeds, formerly thought to be shear zones, are actually (partially aquatically reworked) mafic and ultramafic ash beds, some of them with unambiguous sedimentary structures. Together with the spinifex-textured komatiites and coarse-grained cumulates, these units drew a clear picture of high-volume, high-temperature, low-viscosity Archean magmatism and its interaction with oceans and the atmosphere.

Fine-grained mafic ash in a unit of the Piuoneer Ultramafic Complex, Weltevreden Formation, Onverwacht Group (ca. 3300 Ma) showing cross-bedding stratification.
Tubercled Gecko (Chondrodactylus sp.) found under a rock at the Pioneer Complex.
Leucospermum sp., member of the Protea family, in early bloom. Winter in the Barberton Mountains is coming to an end.
Field photograph showing part of the Pioneer Ultramafic Complex east of Barberton. The Moodies Hills, underlain by resistant quartz-rich Moodies sandstones, can be seen in the background. Light-colored strata consist of dunite, red strata in the foreground of pyroxenite.

The second day brought the field trip to the Buck Reef Chert, basal chert of the Kromberg Formation in the Onverwacht Group. Don Lowe presented the regional geology of the Onverwacht Anticline and then led the group through a deepening-upward section of this unit, in places up to 300 m thick and thought to be the thickest Archean chert. He presented clear evidence of initial evaporitic deposition in shallow lagoons on a large volcanic plateau, overlain by abundant microbially-laminated cherty sediment (some of the earliest microbial body fossils) which graded in ferruginous chert. The group examined evidence for the Quaternary (not Archean) nature of iron-oxide precipitation near the top of the unit before returning to Barberton.

Don Lowe (right) explaining central BGB geology to field trip participants Mike Daly, Chungjing Wei, Tim Pharaoh, and Stefano Zincone (left to right). Person second from left is unidentified.
Silicified crossbedded and wave-rippled sandstone overlain by chert-slab conglomerate; Buck Reef Chert (Kromberg Formation), central Barberton Mountain Land.
Chinese field trip participants relax at the Shellhole, the local MOTH Club, and admire the vintage militaria on display there.

Meteorite impacts and sedimentary barite sedimentation stood in the centre of attention of the third day. A series of large but distant meteorite impacts had generated melt droplets which had rained out from near-Earth orbits as decimetre-thick fallout deposits, blanketing the sea floor and preserved in strata of the basal Fig Tree Group. Don Lowe explained the multifold consequences of these nonuniformitarian events to the early Earth, including shattering of the sea floor, partial evaporation of the oceans, reworking by tsunamis and partial extinction of life.

Close-up image of spherule bed S3 in the basal Mapepe Formation of the Fig Tree Group. Nearly undeformed spherical grains show quench structures and consist of variably silicified Cr-diopside and sericite. They represent former melt droplets from a major distant meteorite impact.
Don Lowe (right) explains geologic map pattern to (from left) Prof. Chungjing Wei (Beijing University, PR China), Barbertonian Roland Jones, Prof. Mike Daly (Oxford, UK), and Barbertonians Dave Mourant and Terry Bellington.
Geologists set out to climb a hill exposing strata in the Barite Valley of the central Barberton Greenstone Belt.
Research student Nadja Drabon (right) is interviewed at the rock outcrop by a Nelspruit-based SABC TV crew. Emlembe, highest peak of Swaziland (1862m), is visible in the center background.
Sawed and polished rock slabs of typical Barberton Greenstone Belt rock types on display at the group accommodation for close-up study and photographing.

Christoph Heubeck presented the geology of the central BGB along the transect of the R40, a scenic paved road across the range from Barberton to the Josefsdal/Bulembu border post. This road, known as the Barberton-Makhonjwa Geotrail, features about a dozen parking lay-bys with well-designed panels explaining geological perspectives of early Earth. A second focus of the day lay on the depositional environment and ecological niches of abundant widespread photosynthetic microbial mats in high-energy tidal zones of the Moodies Group, ca. 3220 Ma, clearly demonstrating how early life had adapted to cope already with challenging environments.

Microbial laminations interspersed with sandstone and overlying conglomerate of a fluvial-supratidal sandplain, exposed in the Moodies Group of the Saddleback Syncline, central Barberton Greenstone Belt. Angular green clast is composed of altered ultramafic rock.
Panoramic view from the Lebombo Overview viewpoint, looking westward along the Ushushwana and Lomati River valley into northernmost Swaziland. Generally, resistant strata in this image are composed of quartz-rich Moodies Group sandstones, slopes of Fig Tree shales and greywacke, and valleys of soft Onverwacht Group ultramfic rocks.
Geotrail panel at the Lebombo Overview stop of the Geotrail, the R40 between Barberton and Bulembu. The panel is one of a series explaining the natural history of the Makhonjwa Range, visible in the background.

