2014 Diary: Day 17 - Tillites are not a girl's best friend

Today was the final mapping day of the Field School 2014! Our participants were visibly upset and some even chose to stay out in the field for longer than usual. A large team that included Taufeeq, Ngqondi, Sims, Eve, Judy, Ndidi and Olma set out to answer some questions and recap the Gariep Belt history. This team travelled the top of the basal thrust zone, exposed within the Western region of the mapping area. This zone includes the tectonic melange, recently mentioned and comprises of large (meter-thick) thrust faults that have stacked diamictite and quartzite in a repetitive nature along at least five noticeable thrust packages. Based on the steepness of these faults, the team opted to spend some time practising their 4X4 skills before trying to quell some lithostratigraphic issues.

4X4 training was done with the basal thrust packages as a backdrop, literally. Participants engaged the low-range gear of our VW Amarok and attempted to crawl gently up the steep, rock slopes. All of this before having to perform a three-point turn and crawl back down. This is vital skill needed when driving in this kind of terrain and since we’re all alive now, everybody passed this test with flying colours!

Slow and steady, keeps you alive! Low-range crawl down a slope

Now, problem number one: what is a diamictite; what is a tillite:

A diamictite is essentially a sedimentary rock that has a matrix (fine-medium grained) and at least two different kinds of clast. This is as opposed to a conglomerate that mostly has one type of clast, commonly quartz pebbles. Now, a tillite is a diamicitite (i.e. matrix with at least two different kinds of clasts), but it has been deposited through glacial processes. Moreover, how do we distinguish these two rock types in the field?! Well, without the presence of proper drop stones (clasts that have “dropped” into sediment and caused soft-sediment deformation) this is very difficult- thus geology.com recommends diamictite as a safe name for “careful” geologists.  It will be interesting to see how our team handles this based on what they've seen in the field and what the literature elucidates to.

Spot the Diamicite/Tillite? Notice the dropstone!

Tomorrow further fears and questions will be quelled as we slowly make our exit from the Richtersveld!

2014 Diary: Day 17 - Diamictites are a girl's best friend

By: Nick, Tebogo, Penester and Khaya

Tebogo showing that the diamictites can be your friend in certain circumstances!

Well only in the Richtersveld perhaps, and only on another sun-drenched day, but after the singular (but expected!) non-success in earlier diamond quests among Orange River gravels, team attention went beyond the terraces to the rocks just above the basement of the last few days. The barren landscape is often almost surreal with quartz-covered plains contrasting with the dark desert varnish-covered outcrop ridges, though the geology today was relatively consistent overall. Largely Kaigas diamictite, with variably-sized clasts throughout, in this area the largest being carbonates along with various granitoids. The fine-grained matrix is generally sheared, with spaced phyllitic zones a feature, forming knife-edged outcrops protruding through the prevailing scree. The contact zone with the basement granodiorites and conglomerates and siltstones of the Vredefontein Formation is however marked by a gap in outcrop rather than spectacular thrust.

Clockwise from top left: the thorn among the roses; the contact between the basement granodiorites in the left foreground and the metasediments of the Vredefontein Formation; the zone proximal to the thrust though is marked by numerous quartz veins; a drawback of removing your boots during lunch break is that they may disappear!; you’re not looking for these boots are you? 

2014 Diary: Day 10 - First Day at School

Today the team split into smaller groups and started mapping individual areas across the Gariep Belt. All teams began within the Richtersveld National Park following the southern route along the Hellshoogte Pass. This road provides some interesting scenery straddling tall quartzitic and dolomitic lithologies. The Field School of 2014 has split the final portion of the Gariep Belt into several smaller sheets (approximately 5 X 6 km). It is envisioned that everybody within the team will have their own sheet to complete, however with geological data being shared across all completed sheets. With this, team members will aim to ensure and motivate their partners to collect the data of the highest quality (not knowing who might be providing the bulk of their data).

Today we follow the progress of Tebogo, Belinda and Haajiera. This team aimed to complete a section across Zone 20. Preliminary remote sensing suggested that this would be a relatively simple sheet, however it was anything but. This team started in a strange clast-supported diamictite (previously not seen by this group). This is highly foliated and coarse-grained (ca. 1-2 cm), with clasts of quartzite, arenite and dolomite. Of further interest was the presence of basaltic dykes cutting across, and being entrained within this unit. The dyke had phenocrysts of plagioclase, showing a strong mineral elongation lineation and in some places amygdaloidal texture. This unit bares remarkable similarities to the volcanics within the Vredefontein, however provided this group with a few things to consider: 1. If this was older (Vredefontein being older than the Numees/Kaigas), why does this show no foliation/evidence for strong deformation; 2. If this was younger, why are there also rafts of dyke entrained in the foliated diamicitite?

Clockwise: Basaltic dyke entrained and cutting across a highly sheared, clast-supported diamictite; Kaigas-proper dimictite and Numees-proper varved carbonaceous shale with dropstone; our team hard at work

Thereafter, they proceeded to climb some mountains and encountered the Wallekraal, within the Hilda Subgroup. This unit, similar to what was seen during day 2 of the show-and-tell displays both a bedding plane and foliation plane. This is possible due to the bedding being preserved at the contact between the carbonate and phyllite layers. Deformation was concentrated due to strain partitioning in the phyllite, thus forming a prominent foliation in this rock while the carbonate would simply recrystallise in response.

Further along, the group encountered Vioolsdrift basement rocks of this area. This mostly comprised of metavolcanics and granite-gneisses. The metavolcanics in particular displayed strain markers highlighting a top-to-the-west sense of shearing. There were also several large thrust faults, forming tectonic contacts with the Rosyntjieberg quartzites; and shear zones forming a prominent crenulation cleavage in places.

In total this group consolidated their knowledge of identifying and measuring structural elements expressed in the various lithologies, in addition to completing thorough rock descriptions and producing detailed field sheets. They will now look forward to sharing/learning from others tomorrow as the groups will continue to vary and complete the respective sheets.

Click here to see the mapping route of the group: Mapping 1