Terms to understand:
Sedimentary rock: One of the three general rock types (the others being igneous and metamorphic rock) which forms from the deposition and compaction of sediment into layers.
Clast: A clast is a fragment of an already present mineral or rock which forms from the weathering of the original rock.
Mass movement: Mass movement is a geological term used to describe the movement of surface material down a slope and examples include landslides, avalanches and rockfalls.
This EC focuses on the diamictite exposed at GZ which forms part of the Promise Reef of the Government Subgroup within the Witwatersrand Supergroup.
Diamictite and its origins:
Diamictite is quite an unusual rock type due to the fact that normally during the process of sedimentation, particles are sorted according to their size and you don’t often find various sized clasts all jumbled into one rock. It may be hard to see this at the exposure but this diamictite consists of fine silt particles with sand and pebbles. It was thought to have formed through a mudflow that was potentially caused by an earthquake.
Illustration showing sediment sorting in sedimentary rocks. Diamictite falls in the poorly sorted category.
A mudflow is a form of mass movement. Mudflows are differentiated from other types of mass movement by their water content and happen when the soil is saturated with water and earth and rock rushes down a slope in a fluid-like manner. This rock probably formed when rock and earth on the side of a steep slope rapidly moved downwards after being triggered by an earthquake. It deposited in an underwater valley forming thick muddy deposits which over a very long time were compacted and formed the diamictite rock you see here.
The differences between conglomerate, diamictite and tillite:
Conglomerate is the term used to describe sedimentary rocks that consist of large round gravel-sized clasts (greater than 2mm in size) which are imbedded in a cementing material such as sand.
Both diamictite and tillite consists of poorly sorted or unsorted sediments (such as sand and silt and pebbles with various clast sizes) in a mud matrix. The difference between the two depends on how general you want to be. The reporting of diamictites is very confusing in the literature and diamictite is just defined as poorly sorted clasts in a mud matrix. Diamictite is called tillite when the rock has a glacial origin.
Grain size scale for sedimentary rocks.
LOGGING REQUIREMENTS
In order to substantiate your visit, please submit your answers to the following questions to me via the Message centre or an email. In your message/email please mention the Earthcache which you are sending answers for. Any logs not accompanied by answers within 7 days will be deleted.
1) Describe the diamictite exposed here – its various colours, texture and angle of the layers.
2) Where do you think the colours of the rock come from?
3) Why do you think the layers are not horizontal?
4) Looking at the exposure, which type of sediment (sand or silt or pebbles) does it mostly consist of and why?
OPTIONAL: Any non-spoiler photos and/or a photo of you with your GPS by the GSSA sign will be appreciated.
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References:
1) Goodwin, A.M., Precambrian Geology: The Dynamic Evolution of the Continental Crust, Academic Press, 1991.
2) Mendelsohn, F. and Potgieter, C.T., Guidebook to Sites of Geological and Mining Interest on the Central Witwatersrand, The Geological Society of South Africa, 1986.
3) Stow, D.A.V., Sedimentary Rocks in the Field: A Color Guide, Academic Press, 2011.
4) Tankard, A.J., Crustal Evolution of Southern Africa: 3.8 Billion Years of Earth History, Springer-Verlag, 1982.