The program of the fifth and last day led participants along the northern margin of the greenstone belt east of Barberton. The field trip examined the large “gneiss dome” of the Kaap Valley tonalite and, in several stops, its complex contact and structural relationships with adjacent coeval and younger strata. The afternoon was dedicated to historical and active gold mining, during which the group visited illegal alluvial gold mining activities in Fig Tree Creek, the stupendous underground hole of the Golden Quarry of 1885, historical modern extraction and refining techniques at Sheba Mine, and the museum integrated into its Geology office, all enthusiastically presented by Chris Rippon of PanAfrican Resources. An evening farewell party concluded the field trip, and the participants departed the next morning to Cape Town to take part in the icebreaker program that evening.

Field Trip drivers. From left: Terry Bellington, Roland Jones, Pieter Visser, Wynand Engelbrecht, Prof. Christoph Heubeck, Dave Mourant, Pete Wilson. Not pictured: Tyler Robinson.
Field trip participants Nina Bellot and Vinciane Debaille (both University of Bruxelles, Belgium) examine a simple sluice box used by an illegal miner washing sediment in Fig Tree Creek near Barberton for gold. Uncontrolled alluvial gold mining has severely damaged substantial reaches of drainages in this region.
Gravel pits and heaps resulting from uncontrolled alluvial gold mining in the floodplain of Fig Tree Creek. The outcrop in the left background had been described as recording evidence for a Moon orbiting in a closer orbit during the Archean.
Flakes of visible gold in sheared cherty ultramafic rock from Sheba Mine.
Head gear of ZK Shaft of Sheba Mine.
Field trip participants climb a scree slope in Golden Quarry of Sheba Mine.
Chris Rippon, Head Geologist of Sheba Mine (second from left) shows ore samples at the Geology office.

Participants showed themselves impressed by the level of detailed investigation which the excellent preservation of greenstone belt geology made possible, and expressed their satisfaction at the depth and breadth of information they had received through the three field trip leaders and the guidebook. Field trip leaders, in turn, praised the curiosity and the inquisitive questions of the visitors and emphasized that the success of the field trip was to in no small part due to the professional hospitality of the members of the Barbertonians who organized an evening at the local MOTH club, sent a national TV crew to join us in the field, proudly presented their town on impromptu sightseeing tours and always had cold drinks handy. Many thanks to all involved – Barberton presented itself very well!

Barberton Tourism marketing manager, Astrid Christianson, and staff member Daphne serving hot lunch to field trip participants near the Old Sheba cementary.

Christoph Heubeck


Due to its excellent exposure, low-grade metamorphic overprint and considerable extent the Barberton Greenstone Belt, straddling the South Africa - Swaziland border, offers profound insights on Archean processes and events that shaped the evolution of the crust of our planet. These processes include, among others, growth of the early continental crust; the relative roles of horizontal vs. vertical tectonics; controls on the origin, locations, metabolism, and ecology of early life; the nature of magmatism on the early Earth; surface conditions of early Earth, including the composition and physical/chemical state of oceans and atmosphere; non-uniformitarian depositional environments and sediments, early weathering conditions, the early Earth-Moon system, and the role of meteorite impacts in early crustal development and biological evolution.

This trip will introduce participants to many of the principal outcrops, lithologies and styles of deformation of the Onverwacht, Fig Tree and Moodies Groups (3.57-3.21 Ga) and to key lines of evidence on which much of our knowledge of early Earth processes is based. Participants will be based in Barberton and use 4-WD vehicles to access outcrops in the Makhonjwa Mountains. Moderate hiking is required at several stops. The trip begins and ends in Nelspruit from where participants will be shuttled to and from Barberton. The beginning of this trip is set to coincide with the end of the Badplaas Workshop.

Field Trip Leaders: Christoph Heubeck, Don Lowe, Gary Byerly
Start: Participants will be picked up from Nelspruit on Sept 22 and transferred to Barberton
End: Program ends in the evening of Aug. 27. Participants will be transferred from Barberton to Nelspruit early on Aug. 28
Departs: Tuesday, Sept 23, 9 a.m., from Jathira Guest House outside Barberton for first day in the field
Dates: 6 days, Monday 22nd to Sunday 28th August 2016
Price: R 15 000 per person sharing, single supplement not applicable

Info from Trip Leader

resourcing future generations