11th CGS Field School - Mapping the Gariep Belt

And so, finally we have completed another highly successful CGS Field School. This was arguably the most grueling and challenging field school yet. It all came to an end where our team set out to map in the Gariep fold and thrust belt. This of course represents a period spanning the existence of the Supercontinents' Rodinia and Gondwana. Read more about the Gariep Belt here.

Rocks

The team was split into three separate groups and allocated three different maps across the southern portion of the Gariep Belt. Each sheet was unique in its own right and required varying degrees of focus from the various groups. Throughout the mapping period the teams were exposed to several different rocks types characteristic of this region. Some of these include:

TL: Sheared 1,800 Ma Vioolsdrift (Richtersveld Suite) grano-diorite, note the development of spaced cleavage; TR: Magnetite-bearing quartzite of the Stinkfontein Group; ML: Kaigas diamictite with characteristic carbonate clasts; MR: Marble of the Hilda Subgroup; BL: Sheared Numees diamicitite; BR: Iron stone of the Jakkalsberg Member of the Numees.

Tectonics

For those who took the time to hike and closely examine the rocks, there was a continuous sense of dejavu. This was of course due to the continuous repetition of the Gariep sequence. This duplication allowed the team to really consider the tectonic history of this region. 

The tectonics characterises the period after Rodinia broke up, and in the process, formed the mighty Adamastor Ocean; and eventually the growth of Gondwana. This ocean could somewhat be synonymous to the present-day Atlantic, separating the Kalahari (KC) and Rio de la Plata (RC) Cratons. It was within the Adamastor Ocean where much of the Gariep rocks were deposited.

Imagination, and some walking is needed - But, after some time you may begin to exist in a strange space-time continuum bubble where you begin to visualise everything.

Tectonic inversion c. 555 Ma saw the RC and KC encroach, closing the Adamastor Ocean and eventually culminating with the RC accreting atop the KC during the formation of Gondwana. Several structural features were also seen characterising this event:

TL: Fault breccia within Numees diamictite; TR: Pseudotachylite development along the frontal thrust zone; BL: What was the Kaigas diamicitite, now highly deformed schist; BR: Mylonite developed along thrust zone.

Life

Another great thing about spending time in the Richtersveld is interacting with the local people, and clearing your troubled mind in vast and endless landscape.

Sometimes the roads are long and tough. Only for you to discover that this is only half the journey.

Sometimes you chose the easiest path home. Only to discover this means walking over isoclinally folded carbonates interlayered with thrust-bounded phyllite.

But, there's a story to be founded...

And history to be uncovered...

And, some crazy weather to Moer you

Come the third day of battle, look to the East!

And you'll find yourself sad to leave, because you've left a part of yourself behind...

The Gariep Belt

Overview

The CGS Field School has been mapping the Gariep Belt since 2008 in an attempt to gather accurate field data across this orogenic zone. It is envisioned that this data will be used to produce a special edition Gariep Belt map. The Gariep Belt is an arcuate orogenic belt that extends several hundred kilometers from southwestern Namibia, into northwestern South Africa. The Gariep Belt can be subdivided into two separate terrains, namely, the Port Nolloth Zone and the Mamora Terrain. The Port Nolloth Zone consists of various sediments and volcanics that were deposited in a passive rift margin.

Simplified map of the Gariep Belt (within the stippled lines); note the field area for the Field School 2014 (yellow)

Mamora Terrain

The Mamora Terrain is considered to be an allochthonous zone, comprised of several tectonic units. These include (from lowermost to uppermost): The Schakalsberge (mafic volcanic rocks and dolomite), Oranjemund (turbidites) and Chameis Complexes (mafic rocks of intra-plate and mid oceanic ridge origins). The units making up the Mamora Terrain was thrusted onto the Port Nolloth Terrain in a east-southeast direction during the formation of the Gondwana supercontinent (ca. 545 Ma). The 2014 Field School will have minimal exposure to the Mamora Terrain and rather focus on the Port Nolloth Zone.

The Port Nolloth Zone

The Port Nolloth Zone comprises of rocks that were deposited in a volcano-sedimentary rift graben associated with the breakup of the supercontinent Rodinia. Here, periods of sedimentation was accompanied by periods of volcanism, as evident by the abundance of sedimentary rocks closely associated with volcanics (as will be seen during the field school, specifically within the Vredefontein). As the rift continued to evolve, together with a marine facies change associated with sea-level rise, a rising fault scarp led to the deposition of alluvial-fan type rocks. These now make up the bulk of the Stinkfontein Group. Shallow marine conditions prevailed along the continental margin of this rift basin, allowing for the deposition of carbonaceous rock, namely the Hilda Subgroup. The history of this region can be capped off with the development of a glacial event. This led to the deposition of the Numees, and the earlier Kaigas (prior to the Hilda Subgroup). The former being demarkated as a Tillite, and the latter a glacial diamictite. Later, associated with the formation of Gondwana, was the emplacement of the Kuboes pluton (based on field evidence, syn-post tectonic).

Simplified legend of the Port Nolloth Zone, with typical thicknesses shown in parenthesis

Here are a few examples of some of the rock types that will be encountered during the field school:

Lekkersing: flaggy quartzite (base of the Stinkfontein marked by a thick conglomerate)
Vredefontein: quartzo-feldspathic arenite, phyllite, volcanic (basalt, tuff)
Kaigas: diamictite
Pickelhaube: limestone, dolomite
Wallekraal: siliclastic rocks
Numees: tillite (sheared diamictite), iron stone (Jakkalsberg Member)
Holgat: metapelite, feldspathic arenite, conglomerate, limestone (Bloedrift Member)
Kuboes: granite, monzonite, gabbro

(A): Highly sheared NNMB basement; (B): Flaggy quartzites of the Lekkersing Formation; (C): Vredefontein arenitic quartzite, showing cross bedding; (D): Vredefontein amygdaloidal basalt with plagioclase laths; (E): Kaigas diamictite, with various clasts; (F): Pickelhaube limestone