Location
The location of this Earthcache is a disused quarry in the Umkomaas area.
Umkomaas Quarry (disused)
Dwyka Tillite
The Dwyka Group forms the lowermost and oldest deposit in the Karoo Supergroup basin. This basin extended across much of southern Gondwana and records many million years of geological history.
The rocks overlying the Natal Group are a thick mass of tillite that was deposited in a glacial environment by retreating ice sheets about 300 million years ago.
At this time South Africa was part of the supercontinent Gondwana, which was situated near the south pole and covered with ice. Rocks imbedded in the slowly moving ice sheets scoured and polished the underlying older rocks giving rise to glacial pavements. Striation directions indicate that ice flow was from north to south - valuable information when it comes to reconstructing Gondwana.
Tillite is mostly a very fine-grained, blue-grey sedimentary rock comprised of clay matrix with inclusions (or clasts) of many other fragments picked up by glaciers during their travels.
Dwyka tillite is by far the most abundant parent material in the area. It consists of sediments laid down by receding glaciers, which left behind fine-grained glacial moraine or mud with rock fragments. It is about 350 million years old and is up to 350 m thick.
Dwyka Tillie Distribution in Kwazulu Natal
Dwyka tillite quarries in the Province of KwaZulu-Natal have shown remarkably stable slope faces even though some of them were last quarried over 30 years ago. This can be attributed to their resistance to weathering, the high degree ofjoint surface roughness and the general lack of any joint infill.
View of Umkomaas Quarry
Formation of Tillite
During the Late Carboniferous period the lithosphere underlying what is now the Karoo Basin migrated over the South Polar Region. This resulted in southern Gondwana being covered by a major ice sheet. As the ice sheet and subsequent glaciers melted, the sediments of the Dwyka Group were deposited in the newly formed basin. These glacial deposits include diamictite, varved shale and mudstone with dropstones, fluvioglacial gravel and conglomerates. The total thickness of the group ranges from 600 m to 750 m.
The Dwyka Formation is considered to be Permo-Carboniferous in age, but due to ambiguities in the fossil record, more precise dating is not available. Maximum age inferred from fossils found in underlying strata is Late Devonian or Early Carboniferous, and minimum age inferred from fossils in the upper glacial deposits is Early Permian.
Weathering
Dwyka tillite weathers to form shallow, grey and dark grey, fine sandy clay loams and fine sandy clays. All these soils have a slow steady intake rate and high rates of runoff, hence the deeply incised valleys of the Kwazulu Natal South Coast.
Glacial Striation
Glacial Striation is the formation of one or several, long, straight, parallel lines or grooves in a bedrock surface, formed by boulders, gravel, and pebbles embedded in a glacier that has passed over the surface. See the image below (taken from another site).
Glacial Striation
The Outcrop at GZ
The Tillite outcrop at the published co-ordinates is particularly interesting because of the size and shape of the clasts (inclusions) in it. It is interesting to examine these and ponder on where the inclusions came from and how they got there. Their shape, colour, angularity and size are also all different.
Dwyka Tillite Outcrop
The Earthcache
In order to claim a find on this Earthcache, please complete the tasks below and email your answers to me via my gc.com profile. Answers should be received within 5 days, otherwise your found log may be deleted.
Task 1
Standing at the published co-ordinates, describe in your own words the tillite outcrop at your feet (don't fall into the water) with regard to colour and texture.
Task 2
Comment on amount of striation (glacial scour lines) present in the outcrop, as well as the number and size of clasts (inclusions) present. See above for examples of striation at another site.
Task 3
Estimate the maximum height of the quarry face from the outcrop referred to in tasks 1 & 2.
Task 4 (Site visit verification).
At the second waypoint (S30 11.529 E30 46.562) you can see some painting marks on the rocks. Were these marks made by geologists or non-geologists? Give an opinion for your answer.
Task 5 (optional)
Please post a photo in your online log, showing you and your GPS at the published co-ordinates.
References
http://en.wikipedia.org/wiki/Dwyka_Group
http://www.stec.ukzn.ac.za/geologyeducationmuseum/kzngeology/DwykaGroup.aspx