What is Conglomerate?
Conglomerate is a clastic rock (fragments of pre-existing minerals and rock sedimentation) that contains large (greater than two millimeters in diameter) rounded clasts. The space between the clasts is generally filled with smaller particles and/or chemical cement that bind the rock together.
How Does Conglomerate Form?
Conglomerate forms where a sediment of rounded clasts at least two millimeters in diameter accumulates. It takes a strong water current to transport and shape particles this large. So the environment of deposition might be along a swiftly flowing stream or a beach with strong waves. There must also be a source of large-size sediment particles somewhere up current. The rounded shape of the clasts reveal that they were tumbled by running water or moving waves.
Most conglomerates are composed of angular to subangular sandstone intraclasts, the polygonal plan view of which is similar in outline to the areas between cracks in the sandstones. The deformation implied by the cracks, sporadic distribution of conglomerates, high degree of scouring indicated, and angularity of the intraclasts argue that, rather than storms, the conglomerates were generated by occasional tsunamis. The sweep of these tsunamis across the shallow intracratonic sea created extraordinarily strong oscillating bottom currents at the deeper reaches of storm wave base. “Tsunamites” are therefore identified with confidence virtually for the first time in shallow subtidal shelf deposits.
This site contains the Tsunami Conglomerates.
At this point about 3.2 billion years ago this was a sloping, slightly uneven, deep ocean floor, below the effects of the ocean waves and currents. Silica from volcanoes or weathering form flakes that settled on the muddy sea floor and solidified forming chert. An undersea volcanic explosion, earthquake or even a distant meteorite impact may have churned this part of the ocean. Such a violent event may have caused a tsunami. The thin chert layer was ripped up and it's fragments were carried in a powerful turbulent current for hundreds or perhaps thousands of meters. As the current slowed, the fragments settled in the sea floor channel. Cherts is hard and more resistant as the surrounding shales so we see these chert deposits as boulders today.
To log this EarthCache you have to e-mail me the answers to the following questions:
1. According to the panel, the rock face behind the boulder is made of what quiet water deposits?
2. What is the difference between an alluvial conglomerate and an tsunami conglomerate?
3. Take a look at the rock in front of the information plate. What is the average thickness of the different layers?
4. When was this geological formation formed according to the panel?
Uploading photos to the cache page is the best way to say thank you to the cache developer and to encourage others to visit the location but is only optional.
NOTES: - There are no other facilities along the trail except for very nice picnic spots.
- The distance is about 40km from Barberton on a narrow and winding road up to the Bulembu border post to Swaziland.
- Little or no cellphone reception is available on the route.
- You can email me your answers in either Afrikaans or English.
Note that a handy guidebook specially compiled for this route is on offer at Barberton Tourism in town.
Enquiries: 013-712 2880 or www.barberton.co.za
Guided tours by Tony Ferrar on 072-376-2581 and Dave Mourant on 082-923-5366.
Thank you to the Barberton Chamber of Business and all those involved to develop this geotrail.
