Geology > Pre-Karoo > Witwatersrand Supergroup

Witwatersrand Supergroup

Introduction

Sedimentary deposition across the stable granite-gniess basement commenced around 3 billion years ago. In stratigraphic terms the Witwatersrand sequence is divided into two divisions, the lower dominantly marine, slate rich West Rand Group and the upper dominantly aluvial sandstone rich Central Rand Group. Combined, these units make up a sedimentary package up to 7km thick.

Sedimentary deposition ended approximately 2,700 million years ago when widespread faulting resulted in extensive lavas of the Ventersdorp supergroup covering the Witwatersrand Basin. Reef groups located on the West Rand in order of oldest to youngest are the Main, South, Johnstone, Livingstone, Bird, Kimberley, Elsburg and Ventersdorp Contact Reefs.

The Supergroup consists of quartzites, shales (iron rich), conglomerates (pyrites), diamictite, pyritic sands and gold in quartz-pebble conglomerates.

The main areas underlain by this Supergroup are: Witwatersrand, Heidelberg-Nigel-Evander, Klerksdorp-Carltonville, Parys (Vredefort dome), Welkom, Piet Retief and Paul Pietersburg (Pongola).

Geohydrology

•	They have water bearing properties in shear zones, weathered areas and intrusive rock formations.

•	Weathering of sedimentary formations (Wits) before the deposits of the Black Reef is important in water supply – connate water.

•	Water is found mostly in contact between weathered and fresh diabase (dark coloured altered dolerite), contact between quartzite and shale, and on contact between shale and underlying diabase sill.

•	Fault zones in mines very important, and can be a few hundred meters wide –results in millions of liters of water flowing in depth (up to 2000m).

•	Intrusions important for water seams in mines and siting of boreholes.

•	Shear zones creates preferred pathways and can store water.

•	Pongola complex has in general a low yield due to contact zones with metamorphic country rock.

Witwatersrand1

Source: http://www1.newark.ohio-state.edu