Geology of the Tswaing Crater

(Above) Cross-section of the Tswaing crater modified after Brandt, D., (1994), Brandt & Reimold (1999), Partridge & Reimold (1990).

Morphology
The Tswaing crater has a simple bowl shape with a 1.13 km diameter. The outer rim is composed of shattered rock called breccia and is elevated nearly 60m above the surrounding plains. Most of the original ejecta blanket has been eroded away, however numerous large granitic blocks can be found out to hundreds of meters from the crater rim.

Geology of Tswaing
Igneous Rocks: The most common rock type at the Tswaing crater and in the surrounding region is the Nebo Granite. The Nebo Granite is part of the Bushveld complex - a large layered igneous intrusion. The Bushveld itself is a geological novelty, consisting of several vertical kilometers of layered magmatic rock which extends horizontally for several hundred kilometers.  The 2.06 billion year-old Bushveld complex is also one of the world's largest sources of platinum group metals.

Various small intrusions and dykes also cut into the granitic rock that forms the crater rim.These small intrusions are much older than the crater (1.3 billion years old).

Projectile: Unlike other small impact craters (e.g. Barringer Meteor Crater), the Tswaing projectile is believed to be chondritic (stony meteorite) in composition. Other small impact craters are all associated with iron projectiles.

Breccia and impact generated rocks: A sandy breccia layer, sampled in a drill core of the crater,  contains abundant shock-metamorphosed quartz and feldspar grains, in addition to melt and glass fragments (after Reimold et al., Pretoria Saltpancrater: Impact Origin Confirmed,Geology, 20, 1992). This type of impact generated rock, known as suevite, consists of impact breccia clasts with inclusions of impact melt.

Sedimentary Rocks: An important group of regional sedimentary rock units known as the Karoo Supergroup is found near the crater. These rocks were thought to cover the granite in the local vicinity during the time of the impact. The sediments which formed these rocks were deposited 220 million years ago during the Triassic. Karoo sedimentary rocks consist of shale, sandstone and a gritty sediment.

Lacustrine Sediments: The center of the Tswaing crater floor is covered with a hyper-saline lake. The high salinity of this lake made it a source for the commercial extraction of soda brine from 1912 to 1956.  At various times in the past, this lake has deposited carbonate mud, limestone, and evaporites on the crater floor to a current depth of nearly 90m.

This web site is based on information originally created for the NASA/UA Space Imagery Centerís Impact Cratering Series.
Concept and content by David A. Kring and Jake Bailey.
Design, graphics, and images by Jake Bailey and David A. Kring.
Any use of the information and images requires permission of the Space Imagery Center and/or David A. Kring (now at LPI).