2014 Diary: Day 9 - Cooking the Kuboes

As temperatures increase, so too does the level of geological knowledge being exchanged during the field school. Today was the second part of the show-and-tell day and the team began by looking at the Kuboes pluton. This large (ca. 35 km wide) pluton formed during the Pan-African (ca. 507 Ma) after the closure of the Adamaster Ocean and the amalgamation of Gondwana. The team closely examined the minerals and did a mineral proportion analysis. Once in agreement, they plotted this on a QAP diagram. A QAP (quartz, alkali-feldspar, and plagioclase) provides a mechanism for accurately labelling intrusive/extrusive rocks. Based on their assessment, the team decided that the Kuboes is a quartz-bearing monzonite. In addition, they noticed the interesting Rapakivi texture. This texture is formed after the exsolution of plagioclase along the grain boundary of the alkali-feldspar.

Clockwise: Kuboes quartz-monzonite with Rapakivi texture; Numees tillite, Wallekraal phyllite-carbonate layers; Kaigas diamictite

The team then moved on and described the Numees formation tillite. This was of particular interest to the group following their visit to the Dwyka, near Laingsburg. The Numees displays well-preserved varved sediments with many exotic clasts and drop stones. The Numees was deposited during the Marinoan glaciation where most of the Earth was covered in ice. The Numees within the contact aureole with the Kuboes pluton (consider that the Kuboes would have been emplaced with temperatures up to ca. 1200 degrees Celcius) underwent contact metamorphism to upper hornfels conditions. This, together with large quantities of fluids passing through the Numees made it highly prospective for gold and fluorite, however no economically viable commodities was ever located. Deformation is clear in the Numees with the presence of numerous shear sense indicators (i.e. rotated porphyroblasts), highlighting top-to-the-east sense of direction. A sense of direction that highlights the regional thrust direction.

 The Pickelhaube (Hilda Subgroup) was thereafter investigated. This relates to a period when glacial conditions had subsided, allowing for the introduction of shallow-marine conditions. During this time the limestone of the Pickelhaube were deposited; characteristically showing elephant-skin weathering. There are also two particular variations within this unit, a lighter variety and darker variety. This highlights changes in the mineral chemistry and concentration of iron in these rocks.

 The Wallekraal (together with the Pickelhaube) also within the Hilda Subgroup was the final unit investigated today. This was of most interest to the team since it showed bedding and foliation planes. Here the team had an opportunity to test their abilities with the new Silva compasses and attempt measuring both these features.

In addition to the rocks, the team also visited the Primary School in the town of Kuboes, a short visit to deliver some posters. The team will return on Friday to continue the Richtersveld Science Day and continue a lecture series and workshop.

Some of the guys and girls in the field: Clockwise: Ndidi measuring lineation; note taking; sisters Belinda and Tebogo share a joke; Neo and Shane capturing data using the Geotac field computer

Tomorrow the team will split into smaller groups and start the mapping programme, under the supervision of our team of experts!