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