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1) Is phyllite sedimentary or metamorphic rock?
2) How does dune rock form?
3) What gets metamorphised to form phyllite?
4) What causes the sedimentation that forms stromatolite?
5) At the published coordinates is a quartzite rock. What feature of this rock do you immediately notice? (Locational question).
It would be great if you could confirm if you managed to identify samples of all five rock types at the GZ
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There are at least five rock type in a very small area around the GZ. The different rocks are identified below.
Consolidated Calcareous Dune Rock
At the published coordinates the dominant formation from a visual point of view is the large rock with a sandy appearance. This is aeolian calcareous dune rock. Sand dunes were formed during the Pleistocene period. During this period pieces of mollusc shell was mixed with the sand grains in the dunes. Over time rain water seeps through the sand and dissolves the calcium carbonate of the shell fragments and percolates into the sand layer lower down and "cements" it into harder rock.
This rock is probably the Nahoon Formation, even though this formation does not appear on the geological map of the area. There are pebbles in the rock near the top close to the path. These pebbles indicate that some of the outcrop is probably palaeo-beach deposit and not aeolian. Given the crossbedding in most of the deposit the pebble layer probably represent a storm event where the beach cut into the fore-dune area.
There is an Earth Cache further East at Flat Rocks (Aeolian Nahoon Formation) that contains further information on the Nahoon Formation.
This rock is sedimentary rock.
2 Phyllite
Phyllite is a type of foliated metamorphic rock created from slate that is further metamorphosed so that very fine grained white mica achieves a preferred orientation. It is primarily composed of quartz, sericite mica, and chlorite. There is a great deal of phyllite visible from the published coordinates.
There is also an Earth Cache further East at Schoenmakerskop (Schoenmakerskop Phyllite) that contains further information on phyllite.
If phyllyte is subject to a greater degree of heat and pressure it becomes schist.
Phyllite occurs all along this coastline as part of the Kleinrivier Formation.
Stromatolite
Stromatolites are layered bio-chemical accretionary structures formed in shallow water by the trapping, binding and cementation of sedimentary grains by biofilms (microbial mats) of microorganisms, especially cyanobacteria.
In this area there are stromatolite ponds. The rims of these ponds have stromatolites formed by cyanobacteria precipitating calcium carbonate.
Once again we have a detailed Earth Cache (PE Tufa stromatolite ecosystems) in the area. This Earth Cache gives detailed information on this rock type.
Milky Quartz
Quartz is the second most abundant mineral in the Earth's continental crust, after feldspar. Its crystal structure is a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical formula of SiO2. The cloudiness of milky quartz comes from microscopic inclusions of fluids that have been encased in the crystal from the time the crystal first grew.
Milky Quartz is very common along this section of the coastline and appears in veins. This area has been severely tectonised and these veins are very common in the phyllites. The veins range in thickness from mm to a meter or so. The veins are folded, and in some case the folding is extreme. One can clearly see the milky quartz veins from the published coordinates.
Quartzite
Quartzite is a hard, non-foliated metamorphic rock which was originally pure quartz sandstone. Sandstone is converted into quartzite through heating and pressure usually related to tectonic compression within orogenic belts. Pure quartzite is usually white to grey, though quartzites often occur in various shades of pink and red due to varying amounts of iron oxide (Fe2O3). Other colours, such as yellow, green, blue and orange, are due to other mineral impurities.
Quartzite is also common along this section of coastline. It normally has a reddish colour but the quartzites in this formation are a rather grey in colour (possibly due to muddy material in the original sediment). There is quartzite VERY close to the published coordinates.
References
McCarthy T & Rubidge B (2005). The story of Earth & Life: A Southern African perspective on a 4.6-billion-year journey.
Wikipedia
Nick Norman & Gavin Whitfield (2006) Geological Journeys
Nick Norman Box of Rocks
Mr Callum Anderson MsC - Geology Department NMMU