Contorted Beds in Braamfontein EarthCache
Contorted Beds in Braamfontein
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Located in Melle Street in Braamfontein, there is a safe place to get out of your vehicle and observe the exposed beds. For more beds, look on the other side of Hoerskool Helpmekaar in Jan Smuts Avenue just below Wits University.
The Contorted Bed is one of the important marker beds of the West Rand Group which forms the lower part of the Witwatersrand Supergroup. {NB: These groups are geological naming groups}. It occurs approximately 750m above the base of the West Rand Group and about 3650m below its top; immediately overlying the West Rand Group is the Main Reef of auriferous (gold bearing) conglomerates or reefs that formed the basis of mining on the Central Witwatersrand. The Contorted Bed comprises of a sequence of variably ferruginous, flaggy, reddish to brownish or purply-black slaty shales. In places they are banded or striped with thin interbedded quartzite layers.
The lower parts contain layers which are highly ferruginous, with a magnetite and/or hematite content of up to 48 percent (much higher than normal), and thin siliceous and jasperoidal layers, forming what is known as banded ironstone. The regular alternation of red, white and black stripes has also led to it being given the name of “Calico Rock”. The major features are the conspicuous banding and the highly crumpled and contorted laminations, from which the appropriate name Contorted Bed was derived. Both unique in appearance and its lateral persistence contribute to the immeasurable value of the Contorted Bed as a marker horizon.
The magnetic nature of the Contorted Bed and other ferruginous shales in the West Rand Group has been used extensively as an aid in exploring for the overlying gold-bearing Witwatersrand formations. It was demonstrated as early as 1930 that the magnetism of these beds could be detected under extensive cover of younger formations, and because of the distance between the various marker beds and the Main Reef was known. It became possible to deduce the position of the Main Reef from this magnetic response. This factor led directly to the discovery of the West Wits Line of gold mines below the dolomite. And subsequently the extensive use of the magnetometer in conjunction with other geophysical techniques played a significant role in the discovery of the goldfields in the Oragne Free State and at Evander in Mpumalanga.
The Contorted Bed is resistant to weathering and is commonly ridge-forming. The crest of the ridge to the south of this site is part of the continental divide or watershed, separating waters that flow north into the Limpopo River (and Indian Ocean), from those that flow south into the Vaal River (and Atlantic Ocean).
(Acknowledgments: guidebook to Sites of Geological & Mining Interest on the Central Witwatersrand.; Geological Society of South Africa; 1986).
In order to qualify to log this cache, you need to answer the following questions and s=email the cache owner. Any logs not accompanied by an email will be deleted.
1) Take a photo of you and your GPSr at this spot with the Contorted Bed visible and include in your log [Optional].
2) What mineral is in a magnetite or hematite to make it ferruginous?
3) Explain what a watershed is.
4) Look carefully at the various layers and explain some of the crystals, colours or other interesting components you observed in the Contorted Beds.
Additional Hints
(Decrypt)
Erzrzore gb fraq lbhe rznvy!