The 90-km-wide Vredefort Dome, located an hour’s drive from Johannesburg, is widely regarded as one of the Top 10 geological wonders of the world. It formed 2020 million years ago in response to a massive meteorite strike – the largest yet recorded on Earth – that left a 300 km wide and more than 1 km deep crater. The crater itself has long since disappeared owing to erosion, thus, the Dome exposes the deep levels below the original crater. Here we see exposed an exceptional cross-section through almost the entire crust of the Kaapvaal continent, turned on-end by violent, rebound-induced, uplift of the rocks following the initial shock collision. These rocks, which span nearly one-third of Earth history, contain abundant tell-tale signs of the impact, including shatter cones and dykes of impact melt and pseudotachylite that testify to the exceptional pressures and temperatures that marked the impact.

The tour stops will demonstrate both the pre-impact history of the Kaapvaal craton, from its magmatic birth through the formation of the Witwatersrand gold deposits, a giant flood basalt event that rivals the Drakensberg lavas, to the bloom of oxygen-producing cyanobacteria before the impact, and tell the impact story, from the origin of the asteroid to the impact event that, literally, shook the world and that had global environmental consequences. They will also showcase the development of different ideas about the origin of the Dome and the ultimate emergence of the impact hypothesis.

Woven into the landscape is also the history of our species, from traces of the San people carved into the rock outcrops, to remnants of extensive Iron Age settlements, the first Voortrekker dwellings, and gold and diamond mining. The unusual and highly varied landscape also supports an exceptional biodiversity.

Stops to be visited include: Witwatersrand gold reef, shatter cones, granite intrusion into the Witwatersrand sediments, metamorphism, Archaean basement complex rocks, pseudotachylite and impact melt rock and a petroglyph site.

Prof. Gibson has a few remaining copies of his book about the Vredefort impact, entitled “Meteorite Impact!”, which is now out of print. Should anyone be interested in a copy (R425), please contact him directly roger.gibson@wits.ac.za

• Date: 27 August 2022
• Time: 07h30-08h00
• Meet at: The Origins Centre Car Park at Wits University.
• Duration: About 8.5 hours, we expect to return at about 16:30.
• Charge: Members: R350, Non-members R380 (Maximum 30 participants)
• Contact Anne Raeburn to book: anner@mweb.co.za or 072 349 6507

Biography

Roger Gibson is Professor of Structural Geology and Metamorphic Petrology in the School of Geosciences at Wits University. He is a BSc (Honours) graduate of the University of Natal (Durban) and completed his PhD at Cambridge University in 1989 before joining Wits. He served 2 terms as Head of the School of Geosciences from 2009 until 2019. A past winner of both the Wits Young Researcher and Team Teaching Awards and the Jubilee Medal of the Geological Society of SA (twice), his research work over the past 30 years has covered aspects of the geology of the Namaqua, Limpopo and Damara Belts, Bushveld Complex and various impact structures in SA, Botswana, Ghana, Libya, Mexico, the USA and Brazil, foremost among them being the Vredefort Structure. He has published over 100 scientific papers and has written 3 books on the geology of the Vredefort impact structure, including Meteorite Impact!